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  • View in gallery

    Best track of Hurricane Karl (2010) starting at 0000 UTC 14 Sep with intensity classifications marked every 6 h. The days in September at 0000 UTC are also shown. The green circles denote tropical depression status, open hurricane symbols are tropical storm, and closed hurricane symbols are hurricane status with the category listed in the center. The inset shows the time series of maximum surface wind speed (m s−1) with the Global Hawk flight enclosed by the black lines.

  • View in gallery

    A sequence of GOES IR images of Hurricane Karl (2010) in the Bay of Campeche during an RI episode spanning the GH flights into the storm. The times shown are (a) 1845 UTC 16 Sep, (b) 2215 UTC 16 Sep, (c) 0140 UTC 17 Sep, and (d) 0501 UTC 17 Sep. The white arrow in (a) denotes the environmental vertical wind shear vector valid over the time interval. The star represents the estimated storm center. The track of the GH ± 2 h from the satellite time stamp is shown in white with the large numbers denoting the hour (UTC).

  • View in gallery

    Composite analysis of HIWRAP data averaged over the total GH sampling interval (12–13 h) at 2-km height for (a) Ku-band reflectivity (dBZ) and horizontal wind vectors and (b) horizontal wind speeds (m s−1). The large gray arrow in (a) is the large-scale vertical wind shear vector valid for this time interval with a value of ~5 m s−1.

  • View in gallery

    As in Fig. 3, but for 8-km height. The white circle in (a) shows the location of the low-level (2 km) and time- and azimuthally averaged RMW. The gray circle in (a) shows the maximum azimuthally averaged RMW over the 12–13-h GH sampling period.

  • View in gallery

    (a) The vertical vorticity (s−1) computed from NOAA TA wind retrievals averaged between 0.5- and 4-km height and in azimuth for three different time periods. (b) The perturbation vertical vorticity averaged between 0.5- and 4-km height and in time (1842–2042 UTC). The mean RMW during this time period is ~25 km.

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    NOAA WP-3D flight through the inner core of Hurricane Karl (2010) centered at ~1842 UTC 16 Sep showing (a) LF C-band reflectivity at 3.7-km height overlaid with TA-derived winds at 4-km height and (b) TA-derived divergence (s−1) averaged between 0.5- and 4-km height overlaid with perturbation wind vectors averaged over the same interval. The white arrows in (b) highlight features discussed in the text. The gray line in (b) marks the eye–eyewall interface using the gradient in LF reflectivity. The gray “C” and “A” letters in (b) denote the centers of cyclonic and anticyclonic mesovortex circulations, respectively.

  • View in gallery

    GH overpass of the inner core of Hurricane Karl (2010) between 1853 and 1919 UTC 16 Sep showing (a) HAMSR 54-GHz Tbs (K) and (b) HIWRAP Ku-band reflectivity (dBZ). In both panels, horizontal wind vectors from HIWRAP are overlaid and the analysis level is 2-km height. The white arrows and white outline in (a) highlight a protrusion of the warm core discussed in the text. The azimuthal mean RMW at 2-km height of ~30 km is also shown in (a) by the white circle. The reference wind vector in (a) applies to both panels.

  • View in gallery

    HIWRAP vertical cross sections at nadir through the storm center in the north-to-south direction for the GH overpass between 1853 and 1919 UTC 16 Sep. The data shown are (a) Ku-band reflectivity (dBZ), (b) meridional (radial) winds (m s−1) with divergence overlaid in black contours (values shown are from −4 × 10−3 to −1 × 10−3 s−1), (c) vertical winds (m s−1), and (d) vertical vorticity (s−1). The large gray arrows in (b),(c) highlight features discussed in the text while the white lines in these panels show the zero contours. The black arrows in (c),(d) also highlight features discussed in the text. Note the azimuthal mean RMW at 2-km height at this time is ~30 km.

  • View in gallery

    NOAA WP-3D flight through the inner core of Hurricane Karl (2010) centered at ~1930 UTC 16 Sep showing (a) LF reflectivity at 3.6-km height overlaid with TA-derived winds at 4-km height and (b) TA-derived divergence (s−1) averaged between 0.5- and 4-km height overlaid with perturbation wind vectors averaged over the same interval. The gray line in (b) marks the eye–eyewall interface using the gradient in LF reflectivity. The gray “C” and “A” letters in (b) denote the centers of cyclonic and anticyclonic mesovortex circulations, respectively.

  • View in gallery

    As in Fig. 7a, but for the GH overpass between 1938 and 1957 UTC 16 Sep. The large white circle denotes the azimuthally averaged RMW at 2-km height of ~25 km and the white dot is the storm center.

  • View in gallery

    As in Fig. 8, but for the GH overpass between 1938 and 1957 UTC 16 Sep in the southeast (negative radius) to northwest (positive radius) direction. The data shown are (a) Ku-band reflectivity (dBZ), (b) radial winds (m s−1) with divergence overlaid in black contours (values shown are from −4 × 10−3 to −1 × 10−3 s−1), (c) vertical winds (m s−1), and (d) tangential winds (m s−1). The gray arrows in (b),(c) highlight features discussed in the text while the white lines in these panels show the zero contours. Note the azimuthal mean RMW at 2-km height at this time is ~25 km.

  • View in gallery

    Snapshots of LF reflectivity at 3.6-km height covering a 2.5-min period between 2039:40 and 2042:13 UTC 16 Sep. The times shown are (a) 2039:40, (b) 2040:10, (c) 2040:42, and (d) 2042:13 UTC. The gray “C” letter in each panel denotes the center of a cyclonic mesovortex circulation identified in HIWRAP and TA wind fields during this time period (see Figs. 13b and 14).

  • View in gallery

    As in Fig. 9, but for the NOAA WP-3D transect centered at ~2042 UTC 16 Sep.

  • View in gallery

    As in Fig. 7a, but for the GH overpass between 2009 and 2055 UTC 16 Sep with a center crossing at ~2040 UTC. The large white circle denotes the azimuthally averaged RMW at 2-km height and the white dot is the storm center. The gray box shows the region where data are averaged in the y direction for subsequent figures. The “C” letter denotes the center of a mesovortex cyclonic circulation.

  • View in gallery

    Vertical cross sections of radar data averaged between ~0 and 6 km in the +y direction (see Fig. 14) from (a) HIWRAP Ku-band reflectivity (dBZ) valid at ~2040 UTC 16 Sep, (b) HIWRAP-derived radial winds (m s−1), (c) NOAA WP-3D–derived radial winds (m s−1) valid at ~2042 UTC 16 Sep, (d) HIWRAP-derived vertical winds (m s−1), and (e) NOAA WP-3D–derived vertical winds (m s−1). Note that there are no data on the right side of the HIWRAP panels owing to the coverage and cross section cut. The vertical gray line in (b)–(e) denotes the western eye–eyewall interface using the gradient in reflectivity. The large gray arrows in (d),(e) highlight features discussed in the text.

  • View in gallery

    HIWRAP vertical cross sections of Ku-band reflectivity (shading; dBZ) and tangential winds (contours; m s−1) at nadir for the GH overpasses centered at (a) 0012 UTC 17 Sep in southeast-to-northwest direction, (b) 0345 UTC 17 Sep in southwest-to-northeast direction, (c) 0550 UTC 17 Sep in southeast-to-northwest direction, and (d) 0805 UTC 17 Sep in southeast-to-northwest direction.

  • View in gallery

    Conceptual diagram highlighting the measurements and analysis from the HIWRAP, HAMSR, and WP-3D instruments during the Hurricane Karl (2010) sampling. The arrows at lower levels represent the mesoscale flow and the arrows in the clouds represent the convective-scale flow. Red arrows indicate anomalously warm, buoyant air with blue arrows the opposite.

  • View in gallery

    Scatterplot of HIWRAP horizontal wind speed errors (|HIWRAP − WP-3D flight level|) vs WP-3D flight-level wind speeds for the coordinated flight during HS3 on 25 Sep 2013. The points are colored by HIWRAP Ku-band reflectivity. Note the HIWRAP winds are computed using Ku-band Doppler velocities. See text for more details.

  • View in gallery

    As in Fig. A1, but for the coordinated flight during GRIP (sampling of Hurricane Karl at ~2040 UTC 16 Sep 2010). The points are colored by HIWRAP Ku-band reflectivity. Note the HIWRAP winds are computed using a combination of Ku- and Ka-band Doppler velocities. See text for more details.

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The Rapid Intensification of Hurricane Karl (2010): New Remote Sensing Observations of Convective Bursts from the Global Hawk Platform

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  • 1 Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, and NASA Goddard Space Flight Center, Greenbelt, Maryland
  • | 2 NASA Goddard Space Flight Center, Greenbelt, Maryland
  • | 3 NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division, Miami, Florida
  • | 4 NASA Goddard Space Flight Center, Greenbelt, Maryland, and Oak Ridge Associated Universities, Oak Ridge, Tennessee
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Abstract

The evolution of rapidly intensifying Hurricane Karl (2010) is examined from a suite of remote sensing observations during the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment. The novelties of this study are in the analysis of data from the airborne Doppler radar High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) and the new Global Hawk airborne platform that allows long endurance sampling of hurricanes. Supporting data from the High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (HAMSR) microwave sounder coincident with HIWRAP and coordinated flights with the NOAA WP-3D aircraft help to provide a comprehensive understanding of the storm. The focus of the analysis is on documenting and understanding the structure, evolution, and role of small-scale deep convective forcing in the storm intensification process. Deep convective bursts are sporadically initiated in the downshear quadrants of the storm and rotate into the upshear quadrants for a period of ~12 h during the rapid intensification. The aircraft data analysis indicates that the bursts are being formed and maintained through a combination of two main processes: 1) convergence generated from counterrotating mesovortex circulations and the larger vortex-scale flow and 2) the turbulent (scales of ~25 km) transport of anomalously warm, buoyant air from the eye to the eyewall at low levels. The turbulent mixing across the eyewall interface and forced convective descent adjacent to the bursts assists in carving out the eye of Karl, which leads to an asymmetric enhancement of the warm core. The mesovortices play a key role in the evolution of the features described above. The Global Hawk aircraft allowed an examination of the vortex response and axisymmetrization period in addition to the burst pulsing phase. A pronounced axisymmetric development of the vortex is observed following the pulsing phase that includes a sloped eyewall structure and formation of a clear, wide eye.

Corresponding author address: Stephen R. Guimond, NASA Goddard Space Flight Center, Code 612, Greenbelt, MD 20771. E-mail: stephen.guimond@nasa.gov

Abstract

The evolution of rapidly intensifying Hurricane Karl (2010) is examined from a suite of remote sensing observations during the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment. The novelties of this study are in the analysis of data from the airborne Doppler radar High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) and the new Global Hawk airborne platform that allows long endurance sampling of hurricanes. Supporting data from the High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (HAMSR) microwave sounder coincident with HIWRAP and coordinated flights with the NOAA WP-3D aircraft help to provide a comprehensive understanding of the storm. The focus of the analysis is on documenting and understanding the structure, evolution, and role of small-scale deep convective forcing in the storm intensification process. Deep convective bursts are sporadically initiated in the downshear quadrants of the storm and rotate into the upshear quadrants for a period of ~12 h during the rapid intensification. The aircraft data analysis indicates that the bursts are being formed and maintained through a combination of two main processes: 1) convergence generated from counterrotating mesovortex circulations and the larger vortex-scale flow and 2) the turbulent (scales of ~25 km) transport of anomalously warm, buoyant air from the eye to the eyewall at low levels. The turbulent mixing across the eyewall interface and forced convective descent adjacent to the bursts assists in carving out the eye of Karl, which leads to an asymmetric enhancement of the warm core. The mesovortices play a key role in the evolution of the features described above. The Global Hawk aircraft allowed an examination of the vortex response and axisymmetrization period in addition to the burst pulsing phase. A pronounced axisymmetric development of the vortex is observed following the pulsing phase that includes a sloped eyewall structure and formation of a clear, wide eye.

Corresponding author address: Stephen R. Guimond, NASA Goddard Space Flight Center, Code 612, Greenbelt, MD 20771. E-mail: stephen.guimond@nasa.gov

1. Introduction

The intensification of tropical cyclones (TCs) is a complex process that is governed by nonlinear coupling of physics across a vast array of space and time scales. On the slow/large scales, a sufficiently warm ocean and low vertical wind shear have been identified as providing favorable environmental conditions for the intensification of TCs (e.g., Kaplan and DeMaria 2003). On the fast/small scales, a large body of evidence has shown that deep, rotating, convective towers are responsible for the intensification, including rapid intensification (RI), of TCs (Steranka et al. 1986; Simpson et al. 1998; Heymsfield et al. 2001; Kelley et al. 2004; Montgomery et al. 2006; Braun et al. 2006; Reasor et al. 2009; Guimond et al. 2010; Molinari and Vollaro 2010; Rogers et al. 2015). However, recent studies by Jiang (2012) and Tao and Jiang (2015), using statistics from a large satellite database, have stated that shallow and moderate precipitation is more important for the RI process than deep convective precipitation. Uncertainty remains in these satellite-based studies as a result of the inherently coarse temporal (and spatial) resolution for analyzing the cloud/precipitation life cycle as well as the definition of RI relative to other studies.

It is the fast/small scales that are the most challenging for the observation, numerical modeling, and understanding of TCs. Deep convective towers in TCs have lifetimes of an hour or less with horizontal scales of ~10 km (Montgomery et al. 2006; Houze et al. 2009; Guimond et al. 2010), making it difficult to observe their kinematic properties, especially from conventional aircraft, which can only sample storms for short periods of time (~5 h). The turbulent, highly nonlinear character of deep convective towers and their interaction with the TC vortex are major challenges for numerical models and our physical understanding because those scales not explicitly resolved must be parameterized, which are not always adequate (e.g., Persing et al. 2013) and there can be considerable sensitivity to the algorithms used to solve the fluid-flow equations (e.g., Guimond et al. 2016, manuscript submitted to J. Atmos. Sci.).

The dynamics responsible for the rapid intensification of TCs from localized, deep convection project onto two classes of modes relative to the storm center: axisymmetric and asymmetric. In the axisymmetric framework, the projection of localized heat forcing onto the azimuthal mean results in rings of heating typically maximized inside the radius of maximum winds for intensifying storms. Rogers et al. (2013) analyzed a large set of airborne Doppler radar composites of intensifying and steady-state TCs and discovered that a key characteristic of intensifying TCs is the location of deep convective towers inside the radius of maximum winds (RMW). Earlier studies by Schubert and Hack (1982), Nolan et al. (2007), and Vigh and Schubert (2009) have elucidated the dynamics of intensifying hurricane vortices, finding that convective heating placed inside the RMW enables more efficient conversion of potential to kinetic energy owing to the increased inertial stability of the vortex.

The heating rings drive an axisymmetric secondary circulation with radial inflow at low levels, updrafts through the core of the heating, and radial outflow aloft. In the azimuthal mean, the vortex intensifies through the radial convergence of absolute angular momentum, which is materially conserved above the boundary layer. This framework has been understood for many years (e.g., Shapiro and Willoughby 1982). Other axisymmetric theories for TC intensification have been presented such as the work of Emanuel (1986) and Rotunno and Emanuel (1987), which focus on the cycling of energy extracted through the thermodynamic disequilibrium at the air–ocean interface.

In the asymmetric framework, the heating and vorticity asymmetries generated from localized convective forcing interact with the mean flow through eddy heat and momentum fluxes, which can lead to intensification of the vortex for upgradient transport (Montgomery and Kallenbach 1997). This process is generally called “axisymmetrization” and has been shown to occur in observational (e.g., Reasor et al. 2000, 2009) and modeling (e.g., Montgomery et al. 2006; Persing et al. 2013) studies. In nature, the axisymmetric and asymmetric modes are coupled to one another with axisymmetric processes often playing the largest role (e.g., Nolan and Grasso 2003), but with asymmetric dynamics contributing a significant, nonnegligible component of the overall system intensification (e.g., Montgomery et al. 2006; Persing et al. 2013; Guimond et al. 2016, manuscript submitted to J. Atmos. Sci.).

In addition to these effects, deep convective towers have also been observed to initiate localized interaction between the eye and eyewall. For example, the studies of Heymsfield et al. (2001) and Guimond et al. (2010), which analyzed very-high-resolution airborne radar data (along-track sampling of 100 m), showed that deep convective towers intensified the warm core through compensating subsidence around strong updrafts and its turbulent transport toward the eye. This intense, localized transport of air from the eyewall to the eye has important implications for storm intensification through the attendant inward flux of angular momentum. Finally, Wang and Wang (2014) simulated the intensification of Typhoon Megi (2010) and found that convective bursts in the eyewall helped to intensify the TC warm core at upper levels by detraining high potential temperature air from the stratosphere.

The purpose of this paper is to analyze the RI of Hurricane Karl (2010), which coincided with a convective burst episode, from a suite of remote sensing observations to understand more details of the dynamics occurring on the fast/small scales. The novelties of this study are in the use of a new airborne radar and a new airborne platform for hurricane research that allows long endurance (up to 24 h) sampling. Details of these new technologies will be discussed in the next section.

2. Data and processing

a. HIWRAP

The High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) is an airborne Doppler radar that was developed at the NASA Goddard Space Flight Center (GSFC) with the goal of studying hurricanes and other precipitating systems. One of the unique features of HIWRAP is its ability to fly on NASA’s Global Hawk (GH) unmanned aircraft, which operates at ~18–19-km (60–62 kft) altitude and can remain airborne for ~24 h. The long endurance of the GH is a significant capability for hurricane research. Hurricanes form over remote regions of the ocean with important physical processes occurring on fast time scales that can be easily missed by conventional aircraft that can only remain airborne for ~6 h. Satellite measurements can reach these remote areas, but the observational capabilities, including spatial/temporal sampling, is less than optimal.

HIWRAP is a dual-frequency (Ku and Ka band), single-polarized (vertical for inner beam, horizontal for outer beam), downward-pointing and conically scanning (16 rpm) Doppler radar with two beams (~30° and 40° tilt angles) and 150-m-range resolution. The GH aircraft has an airspeed of ~160 m s−1, which yields ~600-m along-track sampling for HIWRAP. More details on HIWRAP can be found in Li et al. (2016).

The NASA Genesis and Rapid Intensification Processes (GRIP) experiment in 2010 was the first time HIWRAP collected significant data and some issues with the data quality (e.g., excessive noise at Ku band due to a variety of issues including pulse processing) were found. To address these issues, we have done two things: 1) pulse pair estimates at Ku band were reprocessed with 128 pulses averaged (azimuthal resolution of ~2.8°), which improves the signal-to-noise ratio (SNR) over the original averaging interval of 64 pulses, and 2) Ku-band wind retrievals below the noise saturation level (determined using a power threshold, which translates to ~25 dBZ at 3-km height) were replaced with the corresponding Ka-band wind retrievals, which provide a higher SNR and, thus, lower uncertainty in the Doppler velocities in these regions. In the flights over Karl presented in this work, only the inner (30°) beam was functional, which provides a swath width at the surface of ~20–22 km.

Retrievals of the three-dimensional wind vector over the entire radar-sampling volume are performed with the three-dimensional variational data assimilation (3DVAR) algorithm described in Guimond et al. (2014). The 3DVAR method combines an observational error term as well as constraints that include the anelastic mass continuity equation, a Laplacian filter, and the impermeability condition at the surface. No background field is included in the algorithm because a quality wind field can be obtained using the observations and dynamic constraints alone. A coefficient of 2Δx2 was used for the mass continuity constraint and 0.5Δx4 was used for the filtering constraint with Δx representing the horizontal grid spacing. These values were chosen based on wind vector solution sensitivity tests that provided reasonable accuracy and damping characteristics. The retrievals are performed on a storm-following grid with a horizontal grid spacing of 1 km and vertical spacing of 1 km. Retrievals with vertical spacing of ~150 m are possible, but 1-km spacing was deemed sufficient for the present study. The first level of useful data is ~1-km height for the inner beam. Below this height, antenna side lobes interacting with the surface obscure the precipitation signal. NOAA’s Hurricane Research Division (HRD), using the Willoughby and Chelmow (1982) method, provided storm center estimates. The mean storm motion vector averaged over the aircraft-sampling period was removed from the HIWRAP-derived horizontal winds.

Guimond et al. (2014) showed that simulated and in situ errors for the horizontal wind components were ~2.0 m s−1 or ~7% of the hurricane wind speed. The errors in the vertical velocity were strongly dependent on the across-track location of the measurements with comparisons to in situ data revealing errors of ~2.0 m s−1 at nadir. Simulated off-nadir vertical velocity estimates were still quite accurate within ±5 km from nadir (Guimond et al. 2014). The in situ errors noted above used data from the Imaging Wind and Rain Airborne Profiler (IWRAP) flying on the NOAA WP-3D aircraft, which has a similar scanning geometry to HIWRAP. The appendix presents comparisons of in situ data to HIWRAP retrievals, which reveal that for wind speeds >10 m s−1 the mean error in the computed wind speed and direction is ~1–4 m s−1 and ~10°–20°, respectively.

b. NOAA WP-3D radars

The NOAA WP-3D tail (TA) radar is an X-band airborne Doppler radar that scans in a cone 20° fore and aft of the plane perpendicular to the aircraft with a scan rate of 10 rpm and along-track sampling of fore/aft sweeps of ~1.6 km (Gamache et al. 1995). Retrievals of the three-dimensional wind vector are performed using the variational methodology outlined in Gamache (1997) and Reasor et al. (2009) at a grid spacing of 2 km in the horizontal and 0.5 km in the vertical. Quality control procedures on the raw observations of reflectivity and radial velocity can be found in Gamache (2005). The mean storm motion vector averaged over the aircraft-sampling period was removed from the TA-derived horizontal winds.

The NOAA WP-3D aircraft also carries a C-band lower fuselage (LF) radar that provides a scan of radar reflectivity every 30 s at approximately the flight-level height. These data are useful for identifying and tracking vortex- and convective-scale features of TCs close to the aircraft. The large vertical beamwidth of 4.1° can cause smearing of features and inadequate beam filling for ranges greater than ~60 km (Marks 1985). Analysis of the LF data is confined to ranges less than 50 km to avoid these problems.

c. HAMSR

The High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (HAMSR) is a passive microwave sounder measuring upwelling radiation from the atmosphere at frequencies sensitive to temperature (~50 and ~118 GHz) and water vapor (~183 GHz). The intensity of convective clouds can also be estimated in regions where upwelling radiation is scattered out of the beam by ice particles, which results in anomalously low brightness temperatures (Tbs) at the instrument receiver. The HAMSR instrument scans ±60° across track providing a swath width of ~65 km from the height of the GH aircraft. However, we remove data greater than ±45° because larger errors are found beyond this range (Brown et al. 2011). The HAMSR Tbs exhibit across-track dependence, which is due to the viewing geometry of the instrument. As the radiometer scans away from nadir, the optical pathlength becomes larger, which pushes the weighting function peak higher in altitude, inducing a cooling effect. Goldberg et al. (2001) analyzed satellite radiometer data to show that this limb effect is relatively small within the inner portion of the swath. The analysis of HAMSR data in this paper focuses on the inner portion of the swath to avoid larger errors associated with the limb effect.

The footprint of HAMSR at nadir from the GH altitude is ~2 km with an increase in size as the instrument scans off nadir. The along-track sampling of HAMSR measurements is ~250 m. In this study, the HAMSR Tbs are mapped to a grid with 1-km spacing to match the HIWRAP wind retrievals. The vertical resolution of the HAMSR data is dictated by each channel’s weighting function, which amounts to ~2–3-km intervals in height. More detailed information on HAMSR can be found in Brown et al. (2011).

3. Overview of Hurricane Karl

During the summer of 2010, NASA conducted the GRIP field experiment in the Atlantic Ocean basin to study the physical processes controlling hurricane formation and intensity change. A total of three NASA aircraft were deployed during GRIP with instruments on board to measure properties of the hurricane environment and inner-core region. In this study, we focus on the inner-core aircraft (GH) and instruments (HIWRAP and HAMSR) described in the previous section. Further information about GRIP can be found in Braun et al. (2013).

Hurricane Karl began from a combination of a tropical wave moving off the African coast and an elongated trough of low pressure situated over the southwestern North Atlantic Ocean. Figure 1 shows the best track of Karl and intensity classifications starting at 0000 UTC 14 September. Over several days’ time, deep convection located near the wave axis became more organized and by 1200 UTC 14 September a tropical depression formed in the northwestern Caribbean Sea (Stewart 2011). Not long after, Karl intensified to a tropical storm and made landfall on 15 September on the Yucatan Peninsula with surface winds of ~27 m s−1. Karl weakened while crossing land but was able to maintain tropical storm classification (~20 m s−1 surface winds) with a well-organized circulation.

Fig. 1.
Fig. 1.

Best track of Hurricane Karl (2010) starting at 0000 UTC 14 Sep with intensity classifications marked every 6 h. The days in September at 0000 UTC are also shown. The green circles denote tropical depression status, open hurricane symbols are tropical storm, and closed hurricane symbols are hurricane status with the category listed in the center. The inset shows the time series of maximum surface wind speed (m s−1) with the Global Hawk flight enclosed by the black lines.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

After emerging into the Bay of Campeche, Karl rapidly intensified from a ~20 m s−1 tropical storm at 0600 UTC 16 September to a ~57 m s−1 hurricane at 1200 UTC 17 September (Fig. 1). This equates to a ~37 m s−1 increase in surface winds in a 30-h period, which is more than double the typical RI rate of ~15 m s−1 in 24 h (Stewart 2011). Our focus in this study is the inner-core structure and dynamics during this RI episode that was sampled by the GH aircraft between ~1900 UTC 16 September and ~0800 UTC 17 September (see Fig. 1).

From an environmental perspective, Karl was primed for RI with high sea surface temperatures of ~30°C in the Bay of Campeche, relatively low vertical wind shear of ~5 m s−1 with the vector pointing mostly toward the southwest over the RI interval, and moist midlevel air. The large-scale vertical wind shear impacting the storm was determined from CIMSS satellite analyses and verified using NCEP–NCAR reanalyses.

4. Convective burst remote sensing observations

a. Satellite evolution

Animations of GOES IR satellite data indicate that localized convective bursts in Karl were actively pulsing for a ~12-h period between 1200 UTC 16 September and 0000 UTC 17 September. After this time period, the convective forcing is less frequent and a more axisymmetric presentation of the cloud field emerges.

Figure 2 shows a sequence of GOES IR images of Karl spanning the period of GH observations during the storm’s RI. The GOES IR data have a resolution of ~4 km. At 1845 UTC 16 September (Fig. 2a), a region of asymmetric cold cloud tops (~−80°C) associated with a pulsing convective burst is located in the downshear to downshear-left portions of the storm. No apparent eye is visible at this time owing to the presence of clouds. At 2215 UTC (Fig. 2b), the convective burst episode is still evident in the IR imagery with deep convection located in the downshear-left sector of the storm and the appearance of a cloud-filled eye. A few hours later at 0140 UTC 17 September (Fig. 2c), the cold cloud-top region has wrapped around to the upshear quadrants of the storm. A clearer depiction of an eye is present at this time although it is still not cloud free. Toward the end of the aircraft observation period at 0501 UTC (Fig. 2d), the cold cloud tops have diminished and spread around the storm in a more axisymmetric pattern along with the development of a large, clear eye. Karl is nearing landfall at this point, but the core region of the storm sampled by the GH (Fig. 2d) is still well offshore (see Fig. 1).

Fig. 2.
Fig. 2.

A sequence of GOES IR images of Hurricane Karl (2010) in the Bay of Campeche during an RI episode spanning the GH flights into the storm. The times shown are (a) 1845 UTC 16 Sep, (b) 2215 UTC 16 Sep, (c) 0140 UTC 17 Sep, and (d) 0501 UTC 17 Sep. The white arrow in (a) denotes the environmental vertical wind shear vector valid over the time interval. The star represents the estimated storm center. The track of the GH ± 2 h from the satellite time stamp is shown in white with the large numbers denoting the hour (UTC).

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

The satellite presentation of Karl’s RI with localized convective bursts pulsing in the downshear quadrants of the storm, their rotation and dissipation into the upshear quadrants, and development of an axisymmetric cloud structure with a clear eye at late times is common (e.g., Reasor et al. 2009; Guimond et al. 2010; Stevenson et al. 2014). In addition, the presence of lightning associated with convective bursts has become more commonly recognized. Reinhart et al. (2014) analyzed satellite data and several GRIP datasets and found that some of the more intense convective burst activity in Karl produced significant lightning.

b. Radar time-averaged structure

The spatial and temporal evolution of convective bursts is very turbulent in nature and requires high-resolution aircraft measurements to accurately describe their structure. A time-averaged view of the storm from HIWRAP and TA radar measurements is first presented and then individual overpasses are analyzed from several data sources to highlight the detailed structure of convective bursts during the pulsing phase. Finally, we briefly show the structure of the vortex during the axisymmetric response phase.

Figure 3a shows HIWRAP Ku-band reflectivity overlaid with horizontal wind vectors at 2-km height on a storm-relative grid averaged over the entire GH sampling interval (from ~1900 UTC 16 September to 0800 UTC 17 September). A broad cyclonic circulation is evident with a reflectivity-filled eye, which is weighted toward early time periods. There are gaps in the azimuthal coverage of the storm owing to the small swath width of HIWRAP. These gaps decrease toward the storm center where estimates of the low-wavenumber components of the flow are best suited.

Fig. 3.
Fig. 3.

Composite analysis of HIWRAP data averaged over the total GH sampling interval (12–13 h) at 2-km height for (a) Ku-band reflectivity (dBZ) and horizontal wind vectors and (b) horizontal wind speeds (m s−1). The large gray arrow in (a) is the large-scale vertical wind shear vector valid for this time interval with a value of ~5 m s−1.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Figure 3b shows the horizontal wind speeds at 2-km height averaged over the same time interval. The strongest winds are generally located in the downshear quadrants of the storm with large patches of ~40–45 m s−1 winds in this region. The time- and azimuthally averaged RMW at this level is 20–25 km. An interesting feature appearing in the data is the presence of small clusters of anomalously large wind speeds located in the eyewall. These clusters have a length scale of ~10–15 km and are found most notably in the downshear direction and downshear-left quadrant just inside the RMW.

Figure 4a is similar to Fig. 3a only at 8-km height. At this higher level, the presence of convective burst activity shown by the high reflectivity anomalies between ~25 and 40 dBZ is evident. These bursts are occurring in the downshear to downshear-left portions of the storm with evidence of rotation into the upshear quadrants observed by tracking the HIWRAP measurements with the GOES IR data. The majority of the burst activity over this time interval is located inside the low-level (2 km) RMW (either time mean or time maximum value), which is consistent with the intensifying TC composite of Rogers et al. (2013). The patches of anomalously large wind speeds shown in Fig. 3b are generally well correlated with the high reflectivity anomalies in Fig. 4a, which suggests the connection of the convective bursts to the localized spinup of the low-level wind field. The association of the high reflectivity anomalies aloft to the localized low-level wind spinup is burdened by the 12–13-h time-averaged perspective. It is also important to keep in mind that the number of data points that contribute to the time mean structure in Figs. 3 and 4 are not uniform, especially along the swath edges. Individual overpasses were analyzed and they confirmed the existence of the reflectivity–wind relationship, but uncertainty exists in the timing of the bursts and the anomalously large low-level wind speeds.

Fig. 4.
Fig. 4.

As in Fig. 3, but for 8-km height. The white circle in (a) shows the location of the low-level (2 km) and time- and azimuthally averaged RMW. The gray circle in (a) shows the maximum azimuthally averaged RMW over the 12–13-h GH sampling period.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Figure 4b shows the horizontal wind speeds at 8-km height, which reveals similar cellular structures as the 2-km wind speeds albeit with generally reduced magnitudes. The strongest wind speeds of ~35–40 m s−1 are found in the downshear-left quadrant and the northeast, upshear quadrant at 8-km height. This shows that the enhanced winds associated with the convective bursts extend through a deep layer with the downshear-left quadrant containing the most intense winds. An individual GH overpass in the upshear-left quadrant analyzed later in the paper shows significant tangential wind speeds at upper levels in the convective bursts (Fig. 11d, described in greater detail below), which helps to explain the time-averaged structure in the upshear quadrants shown in Fig. 4b.

The vertical vorticity structure computed from the NOAA TA wind retrievals is shown in Fig. 5 as a prelude to the analysis in the next section. Figure 5a shows the time evolution (1842–2042 UTC 16 September; includes three analysis periods) of the azimuth and height (0.5–4 km) averaged vertical vorticity. At 1842 UTC, the profile of mean vorticity has peak values in the eye and decreases monotonically with radius. At 1930 and 2042 UTC, the peak in mean vorticity moves radially outward to ~15-km radius (inside the RMW of ~25 km) likely because of the stretching of vorticity from the convective burst activity. The change in sign of the mean radial vorticity gradient at a radius of ~15 km for these later periods supports the presence of a vortex-scale instability process likely dominated by barotropic sources (e.g., Schubert et al. 1999; Nolan and Montgomery 2002). However, a more thorough stability analysis is required to determine this conclusively, which is not conducted here.

Fig. 5.
Fig. 5.

(a) The vertical vorticity (s−1) computed from NOAA TA wind retrievals averaged between 0.5- and 4-km height and in azimuth for three different time periods. (b) The perturbation vertical vorticity averaged between 0.5- and 4-km height and in time (1842–2042 UTC). The mean RMW during this time period is ~25 km.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Kossin and Eastin (2001) analyzed flight-level data in several intense TCs and found similar mean vorticity evolution as that shown in Fig. 5a. They demonstrated that the mixing of vorticity from the eyewall to the eye tended to coincide with the end of intensification. However, the observations presented in this study for Hurricane Karl (2010) indicate that a portion of the vorticity mixing process is occurring during the RI of the storm.

Figure 5b shows the perturbation (removing azimuthal mean) vorticity averaged over height (0.5–4 km) and time (1842–2042 UTC 16 September) revealing a prominent wavenumber-1 signal driven by vertical wind shear in the eyewall region (~20–25 km) and smaller-scale structure in the eye region. The large oscillations of vorticity observed in the eye region indicate that vorticity has been mixed from the eyewall into the eye region. This mixing and/or transport process could be due to a combination of the vertical-wind-shear-induced asymmetries as well as mesovortices that develop from the breakdown of the vortex instability (e.g., Schubert et al. 1999; Kossin and Eastin 2001). Detailed observations of the convective burst evolution in relation to the inner-core asymmetries are provided in the next section.

c. Airborne radar and radiometer analysis during the burst pulsing phase

1) First sampling period (~1830–1920 UTC 16 September)

The NOAA WP-3D aircraft sampled the RI of Karl at certain similar time periods as the NASA GH, which allows a more comprehensive study of the inner-core processes owing to the large swath width of the WP-3D measurements. The WP-3D first crossed the storm center at ~1842 UTC 16 September. The LF radar reflectivity at flight level (3.7-km height) along with the TA radar–derived wind vectors are shown in Fig. 6a at this time.

Fig. 6.
Fig. 6.

NOAA WP-3D flight through the inner core of Hurricane Karl (2010) centered at ~1842 UTC 16 Sep showing (a) LF C-band reflectivity at 3.7-km height overlaid with TA-derived winds at 4-km height and (b) TA-derived divergence (s−1) averaged between 0.5- and 4-km height overlaid with perturbation wind vectors averaged over the same interval. The white arrows in (b) highlight features discussed in the text. The gray line in (b) marks the eye–eyewall interface using the gradient in LF reflectivity. The gray “C” and “A” letters in (b) denote the centers of cyclonic and anticyclonic mesovortex circulations, respectively.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

An interesting wavenumber-5 polygon structure is apparent at the eye–eyewall interface in the LF reflectivity, which is indicative of the presence of mesovortices at the locations of the vertices. The study of Hendricks et al. (2012) observed similar reflectivity structures in the rapid intensification of Hurricane Dolly (2008). The formation of mesovortices has been linked to dynamic instability in the eyewall where thin rings of potential vorticity support the phase locking and exponential growth of counterpropagating vortex Rossby waves (e.g., Schubert et al. 1999; Kossin and Schubert 2001; Rozoff et al. 2009; Hendricks et al. 2014). The above studies showed that the development of mesovortices is an effective means of turbulent mixing between the eye and eyewall, which can lead to important consequences for the intensity of the hurricane. Note that when discussing “turbulent” structures observed in the data presented in this paper, we are referring to azimuthal wavenumbers higher than those associated with mean (wavenumber 0) TC structure.

The wind vectors in Fig. 6a show that the strongest winds (~40 m s−1) are located in the upshear (northeastern) quadrant at this time and level (4-km height), which is more consistent with the time-averaged HIWRAP data at 8-km height (Fig. 4b) than at 2-km height (Fig. 3b). Figure 6b highlights the 0.5–4-km-height-averaged divergence with perturbation wind vectors (computed by removing the azimuthal mean radial and tangential winds from the total flow and projecting back to Cartesian space) averaged over the same height interval. The analysis in Fig. 6b shows a significant region of outflow emerging from the eye and entering the southern eyewall (see thick arrow in Fig. 6b) where a band of ~40-dBZ echoes are observed (Fig. 6a). A band of strong convergence extending from the western to southern portions of the eyewall is evident in Fig. 6b, which is consistent with the strongest echoes observed in the LF data (Fig. 6a).

In the northwestern portion of the eyewall, a wide inflow region (see thick arrow in Fig. 6b) with peak magnitudes of ~−8 m s−1 is transporting air across the eye–eyewall interface. The perturbation wind vectors show that a cyclonic–anticyclonic mesovortex couplet is responsible for the transport of air across the eye–eyewall interface on the northwestern side extending down across the southern side. When combined with the vortex-scale inflow (southwestern portion of Fig. 6b radially outside convergence band), the mesovortex-induced outflow across the southern eye–eyewall interface is partly responsible for the convergence band described above. These mesovortices are ~20–30 km in spatial scale, which are larger than convective vortices observed previously in TCs (e.g., Reasor et al. 2005; Houze et al. 2009) and in the present paper, which have scales of ~10–15 km. We do not observe a wavenumber-5 structure in the perturbation winds as was seen in the LF reflectivity field (Fig. 6a). It is possible that the vortex couplet identified in the wind field is distinct from the structure in the reflectivity field, although data gaps and the relatively coarse resolution of the TA analysis prevent a detailed examination of this linkage.

Figure 7a shows HAMSR 54-GHz Tbs overlaid with HIWRAP-computed horizontal wind vectors from the first GH overpass of Karl between 1853 and 1919 UTC 16 September. The aircraft crossed the storm center at ~1910 UTC, which is ~25 min after the WP-3D transect shown in Fig. 6. The data are shown at 2-km height, which is where the HAMSR 54-GHz weighting function peaks, assuming a standard atmosphere. The presence of light precipitation in the eye of Karl at this time allows the flow in the eye and its interaction with the eyewall to be analyzed.

Fig. 7.
Fig. 7.

GH overpass of the inner core of Hurricane Karl (2010) between 1853 and 1919 UTC 16 Sep showing (a) HAMSR 54-GHz Tbs (K) and (b) HIWRAP Ku-band reflectivity (dBZ). In both panels, horizontal wind vectors from HIWRAP are overlaid and the analysis level is 2-km height. The white arrows and white outline in (a) highlight a protrusion of the warm core discussed in the text. The azimuthal mean RMW at 2-km height of ~30 km is also shown in (a) by the white circle. The reference wind vector in (a) applies to both panels.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

In this pass, the warm anomaly of Karl is evident shown by the anomalously large Tbs in the core of the storm (5–10 K above ambient values). For this analysis we are not as interested in the quantitative properties of the warm core as our focus is on the qualitative structure of this feature. The eyewall of Karl with embedded convective bursts is seen by the depressed Tbs in the southern half of Fig. 7a with an intense cell located in the eastern half of the southern eyewall, which is in the downshear-left quadrant. The azimuthal mean RMW at this time and height is ~30 km, which places the cell inside the RMW. The winds in this region are 30–40 m s−1 as computed from HIWRAP data.

An interesting feature of the HAMSR data is a fingerlike protrusion of the warm core sticking out of the southern eyewall and adjacent to the most intense convective activity (labeled with white in Fig. 7a). The HIWRAP winds follow this feature well and show 10–20 m s−1 flow originating in the eye and cyclonically rotating toward the intense convective cell in the eastern half of the southern eyewall. The winds from this warm anomaly protrusion show a convergence signature with the intense convective cell.

Figure 7b (also at 2-km height) shows Ku-band reflectivity from HIWRAP along with horizontal wind vectors for the same overpass as in Fig. 7a. The warm core protrusion observed in the HAMSR data can also be seen in the HIWRAP data through reduced reflectivity in the southern eye/eyewall from values of 35–40 to ~20 dBZ. The analysis above indicates that turbulent mixing between the warm, dry air in the eye with moist air in the eyewall is helping to carve out and develop the eye of Karl. Reducing the reflectivity through mixing and evaporation should help ensuing subsidence to go more directly into warming the eye. This is a general statement not specifically targeted at the structure shown in this overpass. While axisymmetric warming of the eye from the vortex response to convective heating is likely significant in the RI of Karl (e.g., Vigh and Schubert 2009), the asymmetric mixing process described above can certainly assist the total development of the eye and warm core. Note that HAMSR and HIWRAP data at 1-, 3-, and 4-km height showed very similar structure to that illustrated in Fig. 7 at 2-km height.

In addition to the HIWRAP winds in Fig. 7, the LF reflectivity structure (Fig. 6a) and TA perturbation winds (Fig. 6b) observed ~25 min earlier show that the turbulent mixing is a result of mesovortices located near the eye–eyewall interface (also see discussion in section 4b). Small patches of reduced reflectivity (Fig. 7b) in the same locations as the low Tbs in Fig. 7a are the result of attenuation of the HIWRAP Ku-band signal from the convective bursts.

Figure 8 shows nadir cross sections of HIWRAP data for the first GH overpass (see Fig. 7). This cross section is straight through the storm center in the north-to-south direction. Figure 8a shows Ku-band reflectivity through the convective burst in the southern eyewall revealing a deep column of high values reaching ~35 dBZ at 12-km height (x axis = ~−25 km). There is a large region of lower reflectivity (~20 dBZ) filling the eye that is connected with the convective burst in the southern eyewall at x axis = ~−20 km, y axis = ~−8 km. In the eye region, there is a deep layer (1–10 km) of outflow with peak magnitudes from ~−10 to −15 m s−1 (Fig. 8b, x axis = ~−15 km), which is consistent with the warm core mixing into the eyewall shown in Fig. 7a at 2-km height. The outflow from the eye converges with inflowing air, located radially outside the convection, at low to midlevels in the core of the burst (see divergence contours and gray arrows in Fig. 8b).

Fig. 8.
Fig. 8.

HIWRAP vertical cross sections at nadir through the storm center in the north-to-south direction for the GH overpass between 1853 and 1919 UTC 16 Sep. The data shown are (a) Ku-band reflectivity (dBZ), (b) meridional (radial) winds (m s−1) with divergence overlaid in black contours (values shown are from −4 × 10−3 to −1 × 10−3 s−1), (c) vertical winds (m s−1), and (d) vertical vorticity (s−1). The large gray arrows in (b),(c) highlight features discussed in the text while the white lines in these panels show the zero contours. The black arrows in (c),(d) also highlight features discussed in the text. Note the azimuthal mean RMW at 2-km height at this time is ~30 km.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

These data indicate that the maintenance of the convective burst in the southern eyewall (downshear-left quadrant) is driven by a combination of buoyancy (inferred from Fig. 7a) and horizontal, kinematic convergence (Fig. 8b). Both of these mechanisms are facilitated by the turbulent mixing of air, originating in the anomalously warm eye, with inflowing air in the low-to-midlevel eyewall. During this first sampling period it was difficult to determine if the convective burst was in the formation stage or a mature stage and, thus, the mechanisms outlined above more directly point to maintenance of the burst. However, similar mechanisms were operating during the third sampling period where observations more clearly show the burst formation stage.

A significant region of descent with peak values of ~−3 m s−1 is located in the eye of Karl (wide gray arrow in Fig. 8c), which should be helping to clear and warm the eye. This descent appears to be induced by the convective updraft (thin gray arrow in Fig. 8c) occurring on the inner edge of the eyewall (x axis = ~−15 km). There is also a downdraft located radially inward of the convective burst (x axis = ~−18 km, y axis = ~7–10-km height), which is also well positioned to warm and dry the eye. The convectively induced downdrafts observed in this overpass are consistent with prior airborne Doppler radar studies of TCs (Heymsfield et al. 2001; Guimond et al. 2010).

A reasonably strong updraft of ~10 m s−1 (Fig. 8c) in the core of the deep convection (x axis = ~−25 km) is nearly coincident with an anomalously large patch of cyclonic vorticity (Fig. 8d) at ~7-km height (denoted with black arrow). Note that vorticity values are removed above 10-km height because the swath width of the HIWRAP data at these levels is very small, which places the swath edges close to nadir. The computed horizontal winds at the swath edges have larger uncertainty owing to the HIWRAP scanning geometry (Guimond et al. 2014). At low levels on the inner edge of the deep convection (x axis = ~−18 km), a weak–moderate updraft of ~3–5 m s−1 (Fig. 8c) is collocated with an intense cyclonic vorticity anomaly with values of 10−2 s−1 (Fig. 8d). Note that the main updraft for the convective burst may be sloped such that the vertical cross sections in Figs. 8c and 8d may not have perfectly correlated vertical velocity and vertical vorticity structure in the vertical plane. These observations suggest that the convective burst sampled here is rapidly rotating through a deep layer as has been observed in previous studies (e.g., Reasor et al. 2005; Houze et al. 2009).

2) Second sampling period (~1920–2000 UTC 16 September)

Approximately 20 min after the first GH overpass, the NOAA WP-3D aircraft penetrated the core of Karl again with a center crossing at ~1930 UTC 16 September. Figure 9a shows LF radar reflectivity at flight level (3.6-km height) along with the TA radar–derived wind vectors at 4-km height. Intense reflectivity between 45 and 50 dBZ is present on the western half of the storm while the eastern half is ragged without a continuous region of elevated reflectivity. Significant reflectivity is located in the eye of the storm and animations of several LF scans show mesovortex-like features mixing into the eye from the eyewall. Much like the previous transect, the strongest winds are located in the northeastern (upshear) quadrant.

Fig. 9.
Fig. 9.

NOAA WP-3D flight through the inner core of Hurricane Karl (2010) centered at ~1930 UTC 16 Sep showing (a) LF reflectivity at 3.6-km height overlaid with TA-derived winds at 4-km height and (b) TA-derived divergence (s−1) averaged between 0.5- and 4-km height overlaid with perturbation wind vectors averaged over the same interval. The gray line in (b) marks the eye–eyewall interface using the gradient in LF reflectivity. The gray “C” and “A” letters in (b) denote the centers of cyclonic and anticyclonic mesovortex circulations, respectively.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Figure 9b shows the perturbation wind vectors, averaged over the 0.5–4-km height interval, overlaid on the mean divergence field in this same layer. Note that individual levels at 0.5 and 1 km showed very similar structure to the layer mean. A region of strong convergence is located in the western eyewall, which appears to extend down to the southwestern eyewall similar to Fig. 6b, but the data gaps prevent a clear analysis. The convergence region in the western eyewall (downshear quadrants) is consistent with the intense reflectivity band (Fig. 9a), while the eastern eyewall (upshear quadrants) is having difficulty developing perhaps as a result of the vertical wind shear.

The perturbation wind vectors in Fig. 9b reveal a similar cyclonic–anticyclonic mesovortex couplet as the previous transect (Fig. 6b) although a data gap in the southwestern quadrant makes the placement of the anticyclonic circulation rather broad and with some uncertainty. Nevertheless, the counterrotating circulations are consistent with the convergence signature in the western eyewall. The cyclonic circulation at the northern eye–eyewall interface is meeting inflowing air in the northwestern portion of Fig. 9b as well as the flow associated with the anticyclonic mesovortex in the southwestern eyewall region. These mesovortex circulations are consistent with the wavenumber-1 structure in the vorticity field (Fig. 5b) and are directing air across the eye–eyewall interface with inflow to the eye on the western side and outflow to the eyewall on the eastern side.

Figure 10 is similar to Fig. 7a only for the second GH overpass of Karl between 1938 and 1957 UTC 16 September. This is a diagonal pass from southeast to northwest, which covers part of the upshear-left quadrant of the storm where depressed Tbs from HAMSR show the straining/elongation of deep convection by the advective tendencies of the cyclonic flow. The maximum HIWRAP winds at 2-km height are ~50 m s−1 in the northwestern eyewall and ~30 m s−1 in the southeastern eyewall where the flow has a radially outward directed component into the convection. The warmest Tbs are located on the northwestern side of the eye.

Fig. 10.
Fig. 10.

As in Fig. 7a, but for the GH overpass between 1938 and 1957 UTC 16 Sep. The large white circle denotes the azimuthally averaged RMW at 2-km height of ~25 km and the white dot is the storm center.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

The vertical structure of HIWRAP Ku-band reflectivity at nadir for this overpass is shown in Fig. 11a. Deep convection with similar vertical structure to that shown in the previous GH overpass (Fig. 8a) is observed in the southeastern (upshear left) portion of the eyewall with significant reflectivity filling the eye adjacent to this cell. The northwestern portion of the eyewall is not as convectively active and the eye is clear adjacent to this side of the eyewall, which is consistent with the previous overpass and the warmest Tbs shown in Fig. 10. The RMW at 2-km height for this sampling period is 26 km, which is ~3 km smaller than the first sampling period. In both periods, the convective bursts are located just inside the RMW, which is consistent with the RI of Karl and a contracting eyewall.

Fig. 11.
Fig. 11.

As in Fig. 8, but for the GH overpass between 1938 and 1957 UTC 16 Sep in the southeast (negative radius) to northwest (positive radius) direction. The data shown are (a) Ku-band reflectivity (dBZ), (b) radial winds (m s−1) with divergence overlaid in black contours (values shown are from −4 × 10−3 to −1 × 10−3 s−1), (c) vertical winds (m s−1), and (d) tangential winds (m s−1). The gray arrows in (b),(c) highlight features discussed in the text while the white lines in these panels show the zero contours. Note the azimuthal mean RMW at 2-km height at this time is ~25 km.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Figure 11b shows the radial wind speeds for this overpass. The dominant features are a region of midlevel inflow located radially outside the convective burst and a deep column of strong outflow that traverses the eye region and enters the core of the burst (see gray arrows). The flow across the eye is similar to that observed by the WP-3D shown in Fig. 9b and is driven by the counterrotating mesovortex circulations. These winds acquire entropy from the warm anomaly eye (see Fig. 10) likely leading to assistance in convective development in the southeastern eyewall through buoyancy effects. Regions of convergence at low and midlevels (Fig. 11b) are located on the outer edge of the burst (x axis from ~−20 to −30 km). Also note in Fig. 11b that the outflow at low levels on the southeastern side (upshear left) and the inflow on the northwestern side (downshear right) is indicative of the shear-induced anomalies to the secondary circulation shown in Reasor et al. (2013).

The vertical motion structure in Fig. 11c shows a broad region of descent in the eye adjacent to the convective burst with values from ~−2 to −4 m s−1. This descent appears to be generated by the convective activity through compensating motions around convective updrafts (see gray arrows). The broad region of forced descent in the eye is similar to that observed in the previous overpass in the downshear-left quadrant. This robust structure should lead to a drying and warming effect over time, which will be demonstrated with the data in subsequent overpasses.

Finally, instead of showing the vorticity for this overpass, which was somewhat similar to the previous transect, the tangential winds are presented in Fig. 11d. The tangential winds are ~20 m s−1 stronger in the northwestern eyewall up to midlevel regions with peak values of ~50 m s−1 at low levels. In the deep convection, large tangential wind speeds are located at high levels (12–13 km), which is due to strong updrafts transporting high angular momentum air aloft. It appears the convective towers are trying to build a deeper, more intense vortex in this portion of the eyewall.

3) Third sampling period (~2030–2100 UTC 16 September)

The NOAA WP-3D tracked through the center of Karl one last time centered at 2042 UTC 16 September. Figure 12 shows four LF reflectivity snapshots covering a 2.5-min period between 2039:40 and 2042:13 UTC. These images show that the western/northwestern eyewall with embedded deep convective towers is intensifying rapidly (in terms of reflectivity) during this 2.5-min time period. A cyclonic mesovortex identified in the HIWRAP and TA wind fields during this time period (next two figures) is located at the eye–eyewall interface adjacent to this convective burst development.

Fig. 12.
Fig. 12.

Snapshots of LF reflectivity at 3.6-km height covering a 2.5-min period between 2039:40 and 2042:13 UTC 16 Sep. The times shown are (a) 2039:40, (b) 2040:10, (c) 2040:42, and (d) 2042:13 UTC. The gray “C” letter in each panel denotes the center of a cyclonic mesovortex circulation identified in HIWRAP and TA wind fields during this time period (see Figs. 13b and 14).

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Figure 13a shows the LF reflectivity at flight level (3.6 km) along with TA-derived wind vectors at 4-km height and 2042:13 UTC. The western eyewall is the dominant feature with a large region of reflectivity at or above 50 dBZ. The horizontal winds in this region are ~10 m s−1 stronger than those from the previous WP-3D sampling ~1 h earlier at 1930 UTC (see Fig. 9a). The eastern eyewall is still ragged without a coherent eyewall apparent in the reflectivity, while the southern eyewall has increased banding features, which appear to be coalescing.

Fig. 13.
Fig. 13.

As in Fig. 9, but for the NOAA WP-3D transect centered at ~2042 UTC 16 Sep.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

The divergence field for this flight averaged over the 0.5–4-km layer is shown in Fig. 13b with 0.5–4-km-height-averaged perturbation winds overlaid. The cyclonic–anticyclonic mesovortex couplet identified in the previous WP-3D penetrations continues to persist two hours after initial diagnosis. At this time period, the mesovortex couplet has rotated cyclonically with the mean flow placing the cyclonic circulation directly north of the anticyclonic circulation in the western eyewall. These circulations are consistent with a strong region of convergence in the western eyewall (similar to previous transect in Fig. 9b), which is helping to trigger the convective bursts, and a west-to-east flow across the eye. In the eastern eyewall, which is not well defined in the LF reflectivity, another small-scale cyclonic circulation is evident in the perturbation wind vectors. This circulation is helping to direct a southerly flow across portions of the eastern eye–eyewall interface.

The next GH overpass of Karl sampled directly along the shear vector with a southwest-to-northeast transect just north of the storm center at ~2040 UTC 16 September, which is ~2 min behind the WP-3D. Figure 14 shows HAMSR 54-GHz Tbs along with HIWRAP horizontal wind vectors at 2-km height for this overpass. A very intense convective cell located in the downshear direction is present in the HAMSR data with Tbs falling well below 200 K (strong ice scattering) in the core of the ~10-km-wide feature. This cell is located at and just inside the azimuthally averaged RMW at this level.

Fig. 14.
Fig. 14.

As in Fig. 7a, but for the GH overpass between 2009 and 2055 UTC 16 Sep with a center crossing at ~2040 UTC. The large white circle denotes the azimuthally averaged RMW at 2-km height and the white dot is the storm center. The gray box shows the region where data are averaged in the y direction for subsequent figures. The “C” letter denotes the center of a mesovortex cyclonic circulation.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

In the eye of the storm, the HIWRAP winds reveal a cyclonic mesovortex circulation that is directing air out of the northern portion of the eye and into the convective burst. The HAMSR data show that the air being transported into the burst is anomalously warm with Tbs significantly larger than ambient values. The mesovortex circulation identified in the HIWRAP data is also seen at the same location in the TA perturbation wind vectors (see Fig. 13b).

The close coordination of the GH and WP-3D aircraft during this time allows a comparison of the storm kinematic structure from the HIWRAP and TA radars. The appendix also shows error statistics between HIWRAP-computed winds and WP-3D flight-level data.

Figure 15a shows HIWRAP Ku-band reflectivity in a vertical cross section averaged between ~0 and 6 km in the +y direction (see Fig. 14 for averaging domain). HIWRAP shows an intense convective cell with elevated reflectivity to ~15-km height in the downshear eyewall of Karl. The WP-3D reflectivity (not shown) shows similar features albeit with a more diffuse cell. The radial winds from HIWRAP (Fig. 15b) show a strong convergence signature directly below the intense convective cell with outflow of ~5–8 m s−1 crossing the eye–eyewall interface. This outflow from the eye brings warm anomaly air into the eyewall helping to fuel the developing convective cell (see also Figs. 7 and 14). The location of the low-level convergence signature inside the RMW shown in Fig. 15b is consistent with the study of Rogers et al. (2015) for the RI of Hurricane Earl (2010).

Fig. 15.
Fig. 15.

Vertical cross sections of radar data averaged between ~0 and 6 km in the +y direction (see Fig. 14) from (a) HIWRAP Ku-band reflectivity (dBZ) valid at ~2040 UTC 16 Sep, (b) HIWRAP-derived radial winds (m s−1), (c) NOAA WP-3D–derived radial winds (m s−1) valid at ~2042 UTC 16 Sep, (d) HIWRAP-derived vertical winds (m s−1), and (e) NOAA WP-3D–derived vertical winds (m s−1). Note that there are no data on the right side of the HIWRAP panels owing to the coverage and cross section cut. The vertical gray line in (b)–(e) denotes the western eye–eyewall interface using the gradient in reflectivity. The large gray arrows in (d),(e) highlight features discussed in the text.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

A deep column (1–12 km) of inflow (from ~−5 to −10 m s−1) coincident with the convective cell is present in the HIWRAP data (Fig. 15b) and in the TA data at low and high levels (Fig. 15c), which acts to locally spin up the tangential winds through the inward transport of high angular momentum air. The TA-derived radial winds in Fig. 15c show similar features in similar locations to the HIWRAP fields in Fig. 15b, but the intensity of the flow is somewhat reduced in the TA fields.

The HIWRAP-derived vertical winds (Fig. 15d) show a deep updraft is present in the core of the convective cell with a strong pulse approaching 10 m s−1 located at ~10-km height. A downdraft of from ~−3 to −5 m s−1 is located on the inner edge of the eyewall (see gray arrow), which is likely formed through mass conserving motions around the strong updraft. The TA-derived vertical winds in Fig. 15e show similar structures to those from HIWRAP but, again, with reduced magnitudes. The TA data in Fig. 15e also show compensating downdrafts on either side of the updraft with a broad region of descent (from ~−1 to −2 m s−1) located radially inward of the cell (see gray arrow). This broad descent is well positioned to dry and warm the eye as observed in previous overpasses (see Fig. 8c and Fig. 11c) and is a common feature around convective towers located in the eyewall of intensifying TCs (e.g., Heymsfield et al. 2001; Guimond et al. 2010).

The generally favorable comparisons between the HIWRAP- and TA-derived winds shown in Fig. 15 (as well as the comparisons in the appendix) provide confidence in the HIWRAP data and retrievals shown in this paper, which are not as mature as the TA fields. The reduced wind magnitudes in the TA data were found to be largely a result of a Gaussian distance–weighted interpolation used in the TA wind retrieval [see appendix of Reasor et al. (2009)]. A higher-resolution TA product that minimizes smoothing was also analyzed and showed increased wind magnitudes more similar to HIWRAP. These comparisons are not shown for brevity and because this product was only available along the aircraft track.

d. HIWRAP data analysis during the vortex response phase

The main advantage of the GH aircraft is the long duration sampling, which allows continued analysis of the RI of Karl when the WP-3D aircraft returned to base following the 2042 UTC 16 September eye penetration. The GOES IR satellite data analyzed in section 4a showed that the majority of the convective burst activity was finished by ~0000 UTC 17 September. After this time, the vortex went through a response phase that included axisymmetrization of the convective anomalies, which was sampled by the GH aircraft for a period of ~8 h.

Figure 16 shows vertical cross sections of HIWRAP Ku-band reflectivity and tangential wind speed at nadir for a series of overpasses of the inner core of Karl spanning this 8-h period. At 0012 UTC 17 September (Fig. 16a), the vertical structure of the eye/eyewall already looks different than that shown for the burst pulsing phase (e.g., Fig. 8a and Fig. 11a). There is little reflectivity filling the eye, and the beginning of a more sloped structure to the eyewall is observed. The tangential winds peak at ~40 m s−1 in the southeastern quadrant and ~45 m s−1 in the northwestern quadrant with both sides showing contours sloping outward with height. About 3.5 h later at 0345 UTC (Fig. 16b), the axisymmetric structure reflected in the cross section continues to develop with significant sloping of the eyewall reflectivity and tangential winds with height. The eye has also widened, which is indicative of increased subsidence and growth of the warm core (backed by HAMSR data; not shown) in association with an enhanced secondary circulation from the vortex response to the convective forcing.

Fig. 16.
Fig. 16.

HIWRAP vertical cross sections of Ku-band reflectivity (shading; dBZ) and tangential winds (contours; m s−1) at nadir for the GH overpasses centered at (a) 0012 UTC 17 Sep in southeast-to-northwest direction, (b) 0345 UTC 17 Sep in southwest-to-northeast direction, (c) 0550 UTC 17 Sep in southeast-to-northwest direction, and (d) 0805 UTC 17 Sep in southeast-to-northwest direction.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Over the next ~4 h, the trend toward a wider, clearer, and warmer eye with a sloping eyewall structure reminiscent of axisymmetric hurricanes continues to prevail (Figs. 16c,d), except for the presence of a transient convective burst in the northwestern eyewall in Fig. 16c.

5. Summary and conclusions

In this paper, the evolution of rapidly intensifying Hurricane Karl (2010) is examined from a suite of remote sensing observations during the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment. The novelties of this study are in the analysis of data from a new airborne Doppler radar (HIWRAP) and a new airborne platform (NASA Global Hawk) for hurricane research that allows long endurance sampling (up to 24 h). Supporting data from a microwave sounder (HAMSR) coincident with HIWRAP and coordinated flights with the NOAA WP-3D aircraft carrying the lower fuselage (LF) and tail (TA) radars help to provide a detailed analysis of the storm. The focus of the analysis is on documenting and understanding the structure, evolution, and role of small-scale, deep convective forcing in the storm intensification process.

After Karl emerged off the Yucatan Peninsula as a tropical storm, satellite data revealed the presence of deep convective bursts located primarily in the downshear to downshear-left quadrants of the storm. The bursts went through a ~12-h pulsing phase followed by a vortex response phase that included axisymmetrization of the convective anomalies and the development of a wide, clear eye. During this evolution, the surface wind speeds in Karl increased by ~37 m s−1 in a 30-h period, which is more than double the typical rapid intensification rate of ~15 m s−1 in 24 h (Stewart 2011).

The Global Hawk (GH) and WP-3D aircraft data were analyzed from ~1900 UTC 16 September to 0800 UTC 17 September, which covered portions of the convective burst pulsing phase and vortex response phase. The aircraft remote sensing data and analysis indicate the following science results.

The convective bursts formed primarily in the downshear to downshear-left quadrants through a combination of two main processes: 1) convergence generated from counterrotating mesovortex circulations and the larger vortex-scale flow and 2) the turbulent (scales of ~25 km) transport of anomalously warm, buoyant air from the eye to the eyewall at low levels. Calculations of dynamical fields such as vorticity from the TA wind analyses showed the presence of a deep (0.5–4-km height) and persistent (at least 2 h) wavenumber-1 mode at the eye–eyewall interface during the burst pulsing phase consistent with the mesovortex circulations.

Reflectivity snapshots and animations from the LF radar showed a distinct wavenumber-5 structure at the eye–eyewall interface in one WP-3D transect of the storm and the movement of small-scale features in the eye and across the interface during the aircraft-sampling period. These structures and the observed turbulent mesovortex circulations that produce significant eye–eyewall mixing likely form as a result of the vertical wind shear forcing and dynamic instability in the axisymmetric vortex (e.g., Schubert et al. 1999; Kossin and Schubert 2001; Rozoff et al. 2009; Hendricks et al. 2012), which was shown to be present during the rapid intensification of Karl.

Horizontal wind fields computed from the TA and HIWRAP measurements showed that the mesovortex circulations were primarily located in the western and southern (downshear) eye/eyewall region where the most intense convective activity was found. In one GH overpass, a fingerlike protrusion of the warm core observed from HAMSR was observed to rotate cyclonically into the eyewall, likely helping to fuel convective towers observed in this region. Figure 17 shows a conceptual diagram summarizing the remote sensing measurements and the analysis of the mesoscale dynamics described above. Each physical process highlighted in the conceptual diagram can be traced back to specific figures shown in the paper. For example, the cyclonic rotation of warm air from the eye to the eyewall in the downshear-left portion of Fig. 17 is related to Fig. 7a.

Fig. 17.
Fig. 17.

Conceptual diagram highlighting the measurements and analysis from the HIWRAP, HAMSR, and WP-3D instruments during the Hurricane Karl (2010) sampling. The arrows at lower levels represent the mesoscale flow and the arrows in the clouds represent the convective-scale flow. Red arrows indicate anomalously warm, buoyant air with blue arrows the opposite.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

The mechanism for convective burst formation identified in the observations is similar to that determined by Braun et al. (2006) using a numerical simulation of Hurricane Bonnie (1998). In this study, the initiation of convective towers was found to result from convergence between shear-induced asymmetries and the cyclonic flow associated with eyewall mesovortices. Reasor et al. (2009) also found observational evidence for the triggering of convective bursts through the interaction of low-level environmental flow and low-wavenumber vorticity asymmetries in the eyewall of Hurricane Guillermo (1997). The HIWRAP and WP-3D TA radar analysis described in this paper highlights a similar convergence mechanism with the addition of significant transport of anomalously warm, buoyant air from the eye into the eyewall at low levels as indicated by the HIWRAP and HAMSR data. This additional piece of evidence linked to the formation and maintenance of the convective bursts is supported by the trajectory analysis of a numerically simulated hurricane by Cram et al. (2007). In addition, Eastin et al. (2005) analyzed flight-level and dropsonde data in Hurricanes Guillermo (1997) and Georges (1998) and found that the low-level eye was an important source region for buoyant updrafts in the eyewall. They provided circumstantial evidence that mesovortices were the primary mechanism for the transport of the eye air into the eyewall.

The formation of a clear eye and growth of the warm core of Karl are influenced by both asymmetric and axisymmetric processes. The TA and especially HIWRAP data showed that convective-induced descent on the inner edge of the eyewall and in the eye itself was significant, which helps to warm and dry the eye over time. In addition, in one GH overpass the HIWRAP and HAMSR data revealed that turbulent mixing between the eye and eyewall eroded the reflectivity on a local scale. These processes contribute largely to an asymmetric development of the eye and warm core of Karl. During the vortex response phase where the convective bursts are less pronounced and axisymmetrization of the convective anomalies is dominant, the development of the eye has a clear axisymmetric signal shown by the time series of HIWRAP data.

Acknowledgments

We thank Dr. Lihua Li, Matt McLinden, Martin Perrine, and Jaime Cervantes for their engineering efforts on HIWRAP during GRIP. We also thank the JPL HAMSR team for providing level 1B data used in this study, which was obtained from the NASA Global Hydrology Resource Center in Huntsville, Alabama. Discussions with Dr. Scott Braun were useful and helped to clarify the presentation of the data. Dr. Lin Tian helped with early HIWRAP data processing. Author Guimond and coauthors Heymsfield and Didlake were funded under Heymsfield’s NASA GRIP and HS3 funding, through NASA headquarters Program Manager Dr. Ramesh Kakar. Coauthor Reasor was funded through NOAA base funds. The NASA weather program under Dr. Ramesh Kakar supported GRIP. The first author was also partially supported by the Institute of Geophysics and Planetary Physics (IGPP) at Los Alamos National Laboratory. The first author thanks Robert Kilgore for his artistic work on the conceptual diagram. Finally, we thank Rob Rogers and two anonymous reviewers for their very helpful comments.

APPENDIX

Comparison of HIWRAP Wind Retrievals to Flight-Level Data

The HIWRAP radar participated in the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign between the years 2012 and 2014 to study hurricane evolution. As part of this experiment, a coordinated flight between the Global Hawk and the NOAA WP-3D aircraft on 25 September 2013 allowed for the opportunity to validate the HIWRAP wind retrievals with flight-level wind data. The aircraft sampled the end of a large-scale frontal system with a mix of stratiform and weak convective precipitation.

To make the comparisons, all HIWRAP data with a time offset of <10 min and space offset of <1 km from the WP-3D aircraft and with reflectivity >5 dBZ are retained. These data are then interpolated to the locations of the flight-level measurements (height of ~2 km). In an attempt to match the along-track sampling of the flight-level winds (1 Hz or ~100–150 m for a typical WP-3D airspeed) with the HIWRAP wind retrieval grid (1 km) a 10-point running mean filter is applied to the flight-level winds.

Figure A1 shows a scatterplot of the horizontal wind speed error, defined as |HIWRAP − WP-3D flight level|, versus the flight-level horizontal wind speed. In this figure, HIWRAP Ku-band data are shown. There is a clear trend of lower errors for higher wind speeds. For all the points in Fig. A1 (N = 2727) the RMSE for wind speed and direction (not shown) is 7.8 m s−1 and 27°, respectively. When considering points where the wind speed is >10 m s−1 (N = 1077) the RMSE for wind speed and direction is 1.3 m s−1 and 19°, respectively. These errors are slightly lower for Ka-band data likely because of the higher signal-to-noise ratios when compared to Ku band. For example, when the wind speed is >10 m s−1 (N = 1321) the RMSE for wind speed and direction using Ka-band data is 1.1 m s−1 and 15°, respectively. No clear reflectivity dependence is observed in Fig. A1, but the values give an indication of the intensity of precipitation sampled.

Fig. A1.
Fig. A1.

Scatterplot of HIWRAP horizontal wind speed errors (|HIWRAP − WP-3D flight level|) vs WP-3D flight-level wind speeds for the coordinated flight during HS3 on 25 Sep 2013. The points are colored by HIWRAP Ku-band reflectivity. Note the HIWRAP winds are computed using Ku-band Doppler velocities. See text for more details.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

Coordination between the Global Hawk and NOAA WP-3D aircraft also occurred for one overpass of Hurricane Karl during GRIP at ~2040 UTC 16 September 2010. The same procedures described above were applied to these data. The flight-level measurements were located between 3.5- and 3.8-km height and the time offset between the aircraft was ~2–3 min. Figure A2 shows these comparison results for the same kind of scatterplot as that in Fig. A1. A trend for lower errors with increasing wind speeds is not observed with the range of values sampled here, but a slight indication of lower errors for higher reflectivity values is somewhat apparent. For all the points in Fig. A2 (N = 239) the RMSE for wind speed and direction (not shown) is 4.0 m s−1 and 11°, respectively.

Fig. A2.
Fig. A2.

As in Fig. A1, but for the coordinated flight during GRIP (sampling of Hurricane Karl at ~2040 UTC 16 Sep 2010). The points are colored by HIWRAP Ku-band reflectivity. Note the HIWRAP winds are computed using a combination of Ku- and Ka-band Doppler velocities. See text for more details.

Citation: Journal of the Atmospheric Sciences 73, 9; 10.1175/JAS-D-16-0026.1

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