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  • View in gallery

    Best-track 1-min-sustained 10-m wind speed (m s−1, left ordinate, solid diamonds) and surface pressure (hPa, right ordinate, open squares) as a function of time for Guillermo. Two gray columns identify times when NOAA aircraft sampled the TC and vertical lines delineate an entire period of rapid intensification.

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    Storm-relative location of GPS sondes used in the analysis on (a) 2 and (b) 3 Aug. The black open circle indicates the TC circulation center and the 75-km range ring is shown.

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    Horizontal field of wind speed (solid contours every 5 m s−1) on 2 Aug at 100-m altitude (a) without GPS sonde data in eye and eyewall and (b) with extrapolated aircraft data and extension of GPS sonde winds in the eye from 400 m. The solid dot denotes the circulation center.

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    Mean diameter (km) of the eye based on estimates through cardinal headings for (top) 2 and (bottom) 3 Aug.

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    Four lower fuselage scans (120 km × 120 km) showing an example of the spiraling inward of the eyewall. Times to the nearest minute are 2113, 2117, 2119, and 2123 UTC 2 Aug. The color scale to the right indicates dBZ values. The small white cross in the eye shows the aircraft position at the time of the scan, and the yellow dot shows the circulation center.

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    Lower fuselage plan view of reflectivity (120 km × 120 km) at ∼1909 UTC 3 Aug. The color scale to the right indicates dBZ values. The small white cross in the eye shows the aircraft position at the time of scan, and the orange dot shows the circulation center.

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    The 120 km × 20 km vertical cross sections of reflectivity from the tail radar at (a) 0002:59 UTC 3 Aug in the WSW–ENE direction and (b) 1947:02 UTC 3 Aug in the W–E direction. The small cross in the lower center portion of the image represents the location of the aircraft. Reflectivity values in the eye at the surface depict the sea surface return, not precipitation. The color table in each panel depicts reflectivity values (dBZ).

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    Mean eyewall echo tops of the eyewall (km) derived from the tail radar for 2 (open diamonds) and 3 Aug (solid triangles) as a function of the azimuth. The ENE and WSW azimuths not sampled on 3 Aug.

  • View in gallery

    Net LHR estimates for each pass through the eye on 2 (black diamonds) and 3 Aug (open squares). Reflectivity images used in the analysis are included and colors follow the scale shown in Fig. 5. Images are chosen when the aircraft is near the center of the eye and extend to 60-km radial distance. Approximately 24 h separate each like-numbered pass.

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    Lower fuselage radar views (360 km × 360 km) of rainbands for the east side of the TC early for each mission at (a) 1831:40 UTC 2 Aug, (b) 1846:31 UTC 3 Aug; for late on the west side of the TC (c) 2414:24 UTC 2 Aug, (d) 2437:38 UTC 3 Aug, and again for the east side but late in each mission at (e) 1906:07 UTC 2 Aug and (f) 1920:56 UTC 3 Aug. UTC times exceed 2400 rather than the reset date. The left side is all from 2 Aug and the right side is from 3 Aug. Colors depicting reflectivity follow Fig. 5.

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    Difference field (contours are every 2.5 m s−1 until 5.0 m s−1, then every 5 m s−1) for the tangential wind component (3 Aug − 2 Aug) at 100-m altitude. Positive differences (increases) are solid lines and negative (decreases) are dashed. Distance from the circulation is displayed in kilometers along the x and y axis. The black dotted ring marks the 75-km radius from the circulation center.

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    Radial wind field (contours every 3 m s−1) shown at 100-m altitude for (a) 2 and (b) 3 Aug. Negative values are inflow. The black dotted ring marks the 75-km radius from the circulation center.

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    Inflow depth (contours every 1000 m solid, with 500- and 750-m depth dashed) for (a) 2 and (b) 3 Aug.

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    Mass flux (× 109 kg s−1) through three azimuthal–height surfaces at 200- (open triangles), 150- (solid squares), and 100-km (open diamonds) radial distance for (a) 2 and (b) 3 Aug and for (c) 3 Aug − 2 Aug. Height of the surface for all estimates is 2 km.

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    Temperature field (contours every 1°C unless otherwise labeled) shown at 100-m altitude for (a) 2 and (b) 3 Aug. The black dotted ring marks the 75-km radius from the circulation center.

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    Temperature difference (3 Aug − 2 Aug) at 100-m altitude. Contours are every 0.5°C. Positive (warming) differences are solid contours and negative differences (cooling) are dashed. The black ring marks the 75-km radius from the circulation center.

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    Equivalent potential temperature contoured every 2.5 K, at 100-m altitude for (a) 2 and (b) 3 Aug. The black dotted ring marks the 75-km radius from the circulation center.

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    Equivalent potential temperature difference (K; 3 Aug − 2 Aug) at 100-m altitude contoured every 2 K. Solid contours are positive (increases) and dashed are negative (decreases). The black dotted ring marks the 75-km radius from the circulation center.

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Low-Level Thermodynamic, Kinematic, and Reflectivity Fields of Hurricane Guillermo (1997) during Rapid Intensification

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  • 1 Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

From 0600 UTC 2 August to 1200 UTC 3 August Hurricane Guillermo (1997) deepened by 54 hPa over the eastern North Pacific Ocean, easily exceeding the thresholds that define rapid intensification (RI). The NOAA WP-3Ds observed a portion of this RI with similar two-aircraft missions on consecutive days. The aircraft jettisoned 70 successful global positioning system (GPS) dropwindsondes (or GPS sondes), which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye. Reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km. This behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles. Mixing between the lower eye and eyewall, as detailed by other investigators, appears to have triggered the reduction in the eye diameter. After RI the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located on the east or trailing side of the hurricane and to the left of the deep-layer shear vector. Net latent heat release within 60 km of the circulation center increases 21% from 2 to 3 August and is matched by a 30% increase in the inflow below 2 km at the 100-km radius. The GPS sondes, combined with aircraft in situ data for the eyewall region, reveal that the tropical cyclone (TC) establishes an annulus adjacent to and under the eyewall where the tangential wind component and equivalent potential temperature increase substantially. The radial extent of this annulus is constrained by the rainbands that remain robust throughout RI. The results support the argument that RI is controlled by processes within 100 km of the circulation center, and in particular within the eyewall.

Corresponding author address: G. M. Barnes, Dept. of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822. Email: gbarnes@hawaii.edu

Abstract

From 0600 UTC 2 August to 1200 UTC 3 August Hurricane Guillermo (1997) deepened by 54 hPa over the eastern North Pacific Ocean, easily exceeding the thresholds that define rapid intensification (RI). The NOAA WP-3Ds observed a portion of this RI with similar two-aircraft missions on consecutive days. The aircraft jettisoned 70 successful global positioning system (GPS) dropwindsondes (or GPS sondes), which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye. Reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km. This behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles. Mixing between the lower eye and eyewall, as detailed by other investigators, appears to have triggered the reduction in the eye diameter. After RI the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located on the east or trailing side of the hurricane and to the left of the deep-layer shear vector. Net latent heat release within 60 km of the circulation center increases 21% from 2 to 3 August and is matched by a 30% increase in the inflow below 2 km at the 100-km radius. The GPS sondes, combined with aircraft in situ data for the eyewall region, reveal that the tropical cyclone (TC) establishes an annulus adjacent to and under the eyewall where the tangential wind component and equivalent potential temperature increase substantially. The radial extent of this annulus is constrained by the rainbands that remain robust throughout RI. The results support the argument that RI is controlled by processes within 100 km of the circulation center, and in particular within the eyewall.

Corresponding author address: G. M. Barnes, Dept. of Meteorology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822. Email: gbarnes@hawaii.edu

1. Introduction

Rapid intensification (RI) implies a deepening of minimum sea level pressure of at least 20 hPa or an increase of the maximum sustained winds, commonly located in the eyewall, of more than 15 m s−1 in 24 h. Extreme intensity change such as this continues to be a daunting forecast. Holliday and Thompson (1979), Frederick (2003), and Kaplan and DeMaria (2003) concluded that the most intense tropical cyclones (TCs) in the Northern Hemisphere often have a RI period, and that the heart of the season, August and September, accounts for nearly three-quarters of all the RI events.

Hurricane Guillermo (1997) underwent RI in early August over the eastern North Pacific Ocean and was the first TC to be sampled with global positioning system (GPS) dropwindsondes, referred to hereinafter as GPS sondes. The two National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft sampled the TC over two consecutive days as part of the Vortex Motion and Evolution Experiment. This dataset provides an opportunity to examine how the inner few hundred kilometers of a TC responds to or causes the rapid deepening.

a. Potential causes to RI

Operational meteorologists generally must rely on satellite measurements and synoptic-scale fields to forecast RI. DeMaria and Kaplan (1994) developed the Statistical Hurricane Intensity Prediction Scheme (SHIPS) that incorporates climatological, persistence, and synoptic predictors to produce an operational intensity forecast. Nine years later, Kaplan and DeMaria (2003) enhanced SHIPS by providing forecasters with a simple technique for estimating the probability of RI for the Atlantic basin. This technique includes the examination of five predictors: previous 12-h intensity change, sea surface temperature (SST), low-level relative humidity, vertical shear of the horizontal wind, and the difference between the current intensity and the maximum potential intensity (MPI; Kaplan and DeMaria 2003). The mean values of the predictors at the initiation of RI were found to be more favorable for intensification when compared to times when tropical cyclones were not undergoing RI. The RI tropical cyclones had less deep tropospheric shear (−3.6 m s−1), warmer SSTs (+0.9°C), and more 850–700-hPa relative humidity (+4%) than those TCs that did not deepen rapidly. However, less than 2% of the entire sample satisfied the thresholds for all five predictors and only ∼10% of the RI cases occurred when all thresholds were satisfied. The results demonstrate that a “perfect environment” for RI is quite rare and numerous hurricanes undergo RI despite less than ideal conditions.

Hurricane Opal (1995) in the Gulf of Mexico was an alarming example of RI that has received much attention (Rodgers et al. 1998; Bosart et al. 2000; Shay et al. 2000; Hong et al. 2000). Bosart et al. (2000) considered large-scale environmental influences, storm-scale internal dynamics, and air–sea interaction as factors contributing to RI. Opal crossed over a warm core eddy in the Gulf of Mexico, which provided an increase in oceanic heat and moisture fluxes (also see Shay et al. 2000), an approaching trough from the northwest provided divergent flow aloft that established a favorable outflow channel (similar to Sadler 1978), and there are indications that Opal may have undergone an eyewall replacement cycle as well. Before undergoing RI, Opal was far from its MPI and the vertical shear of the horizontal wind through the 850–200-hPa layer was 0.3 × 10−3 s−1. Upper-level divergence due to an approaching trough preceded eyewall convective growth by nearly 6 h and the onset of RI was attributed to the favorable jet–trough–TC interaction. Maximum surface winds increased by nearly 15 m s−1 shortly before the TC passed over a warm core eddy. Hong et al. (2000) conducted a modeling study of the air–sea fluxes and argued that only 25% (10–15 hPa) of the observed pressure fall during the rapid deepening phase could be explained by the presence of the warm core eddy.

Rodgers et al. (1998) used Special Sensor Microwave Imager (SSM/I) satellite instrumentation to derive volume-integrated net latent heat release (LHR) for the eyewall region during the RI of Opal (1995). Two pronounced spikes of net LHR were observed as Opal traversed across the Gulf of Mexico. The first corresponded with the initiation of rapid pressure decrease and the second occurred simultaneously with the estimated lowest pressure. Invigorated eyewall convection was coincident with intensification.

Changes in the eye, specifically warming due to subsidence, have been correlated with rapid deepening in other TCs. Franklin et al. (1988) noted dramatic warming in the eye during the RI of Hurricane Gloria (1985). Two Omega dropwindsondes (ODWs) were released in the eye less than 5 h apart as the TC deepened at a rate of 2.5 hPa h−1; the temperature (T) just above hub cloud top rose from 14.0° to 28.8°C and the relative humidity fell from 100% to 33%. The limited observations of the eyewall precluded the establishing of a link between the eyewall convection and the subsidence in the eye.

Much recent research associated with rapid intensity changes has focused on the formation of concentric eyewalls and subsequent eyewall replacement cycles (Kossin et al. 2000; Zhu et al. 2004; Wang 2006). Willoughby et al. (1982) first found concentric eyewalls to occur for intense, highly symmetric TCs and later stated that they play a major role in the development of many, but not all, major hurricanes (Willoughby 1990). An outer ring of convection may form and contract, eventually reaching a critical range to the original, inner eyewall where a combination of subsidence, blocking of inflow, and perhaps a reduction in the energy content of the inflow causes the inner eyewall to collapse. Central pressure rises rapidly but then as the outer eyewall continues to contract the hurricane may undergo RI.

Previous research associated with Guillermo focused on the role of mesovortices. Initially, Kossin and Schubert (2001) and Kossin and Eastin (2001) conducted simulations that suggest thin regions of enhanced eyewall vorticity surrounding a nearly irrotational eye may be a precursor for RI. When the annulus of enhanced vorticity breaks down into multiple mesovortices extreme pressure falls occur within the annulus that can lower the central pressure of the TC. In addition to their ability to decrease minimum central pressure, Eastin et al. (2005a,b) argued that the mesovortices may transfer high equivalent potential temperature (θe) in the low-level eye into the eyewall resulting in a more buoyant updraft. The asymmetries in the eyewall were argued to be due to the large-scale shear of the horizontal wind in concordance with the findings of Black et al. (2002). Zhang et al. (2001) and Braun (2002) have conducted simulations showing advection of low-level eye air into the eyewall. Montgomery et al. (2006) argue that this mixing can result in a TC exceeding its theoretical maximum intensity (MPI; Emanuel 1986, 1988; Holland 1997).

Previous climatological investigations concluded that RI is common for high category TCs, but the synoptic environment around a TC often did not provide unambiguous guidance when RI was imminent. Passage over warmer water resulting in an increase in the inflow energy content to the eyewall, a reduction in the vertical shear of the horizontal wind, enhanced upper-level divergence, concentric eyewall cycles, greater subsidence warming in the eye, and convective outbreaks in the eyewall are all factors that can contribute to RI.

b. Questions

The nearly identical sampling on each day provides an excellent opportunity to document the changes of a hurricane’s inner core in the lower troposphere as it deepens from 954 to 929 hPa. GPS sonde data are used to create lower-tropospheric (10–4000 m) composite thermodynamic and kinematic fields for each day. The radar data are used to document changes in eyewall shape, echo-top height, and rainband prevalence, and to provide estimates of net latent heat release, based on the reflectivity field and an appropriate Z–R relationship.

We will concentrate on the following issues:

  1. Does the eyewall reflectivity become more symmetric, with taller echo tops, higher rain rates, a smaller diameter, and greater net latent heat release during RI?
  2. Are the expected increases in θe and the tangential wind component occurring only under and near the eyewall or do these increases extend well into the strength region?
  3. Does the inflow become more symmetric, stronger, and deeper through the intensification period?
  4. How do the rainbands within 250 km of the circulation center evolve as the TC deepens? Does a more robust eyewall suppress nearby bands?
  5. Is there evidence that downdrafts from rainbands are becoming less effective at lowering the boundary layer inflow θe?

To answer these questions we assume steadiness for each 6-h sampling period and produce vortex-scale fields derived from the GPS sondes with respect to the circulation center. Difference fields are created by subtracting 2 August fields from those derived from 3 August. Lower fuselage and tail radar reflectivity views of the inner core of Guillermo are readily comparable given the nearly identical sampling strategy executed on each day.

2. Data and analysis methods

a. Hurricane Guillermo (1997)

Shortly after 0600 UTC 2 August, Guillermo began a period of RI (Fig. 1). Data from the best track show a 31 m s−1 increase and a 54-hPa decrease over the next 30 h. Slow intensification continued after RI and the TC reached a maximum intensity of 919 hPa at 0000 UTC 5 August with wind speeds exceeding 72 m s−1 (Lawrence 1999).

Guillermo was located over 29°–30°C water that exhibited weak meridional gradients. A ridge located northeast of the hurricane steered the system westward at a little over 5 m s−1. Table 1 details the evolution of the large-scale environmental conditions, as well as the intensity and motion of the TC. Note that throughout the RI SST remains high and relative humidity is steady and near 75%. The deep-layer tropospheric shear, calculated from 200 to 800 km from the TC center, is directed to the south and increases during RI approaching a value (∼10 m s−1) where one might expect a negative impact on TC intensity (e.g., Landsea 1993).

b. Sampling

On 2 and 3 August, the two NOAA WP-3D aircraft simultaneously sampled Hurricane Guillermo (Fig. 1, note gray shadings). Sampling occurred shortly after RI commenced on 2 August when the estimated deepening rate was 2 hPa h−1. When the aircraft returned the following day, the TC had deepened 25 hPa from the previous mission and was nearly steady state. Aircraft flew roughly along constant pressure surfaces of 500 and 700 hPa within 300 km of the TC center. Each plane flew the same flight pattern both days such that the lower aircraft visited the center 10 times, while the higher aircraft visited the center 6 times over a ∼6-h period (1830–0100 UTC). This configuration allowed for frequent monitoring of the precipitation fields of the eyewall with the lower fuselage and tail radars. Figure 2 displays the storm-relative location of each GPS sonde used in the 10-m altitude analysis and gives a sense of the straight portions of the track of the upper-level aircraft. Note that the eyewall was not sampled on 2 August with GPS sondes.

c. Data preparation and quality control

The original NOAA WP-3D in situ sensors are described by Jorgensen (1984a) and updates are discussed by Aberson et al. (2006). The radars are discussed by Marks (1985). Nearly 1000 tail and lower fuselage radar images were processed at the Hurricane Research Division (HRD). The images provide a continuous view of the TC inner core during and subsequent to RI and are used to calculate net LHR (Marks 1985).

All of the GPS sonde data were processed through the Atmospheric Sounding Processing Environment (ASPEN) program. This software was developed by the National Center for Atmospheric Research (NCAR) to accept raw data in the form of Airborne Vertical Atmospheric Profiling System (AVAPS) files. (A discussion of the extensive series of quality control algorithms in ASPEN may be found online at http://www.eol.ucar.edu/rtf/facilities/software/aspen/Aspen%20Manual.pdf.)

Equipment malfunctions, such as a break in satellite connection, and sensor failure, which produce erratic profiles, resulted in the discard of 12 soundings. The remaining 70 soundings underwent additional scrutiny after being processed with ASPEN. Brief gaps of missing data were populated with linearly extrapolated values for distances less than 300 m. The wind observations had the highest failure rate near the sea surface with 22 of the 70 (31%) containing no data at 10 m. However, failure rates were only 10% at 50 m. Most of the failures occur in high winds. Franklin et al. (2003) examined 429 eyewall GPS sondes and found that failures quickly increased below 100 m. The cause of the failures was believed to be the GPS sonde’s inability to maintain communication with GPS satellites. Despite the application of ASPEN, there remained some questionable portions of the thermodynamic vertical profiles. The more prevalent of these include a low relative humidity bias and sensor wetting where the relative humidity remains 100% after exiting cloud. The recognition and correction schemes for these problems are discussed by Schneider and Barnes (2005), Sitkowski (2007), and Barnes (2008).

The HRD track file of Guillermo, composed of satellite and aircraft fixes, generates a latitude and longitude pair every 10 min and was used to determine the TC position for a given time. Analysis was centered during the mean time between the first and last GPS sonde deployment for each day. The composite fields take into account the position of the drop relative to the circulation center throughout the sonde’s descent.

GPS sonde data were interpolated to constant height intervals of 10 m and a piecewise cubic Hermite interpolation (Fritsch and Carlson 1980) was performed to analyze the fields for each level. This cubic-spline method preserved the monotonicity and the shape of the data and reproduces the observed value at its observed location. Because of coarse horizontal resolution and the assumption of a steady state over the 6 h to create the composite we can resolve only octant to quadrant-scale features. Subjective hand analyses were compared for several variables at different levels and matched well with the cubic-spline-derived analysis.

The cubic-spline and compositing technique did, however, produce unrealistic structure near the eyewall on 2 August simply because GPS sondes were not deployed in the eyewall on that day. Because the two days had repeatable flight patterns, the same six locations of eyewall drops from 3 August were chosen as the locations to bogus data in the eyewall on 2 August. We bogus T, θe, tangential, and radial wind components from the 700-hPa level to the lower levels using the following assumptions.

For T and dewpoint temperature (Td) we selected a portion of the flight track near or radially inward of the maximum wind speed that was saturated or where the dewpoint temperature (Td) exceeded the T value and vertical velocity was positive. The situation with Td > T is a typical problem with a cooled mirror Td sensor (Zipser et al. 1981). These readings are adjusted following the scheme recommended by Zipser et al. (1981) since the CO2 radiometer was not aboard this aircraft. For T we assume that the vertical profile is moist adiabatic till 300 m and dry adiabatic below that to the sea surface. The adjusted T and Td are used to estimate θe and this value is assumed to be well mixed to the surface. Note that the adjustment probably underestimates the maximum values in the most buoyant portions of the eyewall by a few kelvins (Eastin et al. 2002). This partial fix results in the radial gradient of θe flattening out in the eyewall instead of increasing steadily to the circulation center.

For the wind data we assume that near the inner edge of the eyewall there is a location where radial flow becomes negligible and the wind speed is entirely in the tangential component. Powell (1980) and Franklin et al. (2003) note that surface winds are approximately 0.7–0.9 the value of flight-level observations; we choose 0.9 for the eyewall region. Above 10 m we use the mean vertical eyewall wind profile from Fig. 8 in Franklin et al. (2003) to estimate winds at various altitudes. Figures 3a,b contrast the effect of the bogusing scheme for the tangential winds. A TC-like structure with a radius of maximum winds and a nearly calm eye is produced with the addition of the aircraft data. Based on comparisons with 3 August, where we have winds to the surface from the GPS sondes, we estimate the uncertainty to be 10%–15%. The main improvement is that an eyewall-like structure with a more realistic gradient is created.

The final adjustment to the fields on 2 August was to assume that the radial flow increases to the outer edge of the eyewall by maintaining the observed gradient from 150 to 75 km. It then decreases to zero by the inner edge of the eyewall. This mimics the typical structure seen by aircraft, sampling at 1 Hz, that are flying below 1000 m in both eyewalls and strong convective rainbands (Jorgensen 1984b; Barnes et al. 1983). Applying this method to 3 August as a test reveals that that the adjustment is slightly low, but within 10%–15% of the GPS sonde value. For mass inflow estimates we will constrain our discussion to 100-km radial distance, where we do not rely on the bogusing of the data. The scheme for the radial wind component is used simply to produce a more realistic vortex-scale view that respects the typical changes witnessed in the eyewall with 1-Hz sampling.

The fields derived from the GPS sondes are based on the assumption that the TC is steady during the approximately 6 h of flying during each day. On 2 August this is a bolder assumption because the central pressure decreases by about 11 hPa. The upper aircraft that deployed the sondes does not return to any portion of the TC to compare early and late sonde deployments. However, the aircraft at 700 hPa does. We have examined the wind speeds at the radius of maximum winds, and about 80 km away, in particular for the southeast quadrant where we detect a strong inflow as will be discussed later.

The storm-relative wind speeds for the eyewall, the mean along the flight leg, and at 80 km from the circulation center show only small changes (1–2 m s−1). This leads us to believe that the derived fields are only slightly compromised by the TC evolution.

3. Results

a. Northern portion of the eyewall spirals into eye

The eye diameter is defined as the mean distance between the innermost 29-dBZ contours, passing through the geometric center of the eye on cardinal headings. This estimate was made 131 times, each consisting of a pair of north–south and east–west estimates, with the lower fuselage radar and only when the aircraft was in the eye. The mean eye diameter displays an erratic behavior on 2 August with four episodes starting at 1950, 2113, 2145, and 2330 UTC (Fig. 4, top) where there is a reduction in the eye size. Examination of the reflectivity field shows that the eye has an elliptical shape that is rotating, but there is also a repeated spiraling in of the northern eyewall. The spiraling inward cannot be explained by a simple rotation of the elliptical eye given that it is a periodic behavior.

An example of the eyewall spiraling into the eye (Fig. 5) occurs from 2113 to 2123 UTC. The echoes in the spiral-in feature have tops of 12 km but rapidly dissipate, probably in response to the warm and dry air of the eye that serves as its environment. Dewpoint depressions at 700 hPa exceed 5°C on both sides of the intruding eyewall. Note that this process is not one of uniform contraction akin to concentric eyewall cycles. Throughout 3 August and post-RI the eye diameter is more circular and stable (Fig. 6) with a mean diameter of 43 km (Fig. 4b) and a slight increasing trend during the last few hours of the flight. From 2 to 3 August the mean diameter shrinks 10 km. Note that the views in Figs. 5 and 6 are with the aircraft at the center of the picture; the inward spiral of reflectivity does not reach within 15 km of the circulation center.

b. Eyewall reflectivity remains asymmetrical horizontally and vertically

Figures 5 and 6 also show that despite the RI the eyewall has remained asymmetric with the heaviest precipitation found in the eastern side. Tail radar scans (Fig. 7) also depict a strong asymmetry with echoes on the east and north sides several kilometers taller than those in the west and south portions of the eyewall. The mean of the entire suite of tail scans that are available when the aircraft is in the eye (70 cross sections) reveals that the vertical asymmetry persists throughout RI (Fig. 8). Guillermo is embedded in northerly shear near 8 m s−1 with the strongest reflectivity most often located to the left of the shear vector. The tallest echoes are also to the left and in the upshear quadrant (north). A similar reflectivity configuration was observed by Black et al. (2002) when they examined two TCs embedded in shear >8 m s−1, but these TCs were not undergoing RI. Model studies (e.g., Frank and Ritchie 2001) suggest that shear-induced differential vorticity advection with height may be responsible for the asymmetric structure. The maintenance of such a strong wavenumber 1 asymmetry despite a 25-hPa deepening is counter to our expectations. Only the shrinking diameter of the eyewall matched prior findings on the relationship between contracting eyewalls and intensification (Jordan 1961; Holliday and Thompson 1979; Willoughby et al. 1982; Jorgensen 1984a,b; Weatherford and Gray 1988; Bosart et al. 2000).

c. Net latent heat release around eye

Rain, a measure of net LHR, estimated from satellites or airborne radar, is positively correlated with more intense TCs (Adler and Rodgers 1977; Rodgers and Adler 1981; Marks 1985; Cerveny and Newman 2000). The trends, however, do not always match expectation with TCs either decaying or deepening with no concomitant LHR changes (Marks 1985; Rodgers et al. 1998). Higher rainfall rates concentrate near the TC center during intensification, though there may be a lag of a day or more between maximum net LHR and maximum intensity (Rodgers and Adler 1981; Rodgers et al. 2000). A lag of 1–2 days was also noted by Marks (1985) who used aircraft radar within 1° of the center of TC Allen (1980). Such a long lag is unexpected given the basic concepts believed to control TC intensity (Malkus and Riehl 1960; Emanuel 1986).

We use twenty 120 km × 120 km LF radar reflectivity images (10 for each day) from the 700-hPa aircraft to calculate net LHR on 2 and 3 August. These reflectivity images are taken during each pass through the eye of Guillermo when the aircraft is very near the circulation center and has an unobstructed view of the eyewall with minimal attenuation and beam-filling compromises. The images are composed of 15 reflectivity categories, each covering 2–3-dBZ increments. Given that beam filling becomes a serious problem beyond 60–70 km (Marks 1985), the reflectivity image is cropped to only include reflectivity values within a 60-km radius from the aircraft. This includes the eye, eyewall, and segments of adjoining rainbands on both days. The <15-dBZ range is excluded from the calculation since it produces a near-negligible contribution to net LHR and some of the rain is assumed to have evaporated before it reaches the surface. A tropical Z–R relationship is used (Rosenfeld et al. 1993) to convert dBZ to a rain rate. Other ZR relationships such as that derived by Jorgensen and Willis (1982) produce lower rain rates, especially for greater dBZ values, but we are chiefly interested in the change from one day to the next and different Z–R relationships would yield qualitatively similar results. Net LHR is calculated using
i1520-0493-137-2-645-eq1
where L is the latent heat of condensation (2.5 × 106 J kg−1), A is the area of a particular rain rate, RR, and ρ is the density of rain (assumed to be 1.0 × 103 kg m−3).

Figure 9 shows the estimates for net LHR for the 10 samples for each day and the reflectivity field from which it was derived. The mean for the 10 passes for 2 August is 3.4 × 1013 W, and for 3 August it is 4.1 × 1013 W, a 21% increase. This is expected given the deepening that occurred. What is unexpected is the lack of any increasing trend, at least for 2 August since Guillermo deepens some 11 hPa from the first to last pass. Instead an overall slight decrease could be argued, but with minor peaks in LHR occurring during the second, fifth, and ninth passes. Modest convective bursts occurred near the time of these same passes based on cloud-top temperatures from satellite (M. Eastin 2008, personal communication). The greatest pressure falls from the aircraft (∼6 hPa h−1) were observed near the time of the fifth pass (Fig. 5) when the wrap-in of the eyewall is coupled with one of these convective bursts. However, during 3 August there are small peaks in the LHR that nearly mimic those of 2 August, but RI is not occurring on this day. If convective bursts are indeed responsible for the maxima on the second sample then the importance of an inward spiraling eyewall and its association with eye/eyewall mixing processes maybe the more crucial aspect for RI.

As an eyewall shrinks, and if its width remains constant, one would expect a reduction in rain area and less net LHR, unless there is a corresponding increase in the rain rate. Marks (1985) found that a reduction in eye diameter was approximately balanced by the increased rain rate in the eyewall for TC Allen (1980). When we focus on the specific rain rates and their contribution to Guillermo’s net LHR we observe a 58% increase in area for the 35–40-dBZ range from the second to the third scheme. The most intense rain rates (>43 dBZ), generally found in the main reflectivity arc of the eyewall, actually decrease in spatial extent. The increase of 21% in LHR occurs because the eye itself, which usually contains no reflectivity, covers less area on 3 August. Replacing the precipitation-free eye is moderate rain found radially outward and adjacent to the prominent reflectivity cells in the eyewall. This precipitation is not a convective rainband; instead it may well have originated in the eyewall updraft, but was lifted higher and consequently farther out radially than the droplets that contribute to the heaviest rain in the eyewall.

Estimates of LHR, whether it is from satellite-borne sensors viewing the entire TC or from airborne radar that has finer spatial resolution, albeit over only a portion of the TC, do not yield easily interpreted behavior. If θe increases as the TC deepens, following the arguments of Malkus and Riehl (1960) and Emanuel (1986), then one would certainly expect that the eyewall updraft would follow increasingly higher wet-bulb equivalent potential temperature (θw) lines, condense more water, and thus supply greater amounts of latent heating. The updrafts in eyewalls are modest (Jorgensen et al. 1985) so it seems unlikely that much of the condensate would be advected far from the updraft. Eastin et al. (2005b) rarely found updrafts greater than the typical raindrop fall velocity in Guillermo, and when they did they lasted for only a few seconds.

d. Rainband location and intensity remains constant

The lower fuselage radar cannot view the entire TC accurately at any one location because of beam filling, attenuation, and earth curvature issues. However, the same flight patterns being flown on each day provide a view of each portion of the TC with nearly a 24-h separation. Examination of the radar scans around the TC reveals that the east side of the TC contains the majority of rainbands (Figs. 10a,b). Often there are three to five bands observed on this half of the TC, and the difference between the two days is inconsequential. The east side of the TC persists throughout each day as the locus for band activity (Figs. 10c,d). These two views are near the end of each flight. The west side of the TC often exhibits a rain-free region adjacent to the eyewall with a lone band farther to the west (Figs. 10e,f). The open nature of the west side and the convectively favored east side do not change despite the RI. Tops are usually 10–13-km high and maximum reflectivities as seen with the tail radar are in the 33–37-dBZ range. The intensifying eyewall, with stronger inflow (shown later) and perhaps a circulation that would have produced upper-level sinking and low-level divergence, did not inhibit band activity.

e. Evolution in the kinematic fields

The tangential wind component (VT) increases from 2 to 3 August within the annulus from 25 to 100 km of the circulation center from the sea surface to 2500-m altitude. The difference field for 100 m is typical of this layer (Fig. 11). The large increase in tangential wind near the center is due to the migration inward of the radius of maximum winds (RMW) coupled with an increase in the maximum tangential winds within the eyewall of 11 m s−1. Winds within ∼20 km of the circulation center remain unchanged as this area is still the eye. Note that the tangential wind increase does extend well beyond the eyewall. The increase of VT extends from the inner eyewall edge, about 25 km, to ∼100 km, matching prior findings about how this inner core strength region responds to deepening (Croxford and Barnes 2002). Slight decreases in VT beyond 100 km in all quadrants save due south are seen. The lack of an increase at greater radii in all azimuths save due south largely matches the disconnection between intensity and outer core strength changes that often occurs (Weatherford and Gray 1988).

Inflow that is predominantly from the east-southeast on 2 August becomes more axisymmetric on 3 August (Fig. 12) with a maximum of −20 m s−1 found all around the eyewall. Inflow increases of 10 m s−1 are detected to the west of the eyewall where there is similar coverage on both days by the GPS sondes.

The magnitude of the inflow component near the eyewall has become more axisymmetric in the lowest few hundred meters adjacent to the sea, but the depth of the inflow continues to exhibit an exaggerated asymmetry with the southeast quadrant having inflow of 3–4-km depth, while the northwest side inflow depths are a factor of 5–7 less (Fig. 13). The deepest inflow is found at greater radii to the southeast and lowers with decreasing radius. The rainbands that favor the east side of the TC are dependent on a portion of this deep inflow. The inflow in the southeast quadrant is still over a factor of 2 deeper near the eyewall than other quadrants, which may explain why the deepest cells are also found on that side of the eyewall. The shear vector, pointing to the south for each day, may be the underlying cause for the strong asymmetry in the depth of the flow.

The sonde distribution is dense enough to estimate the net mass inflow from the sea surface to 2 km through three azimuth–height surfaces located 100, 150, and 200 km from the circulation center for each day. Net mass flux is a function of the density, magnitude, and depth of the radial wind component, and the circumference of the chosen circle, and identifies that portion of the inflow that supplies the inner rainbands and the eyewall. Table 2 displays the net mass flux for each range ring for the two days. From 2 to 3 August mass flux increases 30% or more for all three range rings. The mass flux for each ring (Fig. 14) shows that through the RI the inflow becomes more symmetric. Outward flux observed on 2 August in the west, northwest, and north octants turns to either inward or near zero flux and the maximum seen in the southeast, which was responsible for over half the inflow at 200 km on 2 August, becomes more evenly distributed on 3 August. There are similar inflow losses of 25%–30% from the 200- to the 100-km range ring for each day. We expected the TC to become more efficient, perhaps losing less mass to the rainbands during RI, but this is not borne out by the estimates (Fig. 14). Note that the losses occur in the east, southeast, and south where there are several major rainbands (Figs. 10a–d), while there is little or no loss in the regions where rainbands were absent. The mean loss from the 200- to the 100-km range ring is 2.07 × 109 kg s−1, which would be capable of supporting about 20 cumulonimbus clouds (Cb) (Barnes et al. 1991). The reflectivity fields for either day contain about 20–25 cells, each covering at least 40 km2, that we equate with a Cb.

Despite a more than 30% increase on 3 August in the net mass flux below 2 km in the annulus defined by the 200- and 100-km radii there is little or no increase in the tangential wind in that same ring. Increases in VT occur only within 100 km, save for an increase well to the south of the center. Advection of higher absolute angular momentum through the outer ring is being countered by losses through the top, friction, and transport to the inner adjacent ring that contains the eyewall. Are rainbands inefficient in producing spinup while the eyewall is not?

f. Evolution in the thermodynamic fields

The temperature field at 100 m on 2 August (Fig. 15a) shows a wavenumber-1 pattern with cooler air to the east of the circulation center, collocated with the rainbands and the strong eyewall. Downdrafts are the likely cause given that cool air is far from where the strong radial pressure gradient is found (25–60 km). To the west and on the northern periphery of the analysis, where there are few or no bands, the air is 2°–3°C warmer. On 3 August at 100 m (Fig. 15b) the cool air is more symmetrically arranged around the circulation center. Cooling on the west side near the center is collocated with convection on that side of the eye. There is still evidence of cooling associated with rainbands to the east-northeast of the center.

The difference field for temperature at 100 m (Fig. 16) shows that during RI warming of 0.5°–1.2°C dominates the inner 200 km except immediately to the southwest of the circulation center where the reflectivity field of the eyewall has become more complete compared to the prior day. There is also cooling of about 2°C where the eyewall has displaced the eye.

Warming also occurs at 1500-m altitude (not shown) within 150 km of the center in all quadrants except the southwest and where the eyewall has displaced the eye, similar to what was observed at 100 m. Maximum warming (∼1.5°C) occurs 150 km north of the center where the rainbands lose their convective structure and turn more stratiform. The changes in temperature in the lower troposphere support the argument that the downdrafts associated with the eyewall and rainbands located in the eastern half of the TC are less effective at cooling the lowest 1500 m as this layer is about 0.5°C warmer on 3 August.

By 4000 m the primary change is a warming of the eye and inner eyewall (not shown). Temperatures in the 3–4-km layer in the eye did increase by 2.5°C and the aircraft at ∼500 hPa saw an increase of 2°C from 2 to 3 August. Eastin et al. (2005b) also detected warmer downdrafts on 3 August in this TC. By the 35–40-km radius or along the outer edge of the eyewall there is little change (variations within 0.5°C) but the rainband region has warmed 1°–1.5°C. So, as RI progresses the key change in the 2000–4000-m layer is enhanced warming in the eye and inner eyewall region. The primary change in the lower layers is a shift to a more axisymmetric temperature field as the eyewall becomes more complete and a warming on the order of a degree Celsius occurs in the lowest several hundred meters within the 200-km radius.

The equivalent potential temperature (θe) fields for each day at 100 m (Fig. 17) reveal that lower values (≤350 K) are located in the eastern half of the TC and collocated with the active rainband region. On 2 August there is an annulus that stretches from about ∼185-km radial distance to the eyewall where θe increases about 10 K. On 3 August this annulus is narrower, and the increase in θe is greater; the mean change along the east side of the TC is about 17 K over 90 km. This is a factor of 3 increase in the gradient of θe. This annulus (eyewall to 90 km) is where wind speed at 100 m has increased as much as 12 m s−1. The inflow of Guillermo has become much more efficient at both collecting energy from the sea surface and keeping it in the inflow layer.

The difference in the θe fields (Fig. 18; 3–2 August) at 100 m shows an increase of 6–8 K in the eye and eyewall, a decrease of 2–3 K in the rainband region (60–120 km) from the northeast to the southeast moving clockwise, and a remarkable increase on the periphery of the analysis of 8–10 K to the north, east, and south. This outer ring of high θe is caused by an increase in the mixing ratio of over 3 g kg−1. On 3 August the downdrafts associated with the rainbands cooled slightly less (∼0.5°C), but dry the boundary layer more effectively when compared with the previous day. The increase in θe of 7–8 K in the eyewall matches the expectation based on both empirical (Malkus and Riehl 1960) and theoretical arguments (Emanuel 1986). The increase of θe is realized only within 60–80 km of the circulation center, constrained by the rainband downdrafts.

At 1500 and 4000 m θe increases of 6–8 K are seen in the eyewall and eye (not shown). In the rainband region the decreases in θe are small at 1500 m and disappear by 4000 m. The sharply defined annulus of increased θe on the eastern periphery of the analysis loses its identity with height as well.

The remarkable increase in θe along the periphery to the east and south, almost entirely the result of an increase in specific humidity, may play a role in the deepening after 3 August (about 9 hPa by 0000 UTC on 5 August), but is not playing an obvious role in the RI between 2 and 3 August, since this air has not yet reached the eyewall.

4. Conclusions

Seventy global positioning system sondes that provide octant- to quadrant-scale resolution in the lower troposphere are combined with radar and in situ data from the NOAA WP-3Ds to identify the evolving traits of Hurricane Guillermo (1997) as it deepened from 954 to 929 hPa in 24 h. The SST, midlevel relative humidity, and deep layer mean shear as defined by SHIPS (Kaplan and DeMaria 2003) reveal that Guillermo was initially ripe for rapid intensification. However, during RI the deep layer shear increased to over 8 m s−1, demonstrating that unambiguous clues to intensity change are rarely witnessed. Initially Guillermo was far from its maximum potential intensity, but most TCs fail to reach their MPI (Evans 1993; Whitney and Hobgood 1997) and often are tens of hectopascals weaker than the theoretical limit.

The radar, which provided excellent views of the eyewall about every 30 min during each flight, provided no evidence of a remarkable convective burst in the eyewall. Satellite observations do show evidence for a strong cell occurring between the fourth and fifth pass on 2 August (M. Eastin 2008, personal communication) but this event does not survive long enough to alter the reflectivity fields or net LHR dramatically. Explosive growth of one or more cumulonimbi has been implicated in prior intensifying TCs (e.g., Black et al. 1986; Zehr 1989; Rodgers et al. 1998). Through 2 and 3 August, echo tops in the eyewall routinely reached 14–16-km height. Despite RI, asymmetries survived with taller echoes and higher rain rates favoring the east side of the eyewall. This is likely in response to the deep-layer tropospheric shear vector as noted previously for Guillermo by Eastin et al. (2005b).

Radar also reveals that there were multiple periods when the northern portion of the eyewall spiraled into the eye. This spiraling inward is eventually successful resulting in a 10-km reduction of the eyewall diameter and a more circular shape. The process is in contradistinction to the uniform contraction typically associated with concentric eyewalls first documented by Willoughby et al. (1982), but has similarities to the barotropic nondivergent simulations conducted by Schubert et al. (1999).

Rainbands, potential competitors that can rob the inflow and replace it with downdrafts, did not wither during RI. The rainbands were consistently found in the eastern half of the TC and where the wind fields derived from the GPS sondes revealed a reduction in mass inflow with decreasing radius. The rainbands were collocated with cooler temperatures and lower θe in the boundary layer, but the decreases of θe did not achieve the magnitude capable of inhibiting eyewall vigor and subsequent intensity (Barnes et al. 1983; Powell 1990).

During RI the inflow evolved from a favored channel in the southeast quadrant to a more axisymmetric structure. Mass flux increased over 30% from 2 to 3 August in the 200–100-km annulus and was due to both an increase in inflow depth of hundreds of meters as well as a magnitude increase of ∼5 m s−1. However, tangential winds increased only within 100 km of the circulation center despite this enhanced inflow. The decaying correlation between intensity and inner and outer core strength is similar to the findings of Weatherford and Gray (1988) and Croxford and Barnes (2002).

Perhaps the key initial process is the spiraling inward of the eyewall, resulting in the establishment of a smaller diameter, more complete eye. Notable reductions in MSLP are correlated with the eyewall invading the eye, which is not apparent in the coarse best-track record. Two questions then arise, first, what caused the northern portion of the eyewall to spiral inward? Kossin and Schubert (2001), Kossin and Eastin (2001), and Eastin et al. (2005a,b) argue that with certain vorticity profiles mesovortices can form in the eye that allow mixing between the eye and eyewall. A Doppler analysis of Guillermo shows what appears to be flow from the northern eyewall into the eye (Eastin et al. 2005c). The incorporation of lower eye air into the eyewall can boost buoyancy (Eastin et al. 2005b; Montgomery et al. 2006). Can this exchange also cause the eyewall to spiral inward? Loss of lower eye air could allow the eyewall to contract, enhance subsidence in the eye, or both. We do see warming above 2500 m showing that enhanced subsidence is occurring. We view the eye–eyewall exchange as a potential first step in the intensification. The spiraling-in took multiple tries as the invading cells are initially defeated by the warm, dry air in the upper portion of the eye.

The second question is how does Guillermo maintain the more intense state observed on 3 August? An intriguing result is that the rapid deepening has a temporal scale that is far greater than any spiral-in event or convective cell lifetime. The vortex establishes a robust, steady structure by 3 August well after spiraling inward episodes ceased. Maintenance of this refurbished eyewall with a smaller radius of maximum winds depends on the development of an annulus adjacent to it that efficiently receives and traps energy from the sea, and transports this energy to the eyewall. Its presence is revealed by the sharp radial gradients of θe in Figs. 17 and 18. The width of this ring is defined by the inner edge of the eyewall, where the inflow ascends, and the inner edge of the low θe outflow produced by the multiple bands to the east of the TC center. In the future we plan to examine inflow trajectories to Guillermo’s eyewall with the GPS-derived fields.

Acknowledgments

This work would not have been possible without support from the National Science Foundation, Grants ATM02-39648 and ATM-0735867, and the fine field work of NOAA/AOC and NOAA/AOML/HRD. A portion of this work is derived from the master’s thesis of Matt Sitkowski. Klaus Dolling provided some of the computer code to produce the horizontal fields. The manuscript was improved by the thoughtful reviews of Matt Eastin and John Kaplan. Garpee Barleszi’s uncompromising attitude led to a tightening of the prose.

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Fig. 1.
Fig. 1.

Best-track 1-min-sustained 10-m wind speed (m s−1, left ordinate, solid diamonds) and surface pressure (hPa, right ordinate, open squares) as a function of time for Guillermo. Two gray columns identify times when NOAA aircraft sampled the TC and vertical lines delineate an entire period of rapid intensification.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 2.
Fig. 2.

Storm-relative location of GPS sondes used in the analysis on (a) 2 and (b) 3 Aug. The black open circle indicates the TC circulation center and the 75-km range ring is shown.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 3.
Fig. 3.

Horizontal field of wind speed (solid contours every 5 m s−1) on 2 Aug at 100-m altitude (a) without GPS sonde data in eye and eyewall and (b) with extrapolated aircraft data and extension of GPS sonde winds in the eye from 400 m. The solid dot denotes the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 4.
Fig. 4.

Mean diameter (km) of the eye based on estimates through cardinal headings for (top) 2 and (bottom) 3 Aug.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 5.
Fig. 5.

Four lower fuselage scans (120 km × 120 km) showing an example of the spiraling inward of the eyewall. Times to the nearest minute are 2113, 2117, 2119, and 2123 UTC 2 Aug. The color scale to the right indicates dBZ values. The small white cross in the eye shows the aircraft position at the time of the scan, and the yellow dot shows the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 6.
Fig. 6.

Lower fuselage plan view of reflectivity (120 km × 120 km) at ∼1909 UTC 3 Aug. The color scale to the right indicates dBZ values. The small white cross in the eye shows the aircraft position at the time of scan, and the orange dot shows the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 7.
Fig. 7.

The 120 km × 20 km vertical cross sections of reflectivity from the tail radar at (a) 0002:59 UTC 3 Aug in the WSW–ENE direction and (b) 1947:02 UTC 3 Aug in the W–E direction. The small cross in the lower center portion of the image represents the location of the aircraft. Reflectivity values in the eye at the surface depict the sea surface return, not precipitation. The color table in each panel depicts reflectivity values (dBZ).

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 8.
Fig. 8.

Mean eyewall echo tops of the eyewall (km) derived from the tail radar for 2 (open diamonds) and 3 Aug (solid triangles) as a function of the azimuth. The ENE and WSW azimuths not sampled on 3 Aug.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 9.
Fig. 9.

Net LHR estimates for each pass through the eye on 2 (black diamonds) and 3 Aug (open squares). Reflectivity images used in the analysis are included and colors follow the scale shown in Fig. 5. Images are chosen when the aircraft is near the center of the eye and extend to 60-km radial distance. Approximately 24 h separate each like-numbered pass.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 10.
Fig. 10.

Lower fuselage radar views (360 km × 360 km) of rainbands for the east side of the TC early for each mission at (a) 1831:40 UTC 2 Aug, (b) 1846:31 UTC 3 Aug; for late on the west side of the TC (c) 2414:24 UTC 2 Aug, (d) 2437:38 UTC 3 Aug, and again for the east side but late in each mission at (e) 1906:07 UTC 2 Aug and (f) 1920:56 UTC 3 Aug. UTC times exceed 2400 rather than the reset date. The left side is all from 2 Aug and the right side is from 3 Aug. Colors depicting reflectivity follow Fig. 5.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 11.
Fig. 11.

Difference field (contours are every 2.5 m s−1 until 5.0 m s−1, then every 5 m s−1) for the tangential wind component (3 Aug − 2 Aug) at 100-m altitude. Positive differences (increases) are solid lines and negative (decreases) are dashed. Distance from the circulation is displayed in kilometers along the x and y axis. The black dotted ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 12.
Fig. 12.

Radial wind field (contours every 3 m s−1) shown at 100-m altitude for (a) 2 and (b) 3 Aug. Negative values are inflow. The black dotted ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 13.
Fig. 13.

Inflow depth (contours every 1000 m solid, with 500- and 750-m depth dashed) for (a) 2 and (b) 3 Aug.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 14.
Fig. 14.

Mass flux (× 109 kg s−1) through three azimuthal–height surfaces at 200- (open triangles), 150- (solid squares), and 100-km (open diamonds) radial distance for (a) 2 and (b) 3 Aug and for (c) 3 Aug − 2 Aug. Height of the surface for all estimates is 2 km.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 15.
Fig. 15.

Temperature field (contours every 1°C unless otherwise labeled) shown at 100-m altitude for (a) 2 and (b) 3 Aug. The black dotted ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 16.
Fig. 16.

Temperature difference (3 Aug − 2 Aug) at 100-m altitude. Contours are every 0.5°C. Positive (warming) differences are solid contours and negative differences (cooling) are dashed. The black ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 17.
Fig. 17.

Equivalent potential temperature contoured every 2.5 K, at 100-m altitude for (a) 2 and (b) 3 Aug. The black dotted ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Fig. 18.
Fig. 18.

Equivalent potential temperature difference (K; 3 Aug − 2 Aug) at 100-m altitude contoured every 2 K. Solid contours are positive (increases) and dashed are negative (decreases). The black dotted ring marks the 75-km radius from the circulation center.

Citation: Monthly Weather Review 137, 2; 10.1175/2008MWR2531.1

Table 1.

Synoptic conditions, storm intensity, and motion. Data are taken from best-track and SHIPS predictor database (Kaplan and DeMaria 2003).

Table 1.
Table 2.

Net mass flux through range rings at 100-, 150-, and 200-km radius for 2 and 3 Aug. Values are in units of 109 kg s−1.

Table 2.
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