• Andreas, E. L., 1992: Sea spray and the turbulent air-sea heat fluxes. J. Geophys. Res., 97 , 1142911441.

  • Andreas, E. L., 1995: The temperature of evaporating sea spray droplets. J. Atmos. Sci., 52 , 852862.

  • Andreas, E. L., , and K. A. Emanuel, 2001: Effects of spray on tropical cyclone intensity. J. Atmos. Sci., 58 , 37413751.

  • Augstein, E., , H. Schmidt, , and F. Ostapoff, 1974: The vertical structure of the atmospheric planetary boundary layer in undisturbed trade winds over the Atlantic Ocean. Bound.-Layer Meteor., 6 , 129150.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , and M. D. Powell, 1995: Evolution of the inflow boundary layer of Hurricane Gilbert (1988). Mon. Wea. Rev., 123 , 23482368.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , and P. Bogner, 2001: Comments on “Surface observations in the hurricane environment”. Mon. Wea. Rev., 129 , 12671269.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , G. D. Emmitt, , B. Brummer, , M. A. LeMone, , and S. Nicholls, 1980: The structure of a fair weather boundary layer based on the results of several measurement strategies. Mon. Wea. Rev., 108 , 349364.

    • Search Google Scholar
    • Export Citation
  • Bister, M., , and K. A. Emanuel, 1998: Dissipative heating and hurricane intensity. Meteor. Atmos. Phys., 65 , 233240.

  • Bogner, P. B., , G. M. Barnes, , and J. L. Franklin, 2000: Conditional instability and shear for six hurricanes over the Atlantic Ocean. Wea. Forecasting, 15 , 192207.

    • Search Google Scholar
    • Export Citation
  • Bolton, D., 1980: The computation of equivalent potential temperature. Mon. Wea. Rev., 108 , 10461053.

  • Brummer, B., 1978: Mass and energy budgets of a 1 km high atmospheric box over the GATE C-scale triangle during undisturbed and disturbed conditions. J. Atmos. Sci., 35 , 9971011.

    • Search Google Scholar
    • Export Citation
  • Bryan, G. H., , and J. M. Fritsch, 2000: Moist absolute instability: The sixth static stability state. Bull. Amer. Meteor. Soc., 81 , 12071230.

    • Search Google Scholar
    • Export Citation
  • Cione, J. J., , P. G. Black, , and S. H. Houston, 2000: Surface observations in the hurricane environment. Mon. Wea. Rev., 128 , 15501560.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones, Part 1: Steady-state maintenance. J. Atmos. Sci., 43 , 585604.

    • Search Google Scholar
    • Export Citation
  • Fairall, C. W., , J. D. Kepert, , and G. J. Holland, 1994: The effect of sea spray on surface energy transports over the ocean. Global Atmos. Ocean Syst., 2 , 121142.

    • Search Google Scholar
    • Export Citation
  • Fairall, C. W., , J. E. Hare, , and A. A. Grachev, 2004: Sea spray droplet measurements in hurricanes Fabian and Isabel. Preprints, 26th Conf. on Hurricanes and Tropical Meteorology, Miami, FL, Amer. Meteor. Soc., CD-ROM, 3A.3.

  • Firestone, J. K., , and B. A. Albrecht, 1986: The structure of the atmospheric boundary layer in the central equatorial Pacific during January and February of FGGE. Mon. Wea. Rev., 114 , 22192231.

    • Search Google Scholar
    • Export Citation
  • Fitzjarrald, D. R., , and M. Garstang, 1981: Vertical structure of the tropical boundary layer. Mon. Wea. Rev., 109 , 15121526.

  • Foster, R. C., 2005: Why rolls are prevalent in the hurricane boundary layer. J. Atmos. Sci., 62 , 26472661.

  • Frank, W. M., 1977: The structure and energetics of the tropical cyclone. I. Storm structure. Mon. Wea. Rev., 105 , 11361150.

  • Franklin, J. L., , S. J. Lord, , and F. D. Marks Jr., 1988: Dropwindsonde and radar observations of the eye of Hurricane Gloria (1985). Mon. Wea. Rev., 116 , 12371244.

    • Search Google Scholar
    • Export Citation
  • Hawkins, H. F., , and S. M. Imbembo, 1976: The structure of a small, intense hurricane, Inez, 1966. Mon. Wea. Rev., 104 , 418442.

  • Hock, T. F., , and J. L. Franklin, 1999: The NCAR GPS dropwindsonde. Bull. Amer. Meteor. Soc., 80 , 407420.

  • Houze Jr., R. A., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Jordan, C. L., 1958: Mean soundings for the West Indies area. J. Atmos. Sci., 15 , 9197.

  • Jorgensen, D. P., 1984: Mesoscale and convective-scale characteristics of mature hurricanes. Part II: Inner core structure of Hurricane Allen (1980). J. Atmos. Sci., 41 , 12871311.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., , E. J. Zipser, , and M. A. LeMone, 1985: Vertical motions in intense hurricanes. J. Atmos. Sci., 42 , 839856.

  • Khalsa, S. J. S., , and G. Greenhut, 1985: Conditional sampling of updrafts and downdrafts in the marine atmospheric boundary layer. J. Atmos. Sci., 42 , 25502562.

    • Search Google Scholar
    • Export Citation
  • Kloesel, K. A., , and B. A. Albrecht, 1989: Low-level inversions over the tropical Pacific—Thermodynamic structure of the boundary layer and above-inversion moisture structure. Mon. Wea. Rev., 117 , 87101.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., , and W. T. Pennell, 1976: The relationship of trade wind cumulus distribution to subcloud layer fluxes and structure. Mon. Wea. Rev., 104 , 524539.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., , G. M. Barnes, , E. J. Szoke, , and E. J. Zipser, 1984: The tilt of the leading edge of mesoscale tropical convective lines. Mon. Wea. Rev., 112 , 510519.

    • Search Google Scholar
    • Export Citation
  • Malkus, J. S., 1958: On the structure of the trade wind moist layer. Pap. Phys. Oceangr. Meteor., 13 , 2. 47. [NTIS AD121519/3ST.].

  • Malkus, J. S., , and H. Riehl, 1960: On the dynamics and energy transformation in steady-state hurricanes. Tellus, 12 , 120.

  • McCaul Jr., E. W., 1991: Buoyancy and shear characteristics within hurricane tornado environments. Mon. Wea. Rev., 119 , 19541978.

  • Molinari, J., , P. K. Moore, , and V. P. Idone, 1999: Convective structure of hurricanes as revealed by lightning locations. Mon. Wea. Rev., 127 , 520534.

    • Search Google Scholar
    • Export Citation
  • Morrison, I., , S. Businger, , F. D. Marks, , P. Dodge, , and J. A. Businger, 2005: An observational case for the prevalence of roll vortices in the hurricane boundary layer. J. Atmos. Sci., 62 , 26622673.

    • Search Google Scholar
    • Export Citation
  • Nicholls, S., , and M. A. LeMone, 1980: The fair weather boundary layer in GATE: The relationship of subcloud fluxes and structure to the distribution and enhancement of cumulus clouds. J. Atmos. Sci., 37 , 20512067.

    • Search Google Scholar
    • Export Citation
  • Pennell, W. T., , and M. A. LeMone, 1974: An experimental study of turbulence structure in the fair weather trade wind boundary layer. J. Atmos. Sci., 31 , 13081323.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., 1990: Boundary layer structure and dynamics in outer hurricane rainbands. Part I: Mesoscale rainfall and kinematic structure. Mon. Wea. Rev., 118 , 891917.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., , and S. H. Houston, 1996: Hurricane Andrew’s landfall in south Florida. Part II: Surface wind fields and potential real-time applications. Wea. Forecasting, 11 , 329349.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., , P. J. Vickery, , and T. A. Reinhold, 2003: Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422 , 279283.

    • Search Google Scholar
    • Export Citation
  • Reinking, R. F., , R. J. Doviak, , and R. O. Gilmer, 1981: Clear-air roll vortices and turbulent motions as detected with an airborne gust probe and dual-Doppler radar. J. Appl. Meteor., 20 , 678685.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., , and K. A. Emanuel, 1987: An air-sea interaction theory for tropical cyclones. Part II: Evolutionary study using a non-hydrostatic axisymmetric numerical model. J. Atmos. Sci., 44 , 542561.

    • Search Google Scholar
    • Export Citation
  • Schneider, R., , and G. M. Barnes, 2005: Low-level kinematic, thermodynamic, and reflectivity fields associated with Hurricane Bonnie (1998) at landfall. Mon. Wea. Rev., 133 , 32433259.

    • Search Google Scholar
    • Export Citation
  • Wang, Y., , J. D. Kepert, , and G. J. Holland, 2001: The effect of sea spray evaporation on tropical cyclone boundary layer structure and intensity. Mon. Wea. Rev., 129 , 24812500.

    • Search Google Scholar
    • Export Citation
  • Weckwerth, T. M., , J. W. Wilson, , and R. M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon. Wea. Rev., 124 , 769784.

    • Search Google Scholar
    • Export Citation
  • Wilczak, J. M., 1984: Large-scale eddies in the unstably stratified atmospheric surface layer. Part I: Velocity and temperature structure. J. Atmos. Sci., 41 , 35373550.

    • Search Google Scholar
    • Export Citation
  • Wroe, D. R., , and G. M. Barnes, 2003: Inflow layer energetics of Hurricane Bonnie (1998) near landfall. Mon. Wea. Rev., 131 , 16001612.

    • Search Google Scholar
    • Export Citation
  • Wurman, J., , and J. Winslow, 1998: Intense sub-kilometer boundary layer rolls in Hurricane Fran. Science, 280 , 555557.

  • Wyngaard, J. C., , W. T. Pennell, , D. H. Lenschow, , and M. A. LeMone, 1978: The temperature-humidity covariance budget in the convective boundary layer. J. Atmos. Sci., 35 , 4758.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    A schematic of the vertical profiles of θ (K) and θe (K) and the layers usually identified in a sounding over the tropical trade wind ocean region. Label Zi denotes the trade wind inversion at approximately 2-km altitude.

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    Three examples of vertical profiles of θe (K) as a function of height (m) obtained in Bonnie (1998) that exhibit ∂θe/∂z > 0 well below the usual location of the midtropospheric minimum. The base of the positive lapse rate begins at (a) 900, (b) 600, and (c) 500 m.

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    (a)–(c) Temperature (boldface solid) and dewpoint temperature (thin solid) skew T–logp diagrams that correspond to the profiles of θe shown in Figs. 2a–c, respectively. Pressure increases downward on the y axis. Temperature lines (°C, solid gray) slant from lower left to upper right. Mixing ratio lines (g kg−1, gray dashed) also slope from lower left to upper right. Adiabats (°C, thin gray solid lines) slope from lower right to upper left and pseudoadiabats (°C) are the curved gray dashed lines. Winds are shown at the right; a full barb is 5 m s−1, and a triangle is 25 m s−1.

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    Vertical profiles of θe (K) and the relative radial wind component (m s−1) as a function of height (m) for three soundings. Horizontal lines denote layer where ∂θe/∂z > 0. Negative values of the radial component (Vr) denote relative inflow.

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    (a) Skew T–logp diagram of a MAUL-like structure observed in Hurricane Bonnie (1998) that is believed to be sensor error. Plotting convention for the skew T–logp follows Fig. 3. (b) Vertical profile of θe (K) for the same sounding. The plotting convention follows Fig. 2.

  • View in gallery

    Examples of two real MAULs (a) from 964 to 940 hPa and (b) from 960 to 925 hPa. Skew T–logp plotting convention follows that of Fig. 3.

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    Vertical profile of θe (K) with a MAUL denoted by the two horizontal lines.

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    Lower fuselage radar scan of the northwest portion of Bonnie at 1239 UTC 26 Aug 1998. Colors depicting reflectivity values (dBZ) are to the right of the picture. The picture is 240 km × 240 km. The aircraft is in the center of frame and is marked by a white plus sign. The arc of high reflectivity to the southwest of the aircraft is the outer eyewall.

  • View in gallery

    Departure in specific humidity from well mixed as a function of wind speed (m s−1) at the nominal height of 10 m for five speed groups. The thin line with diamonds shows total number of soundings per category, the thicker solid line with squares shows number of unmixed soundings, and the solid line with open triangles shows the percent of unmixed soundings near the surface.

  • View in gallery

    A schematic showing the possible effect that boundary layer rolls and spray may have on the vertical gradient of specific humidity. Main component of flow is into the page with thin lines with arrows depicting rolls. Specific humidity profiles are dark solid with a baseline q value shown as the dashed vertical line.

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    Schematic showing possible range of behavior for the temperature in the lowest 200 m; percentages show what was observed. Dashed line is a stable profile, solid line is dry adiabatic, and dotted is superadiabatic. Spray in layer is depicted.

  • View in gallery

    Vertical profile of θ (K) that manifests slight cooling near the sea surface.

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    Skew T–logp diagrams showing examples of superadiabatic lapse rates adjacent to the sea. Plotting scheme follows Fig. 3.

  • View in gallery

    (a) Temperature departure (°C) from adiabatic at the nominal height of 10 m as a function of wind speed (m s−1). (b) Schematic depicting median conditions for layers that have superadiabatic lapse rates in wind speeds exceeding 30 m s−1.

  • View in gallery

    Schematic summarizing the three atypical structures detected with the GPS sondes placed on the eastern side of a hurricane. Eye is on the far left with radial distance increasing to the right. The circle with an X within depicts predominantly tangential flow while boldface arrows below thin line depict layer with significant inflow. Vertical profiles of θe are shown by boldface lines, and the direction and magnitude of the flux of θe are shown by arrows. Numbers identify positive lapse rate of θe (1), MAUL (2), and steep decrease of θe adjacent to sea (3). Eyewall is shaded, heavy rain is thin clustered parallel lines, and spray is depicted by thin closed lines attached to ocean surface.

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Atypical Thermodynamic Profiles in Hurricanes

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

The global positioning system dropwindsondes deployed in Hurricane Bonnie on 26 August 1998 with supporting deployments in Hurricanes Mitch (1998) and Humberto (2001) are used to identify three unusual thermodynamic structures in the lower-cloud and subcloud layers. Two of these structures impact the energy content of the inflow and therefore the intensity of the hurricane. First, positive lapse rates of equivalent potential temperature are found near the top of the inflow. These layers insulate the inflow from the negative impacts of entrainment mixing and promote rapid energy increases, especially near the eyewall. The second structure is a rapid decrease of equivalent potential temperature adjacent to the sea surface. This is essentially a prominent surface layer that owes its existence to both higher moisture content and a superadiabatic lapse rate. The steep lapse rate most often occurs under and near the eyewall where wind speeds at the surface exceed hurricane force. The author speculates that water loading from spray increases the residence time of air parcels in the surface layer, contributing to the creation of this structure. The third feature is a moist absolutely unstable layer previously identified by Bryan and Fritsch for the midlatitudes. These layers are found adjacent to the eyewall, in rainbands, and in the hub cloud within the eye and are evidence of mesoscale or vortex-scale convergence and the very modest instabilities often found in the core of a hurricane.

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

Abstract

The global positioning system dropwindsondes deployed in Hurricane Bonnie on 26 August 1998 with supporting deployments in Hurricanes Mitch (1998) and Humberto (2001) are used to identify three unusual thermodynamic structures in the lower-cloud and subcloud layers. Two of these structures impact the energy content of the inflow and therefore the intensity of the hurricane. First, positive lapse rates of equivalent potential temperature are found near the top of the inflow. These layers insulate the inflow from the negative impacts of entrainment mixing and promote rapid energy increases, especially near the eyewall. The second structure is a rapid decrease of equivalent potential temperature adjacent to the sea surface. This is essentially a prominent surface layer that owes its existence to both higher moisture content and a superadiabatic lapse rate. The steep lapse rate most often occurs under and near the eyewall where wind speeds at the surface exceed hurricane force. The author speculates that water loading from spray increases the residence time of air parcels in the surface layer, contributing to the creation of this structure. The third feature is a moist absolutely unstable layer previously identified by Bryan and Fritsch for the midlatitudes. These layers are found adjacent to the eyewall, in rainbands, and in the hub cloud within the eye and are evidence of mesoscale or vortex-scale convergence and the very modest instabilities often found in the core of a hurricane.

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

1. Introduction

The Woods Hole Expeditions, conducted after the Second World War, provided the first detailed views of the thermodynamic vertical structure of the tropical atmospheric boundary layer adjacent to the sea (Malkus 1958). The structure is now well established for fair weather conditions (Fig. 1) based on numerous sources (e.g., Augstein et al. 1974; Pennell and LeMone 1974; Nicholls and LeMone 1980; Barnes et al. 1980; Firestone and Albrecht 1986; Kloesel and Albrecht 1989). The typical subdivisions of the boundary layer include the surface, mixed, transition, cloud, and inversion layers for the trade wind regions.

How this vertical thermodynamic structure evolves in a tropical cyclone (TC) is not well documented. Omega dropwindsondes (ODWs) did show some aspects of the boundary layer (Franklin et al. 1988; Powell 1990; Bogner et al. 2000; Barnes and Bogner 2001), but their quality demanded averaging over tens of meters. The hygristor often stayed wet after transiting through even thin clouds, creating the impression of saturated dry adiabatic layers from cloud base to the sea surface.

In 1997, a campaign was initiated to deploy the new global positioning system (GPS) dropwindsondes (sondes) in TCs. The sondes are an economical and safe way to sample the lower-cloud and subcloud layers of a hurricane. The fine vertical resolution (6–7 m) in the lower troposphere reveals new structures that are clues to what processes are becoming important as one approaches the eyewall.

Perhaps the most important difference between an undisturbed tropical boundary layer and the one found in the TC is the magnitude of the near-surface winds. Hurricane force winds produce high interfacial fluxes, generate large ocean waves, and produce copious amounts of spray. The secondary circulation of the TC can result in the juxtaposition of fresh inflow air underneath air that may have been rotating around the center of the TC for a number of hours. This condition is deemphasized in composite axisymmetric views of the TC but may be prominent for any particular hurricane. Convective-scale updrafts and downdrafts in rainbands and the eyewall replace the boundary layer with midlevel air. Mesoscale downdrafts often found under stratiform rain alter what type of air is entrained into the top of the boundary layer.

a. Goals

The objective of this work is to determine how the GPS sondes performed in the challenging wind and humidity of a TC, and identify structures that ultimately might provide insight on processes such as high interfacial fluxes, the ejection of spray into the surface layer, and the reduction of instability as the radial distance decreases to the radius of maximum winds (RMW). In certain instances structures are apparent that affect the gain or loss of energy in the inflow equivalent potential temperature (θe), which ultimately impacts TC intensity (Malkus and Riehl 1960; Emanuel 1986). Three structures will be discussed: 1) positive vertical gradients of θe found well below the typical minimum at 650 hPa, 2) moist absolutely unstable layers (MAULs), and 3) prominent surface layers where the vertical gradient of θe is strongly negative.

2. Data

a. GPS sonde

The sonde falls at 12–14 m s−1 through the lower troposphere, transmitting measurements of air temperature, relative humidity, pressure, and position at a rate of 2 Hz. The pressure sensor is the Barocap, a silicon diaphragm that alters the capacitance of the circuit as it responds to changing pressure. A minor correction is made to account for the corruption of the static pressure measurement by the dynamic pressure produced by the sonde’s descent (Hock and Franklin 1999). The temperature sensor is a solid state thermistor known as the Thermocap. The more robust RS-80 is used for the GPS sondes, which can survive the jettisoning from aircraft. This sensor does have a lag, but this lag is well known and correctable (Hock and Franklin 1999). The moisture sensor is the H-Humicap. This is a thin film polymer that varies its capacitance as it absorbs or releases water vapor as a function of the relative humidity of the air. The H-Humicap, while superior to the older carbon strip that varied its resistance as a function of relative humidity, does have moments of questionable performance. It tends to miss the saturated conditions of very cold, thin clouds, may be slow to return to unsaturated conditions after exiting thicker clouds, and may drift at very high relative humidities. Occasionally there is evidence that a sonde may exhibit a dry bias of several percent. This can be recognized when the derived dewpoint mimics the detailed variations recorded by the temperature sensor, but is offset from saturation by a few percent.

Typical measurement errors for pressure, temperature, and relative humidity are 1.0 hPa, 0.2°C, and <5%, respectively (Hock and Franklin 1999). Maximum error in θe could reach 3.5 K if temperature and humidity sensors have such errors and are in phase.

The Atmospheric Sounding Processing Environment (ASPEN) software developed at the National Center for Atmospheric Research (NCAR) is used to process the raw data files. Full details can be found online (see www.eol.ucar.edu/rtf/facilities/software/aspen/aspen.html). In ASPEN, the thermodynamic variables are quality controlled independently. A variable is first constrained by a chosen, realistic range. Each point is compared to preceding and following points to eliminate suspicious data. A least squares linear fit is done and a chosen multiple of the standard deviation is used to eliminate outliers. The pressure is smoothed and checked to insure that it changes monotonically. The temperature is corrected for the lag inherent in the sturdy thermistor. Vertical resolution is maintained at approximately 7 m.

The corrections done in ASPEN do not eliminate all suspicious data. In particular I will discuss layers that are saturated but dry adiabatic or are saturated but not moist adiabatic. Thermodynamic variables such as specific humidity q, potential temperature θ, and θe are calculated using the equations presented by Bolton (1980).

b. Deployment in Bonnie and Mitch

The primary dataset consists of the 85 successful drops made in Bonnie (1998) as it made landfall on 26 August in North Carolina. Two National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft combined to deploy sondes over nearly 11 h in Hurricane Bonnie (1998) as it was making landfall in North Carolina on 26 August. Highlights of the flight patterns include spiral paths into the eyewall to mimic the inflow and several legs parallel to the shoreline on either side of the landfall point. Bonnie maintained a central surface pressure of 964 hPa, and maximum 1-min near-surface winds of ∼45 m s−1 were estimated with the “HWIND” near-surface wind analysis program of the Hurricane Research Division (see Powell and Houston 1996). Maps showing the sonde deployment are presented by Schneider and Barnes (2005).

Air Force reconnaissance deployed 10 sondes in Mitch (1998) on 26 October when the hurricane achieved a central surface pressure of 905 hPa. Winds measured with the GPS sonde reached ∼75 m s−1 within 100 m of the sea surface. The NOAA WP-3D mission into Mitch on 27 October was concentrated in and around the eyewall with a series of rotating figure “4” patterns with each leg of the figure 4 about 80 km from the circulation center. Twenty of the 31 sondes deployed were in the eyewall. The central pressure of Mitch was ∼930 hPa on 27 October with near-surface 1-min winds of ∼68 m s−1 estimated with HWIND for the eyewall. I will also use GPS sondes deployed on the periphery of Humberto (2001) by the G-IV aircraft to increase the number of drops in winds less than 7 m s−1 at 10-m altitude.

3. Results

a. Positive lapse rate of θe below 700 hPa

Generally θe manifests a minimum in the midtroposphere, typically around 650 hPa. Near the eyewall the midtropospheric minimum is nearly eliminated as moisture content increases, and the entire profile shifts to higher values (e.g., Jordan 1958; Hawkins and Imbembo 1976; Jorgensen 1984) in response to surface fluxes and subsequent redistribution of this energy by convective and mesoscale updrafts. In Bonnie, however, thin layers well below 650 hPa are detected that manifest increasing θe with height. Examples of three such soundings (Figs. 2a–c) show +∂θe/∂z at 900, 400, and a little above 500 m, respectively. Equivalent potential temperature increases by 3, 4, and 8 K, respectively. The skew T–logp diagrams for the aforementioned θe soundings reveal that the primary cause, as one would expect, is an increase in q with height (Figs. 3a,c), but the warm core anomaly associated with hurricanes may also contribute (Fig. 3b).

A survey of the soundings from Bonnie reveals that about half contain layers with ∂θe/∂z > 0. The thickness of the layer where θe increases averages 400 m, with a mean positive lapse of 7.5 K km−1. The typical altitude where the lapse rate turns positive is about 900 m with a standard deviation of 500 m.

Equivalent potential temperature is conserved during both dry and moist adiabatic processes. Energy inputs from the sea, at least at the moment of observation, cannot be responsible simply because the layers of interest are not adjacent to the sea surface. Radiative divergence can only reduce θe. Thus, the key diabatic terms do not offer plausible explanations for the observed structure.

A jettisoned sonde will follow a cyclonic horizontal trajectory, initially moving concentric to the circulation center, then entering into stronger inflow in the last 1–2 km. The gradients in θe occur over a depth of a few hundred meters where the sonde will cover much less than 1 km. This would demand unusually strong horizontal gradients in θe that also would be the reverse of the typical hurricane situation, with θe decreasing inward.

A layer with ∂θe/∂z > 0 could be produced if the sonde passed from an updraft that contained high θe to either downdraft or quiescent air with lower θe. However, correlations between the changes in θe and vertical velocity excursions greater than |1.5| m s−1 that occupy a depth of at least 150 m were poor (r2 = 0.12), demonstrating that the behavior of θe was not due to this possibility.

Differential advection appears to be the cause for the observed structure. The layers with ∂θe/∂z > 0 are correlated with decreasing inflow with height (∂Vr/∂z < 0) 85% of the time. The first two examples (Figs. 4a,b) show that the inflow changes to outflow through the same layer where there is ∂θe/∂z > 0. Other examples have the expected shear that would have the lower portion undercutting the upper portion of the layer (∂Vr/∂z < 0) but do not switch from inflow to outflow (Fig. 4c). The median decrease in the inflow across the layer with ∂θe/∂z > 0 is 3 m s−1.

From about 2–10 km, the flow in composite typhoons is essentially tangential but below 2 km the radial component of the flow becomes increasingly significant (Frank 1977). Hurricane Bonnie has a radial flow that is more asymmetric (Schneider and Barnes 2005) but still has its inflow confined to the lowest 2 km. A situation is created where air that has been in the core region for some time, and has already seen its energy content increased, is undercut by the inflow air. The inflow is still undergoing strong diabatic processes since it is in contact with the sea surface, but it may not yet be equal in energy content to the air that has been in the core region for some time.

The soundings with increasing θe with height are found in preferential locations. Just over half are in and near the eyewall, and another 36% are in or near rainbands. Less than 10% are found farther away, 150 km or more from the circulation center, and far from any active mesoscale feature. Near the eyewall CAPE may be modest or near zero, based on estimates from omega dropwindsondes (McCaul 1991; Bogner et al. 2000) and inferred from lightning observations (Molinari et al. 1999). Little or no instability would certainly favor detrainment of inflow air as it rises. This air, with a weak radial flow component, may have a long residence time near the eyewall. Another possibility is that the inflow simply had higher θe at an earlier time. Later, at the time of the drop, the TC is less intense with a corresponding lower θe in the inflow.

What is the importance of a layer with θe that is higher than or equal to the inflow θe and located directly above the inflow? Mixing between the inflow layer and the high θe layer will not result in a reduction of the inflow energy. In a sense the layer serves as an insulator, helping the inflow to retain a higher percentage of its energy. The insulating layer occurs near the eyewall favoring this region for rapid energy increase. Extreme energy increases over short radial distances have been observed by Hawkins and Imbembo (1976) and Wroe and Barnes (2003) adjacent to the eyewall and adjacent to strong rainbands (Barnes and Powell 1995). Numerical simulations (e.g., Rotunno and Emanuel 1987; Wang et al. 2001) also have been used to identify the region near the eyewall as the most favored location for rapid energy increase.

b. Nonconstant θe in saturated conditions—MAULs

The static stability of the atmosphere has been usually categorized into five states ranging from absolutely stable to dry, absolutely unstable. Recently, MAULs have been identified by Bryan and Fritsch (2000). Such a layer is characterized by saturated conditions, but with a temperature lapse rate that is steeper than moist adiabatic. In a MAUL, as in a dry absolutely unstable layer, any vertical displacement produces accelerations in the direction of the displacement, yielding rapid overturning that should eliminate the instability. Convective clouds should form and eliminate any MAUL; however, Bryan and Fritsch (2000) hypothesize that such a layer could exist when the dynamically driven mesoscale vertical velocity that is responsible for MAUL formation is greater than the buoyancy-driven vertical velocity, typically convective scale, that works to alleviate the unstable situation. Bryan and Fritsch (2000) found MAULs in the inflow region to mesoscale convective systems, where the outflow from the downdrafts could serve as the mechanism to lift the layer that might be tens of kilometers wide.

Hurricanes have all the requisite conditions to form MAULs with high vapor content, strong kinematic forcing as air converges toward the eyewall, and decreased instability as the distance to the eyewall decreases. All three factors would favor situations where mesoscale lifting could be greater than convective overturning and produce a MAUL.

The 85 drops in Bonnie were examined for evidence of MAULs. Forty-two soundings yielded 46 layers exhibiting MAUL characteristics (a lapse rate steeper than moist adiabatic and saturated, and at least 10 mb in thickness). The MAULs appear to be ubiquitous in Bonnie, but there are complications. When a sonde passes through cloud, conditions are recorded as one would expect, that is, moist adiabatic and saturated. However, upon exiting the cloud, here defined by a change of lapse rate to dry adiabatic, the hygristor continues to record 100% relative humidity. I surmise that the sonde has collected some liquid water on its descent through cloud, and the liquid on the hygristor must evaporate before the sensor records true ambient values. In the thin hurricane mixed-layer where relative humidity exceeds 85% there is rarely enough time for evaporation to occur. The result is that many of the so-called MAULs that appear in Bonnie adjacent to the sea are probably from instrument error. An example (Fig. 5a) shows a dry adiabatic layer adjacent to the sea, approximately 30 hPa thick, but with saturated conditions. Note that immediately above, conditions are moist adiabatic and saturated, making a good case for the hypothesis that the sonde passed through cloud resulting in liquid accumulating on the hygristor. The θe profile (Fig. 5b) manifests a steep decrease in the layer where the hygristor has failed, not in itself suspicious save for the fact that the layer is saturated. Twenty-five of the MAULs occur adjacent to the sea in dry adiabatic conditions and appear to be nothing more than a failure of the hygristor to dry out after exiting the cloud base. Another 12 at higher altitudes are suspect in that they appear immediately below a saturated but moist adiabatic layer. So, most (80%) are essentially suspicious structures indicative of a hygristor coated with liquid.

Nine of the MAULs appear to be real. A MAUL in Fig. 6a exists from 964 to 940 hPa. Immediately above this MAUL is a warm, dry, and stable layer that would inhibit the erasing of the MAUL by convective elements. In Fig. 6b another MAUL is located from 960 to 925 hPa. In these cases θe (Fig. 7) decreases rapidly through a saturated layer in contrast to what one expects (θe = constant). This MAUL is also capped by a stable layer. Note that neither MAUL is found below saturated conditions where the hygristor could have collected liquid but failed to dry out. The mean base and top for these few MAULs are 940 hPa (standard deviation = ±35 hPa) and 905 hPa (standard deviation = ±25 hPa), respectively. These MAULs are located either adjacent to rainbands or the eyewall but not collocated with high reflectivity regions. Several of the MAULs are in the northwest portion of Bonnie, between a rainband and the eyewall (Fig. 8) where there is mesoscale convergence but not deep convection that would tend to eliminate the instability. In Fig. 8 the MAUL was found between the eyewall to the southeast and a rainband to the northwest.

The MAULs observed in Bonnie are thinner than those reported by Bryan and Fritsch (2000). There are observational as well as physical reasons that contribute to this finding. First, the relative speed of a rawinsonde is low (+1 to +2 m s−1) when compared with a dropwindsonde (−14 to −17 m s−1) in a mesoscale updraft of a few meters per second. This would tend to produce a short residence time in the MAUL, especially because tropical updrafts tend to have steep slopes (LeMone et al. 1984) and be narrow (Jorgensen et al. 1985).

Deep MAULs demand very strong mesoscale forcing to keep the vertical velocity greater than the buoyancy-driven vertical velocity that tends to destroy it. If a MAUL had only 0.1°C steeper lapse rate than a saturated adiabat, the resulting vertical velocity is 2.9 m s−1 if integrated over a depth of just 500 m. Water loading, which is about 1.0 g kg−1 (500 m)−1 in the typical conditions in a mature TC, would slow the vertical velocity because of buoyancy to about 2.0 m s−1. This means the mesoscale updraft must be at least that value and double that by 1000-m depth to overcome the buoyancy-driven updraft.

In a TC, low-level background convergence rarely produces a mesoscale updraft greater than a few meters per second. Vertical velocities at the top of a 1-km-deep layer with a mean divergence of −3.0 × 10−3 s−1 are +3.0 m s−1. Such strong divergence is associated with the eyewall and convective rainbands (Jorgensen 1984; Powell 1990). Many of the situations discovered by Bryan and Fritsch (2000) are being forced along gust fronts where the resulting vertical velocity is much greater, allowing for a deeper MAUL.

MAULs are more evidence that in the inner core of the TC convective instability is decreasing while mesoscale forcing is increasing. The MAULs tend to occur near a mesoscale reflectivity feature. Subsidence associated with the eye, eyewall, or rainband produces a stable layer that prevents convective-scale motions from achieving vertical velocities that can overwhelm the MAUL.

c. Negative lapse rate of θe adjacent to the sea

The θe profiles in the lowest few hundred meters adjacent to the sea may be categorized either as mixed (∼33%; Fig. 4a), those with a strong negative gradient (∼57%; Figs. 4b,c, 5b), or those with erratic fluctuations (10%). About three-quarters of the well-mixed profiles appear well beyond the RMW where wind speeds at 10 m are less than 20 m s−1. These profiles are similar to undisturbed tropical conditions (Augstein et al. 1974; Pennell and LeMone 1974; Wyngaard et al. 1978; Nicholls and LeMone 1980; Firestone and Albrecht 1986; Kloesel and Albrecht 1989).

The second category with the steep lapse rate of θe adjacent to the sea [∂θe/∂z < −2.5 K (100 m)−1] is unusual and may be split into those that are due to instrument failure and those that appear to be real. Instrument failure, specifically a compromised hygristor, can occur when the sonde passes through a thick saturated layer collecting both cloud and rain drops, exits the cloud base, and remains saturated until it hits the sea (Figs. 5a,b), yielding a thick layer (a few hundred meters) with a steep lapse of θe. This air would be highly unstable according to parcel theory and would appear to be an extreme MAUL. In these situations the cloud base is low, there may be heavy rain, and the relative humidity in the subcloud layer exceeds 90%. There is probably insufficient time to either shed or evaporate the droplets that collected on the Humicap as it passed through cloud. About 25% of the total number of sondes deployed suffer from this problem. Nearly 85% of these failures occur under the eyewall, a convective rainband, or the hub cloud. I recommend correcting these soundings; assume a well-mixed-layer that becomes saturated at the lifting condensation level (LCL). The premise is that the LCL is coincident with the beginning of the moist adiabatic lapse rate. This correction yields a perfectly mixed and perhaps a conservatively dry subcloud layer.

About 33% of the total soundings or the rest of the 57% that are not well mixed have a rapid increase of relative humidity as the sonde approaches the sea (Fig. 6a). The mean increase of θe as the sonde descends is 3.5 K through a depth of ∼125 m. These usually remain unsaturated (Fig. 6a), though not all do (Fig. 3c). In these situations it is apparent that the moisture source is from below, in contrast to liquid collected during the passage through cloud and retained. About two-thirds of these soundings occur in and near the eyewall.

This is a challenging environment for the sensors, but I believe this lower portion of the sounding to be close to reality. First, spray, which has the potential to corrupt the hygristor, appears to be confined in a shallow layer a few tens of meters thick. A search for spray droplets, though limited to very little data, reveals that virtually no large spray droplets are found above 200-m altitude (Fairall et al. 2004). The bulk of the spray is expected to be composed of these larger droplets (Andreas 1992) that would tend to be concentrated in the lowest few tens of meters because of their high fall velocity. The sonde would pass through this layer in just a few seconds, decreasing the odds that the sensor would hit and acquire the droplets and have the readings compromised.

Second, the frequency of occurrence of an unmixed q profile [∂q/∂z < 0.3 g kg−1 (100 m)−1] as a function of wind speed at 10 m reveals that the condition exists 25% of the time even in light winds when there is no spray (Fig. 9). Here I have included soundings around the periphery of Humberto (2001) to increase the number of profiles in light wind conditions. As speeds increase there is an increase in the frequency of encountering a poorly mixed q profile (Fig. 9). A higher latent heat flux and evaporation of spray would contribute, but the latter would play a decreasing role under the eyewall and adjacent regions simply because relative humidity is above 90% (Schneider and Barnes 2005). Note that in the highest wind speed category there is evidence that a well-mixed profile of q can still occur nearly 30% of the time. Spray mixing ratios would be higher at and immediately downwind of the exploding crests and lower in the trough. Only at extreme speeds would one expect to see a uniform sheet of spray overlying the sea surface.

One-quarter of the profiles collected in light winds near the sea surface have an unmixed q profile reaching a few tens of meters in altitude. For the sondes deployed on the periphery of Humberto, the mean deviation in q away from the mixed profile is 0.4 g kg−1 over a mean depth of 80 m. The mean wind speed at the nominal height of 10 m is ∼5 m s−1. There is no spray present that could be responsible for the higher q near the surface.

I speculate that thermals, eddies, or rolls may be the cause of the structure. Khalsa and Greenhut (1985) observed plumes or thermals in the lowest third of the marine boundary layer over the central equatorial Pacific and attributed increases in q at the observing level to the transport of higher q from below. Horizontal rolls have been correlated with variations of q over sea (LeMone and Pennell 1976) and over land (Reinking et al. 1981; Weckwerth et al. 1996). Doppler radar measurements show that the TC boundary layer, at least during landfall, may contain rolls at times (Wurman and Winslow 1998; Morrison et al. 2005) that would contribute to variations in q. Foster (2005) has conducted a theoretical study for hurricane-like conditions and argues for the existence of rolls in TCs; he estimates that virtual temperature variations in the lower portion of the roll could be as much as 0.8 K, equivalent to a q that is 4 g kg−1 higher than background conditions.

Over 90% of the soundings are either well mixed or have positive departure from a well-mixed condition. In the presence of rolls or plumes, sondes that fall in the updraft or downdraft portion of the roll might tend to be well mixed, with the updraft portion of the roll containing a slightly higher q through a greater depth (Fig. 10). Well-mixed q profiles in the upward portions of eddies have been observed with a tower instrumented at multiple levels (Wilczak 1984). Sondes that fall in the middle of the roll, where the horizontal flow dominates, could have a poorly mixed profile. Note that these scenarios would not produce a thin layer of drier air adjacent to the sea surface, nor is it observed.

A sonde falls through the unmixed-layer in 2–10 s. This does not give much time for the sonde to be preferentially channeled into or out of the updraft or downdraft portion of an eddy or roll. I expect that the sonde will more often fall in the horizontal flow given their aspect ratio (Wurman and Winslow 1998; Morrison et al. 2005). There are more unmixed profiles (57%) than mixed (33%) in Hurricane Bonnie (1998) supporting this conjecture.

As a sonde falls from below cloud base into a layer, there are three possible temperature profiles: 1) the layer maintains a dry adiabatic lapse rate, 2) spray evaporation leads to a cooling of this layer and a shift toward a more stable lapse rate, and 3) a superadiabatic lapse rate (Fig. 11). A dry adiabatic lapse rate would not contribute to the unmixed θe layer while a cooling would work against the observed structure. The frequency of cooling and stabilization is rare (<10%), and when it occurs it is a very small magnitude (Fig. 12). A dry adiabatic lapse rate is maintained from cloud base to the sea in 50% of the soundings. The unanticipated finding is the presence of the superadiabatic lapse rate for about 20% of the Bonnie (1998) and Mitch (1998) drops. The superadiabatic layer is often on the order of 50–100 m thick (Figs. 13a,b).

Many of the soundings that manifest a superadiabatic lapse rate are near or under the eyewall. Horizontal advection of the sonde into a warmer region was considered as a possible cause. However, the sonde passes through the superadiabatic layer in about 7 s. Assuming a radial inflow of 15 m s−1, the sonde will traverse a little over a 100-m radial distance, which demands a horizontal gradient of 1°C (100 m)−1, a remarkable and unlikely magnitude.

Plotting the temperature deviation observed at 10 m of only those soundings that depart from adiabatic as a function of the wind speed at 10 m reveals that there is a trend toward a more pronounced superadiabatic layer with increasing wind speed beyond about 25 m s−1 (Fig. 14a). Below that speed there is a tendency to record those profiles that manifest a slight cooling. Most of these slightly cooled, stable soundings are from Bonnie, located beyond the RMW and in the southwest portion of the TC where drier air is flowing offshore, and where spray evaporation is most likely. The most prominent superadiabatic layers appear in and near the eyewall of Mitch (1998), where wind speeds are often greater than 50 m s−1. The average depth and lapse rate that is observed is 100 m with a maximum difference from an adiabatic value of 1.2°C (Fig. 14b).

There are three sources of energy that can contribute to the creation of a superadiabatic layer: 1) enhanced interfacial sensible heat fluxes, 2) transfer of sensible heat from the spray droplets to the air as they cool from SST to air temperature (Fairall et al. 1994; Andreas and Emanuel 2001), and 3) heating from viscous dissipation (Bister and Emanuel 1998). Which process dominates is unknown because all three would increase with wind speed and be maximized at the RMW. Larger air–sea temperature differences would also contribute to higher interfacial fluxes (Cione et al. 2000). Viscous dissipation is expected to be the most nonlinear responding to the third power of the wind speed (Bister and Emanuel 1998); however, recent estimates of the drag coefficient leveling off above 33 m s−1 (Powell et al. 2003) would reduce the magnitude. Sensible heat transfer from spray occurs when the spray droplet rapidly cools from SST to air temperature T in a fraction of a second (Andreas 1995). This process would be positively correlated with wind speeds, but wave height, length, direction, and speed would also affect spray amount.

Spray may be playing an additional role, reducing the instability of the surface layer. Following Houze (1993), buoyancy may be written as
i1520-0493-136-2-631-eq1
where g is the gravitational constant, T ′ is the temperature perturbation from the base state, T0 is the base temperature, qυ is the perturbation in water vapor, and ql is the liquid water in the parcel. Here I have ignored pressure perturbations in the parcel or a pressure gradient that would affect vertical accelerations. Spray would load a parcel, contributing to its density, as liquid and ice do in saturated conditions. A layer with either a superadiabatic temperature profile or decreasing water vapor content with height would be unstable and tend to overturn. But a small amount of spray can counteract this apparent instability. A parcel with a T ′ = 0.5 K and a qυ = 0.5 g kg−1 can be balanced by slightly less than 2 g kg−1 of spray. A little over 5 g kg−1 would counteract the warmest and moistest parcels that we observed in the surface layer. Spray may delay eddies from escaping from the surface layer for a number of seconds, allowing the parcels to achieve the higher temperature and moisture values that create the poorly mixed condition that we observe.

4. Conclusions

The GPS sondes reveal thermodynamic profiles in the lower-cloud and subcloud layers on the periphery of a hurricane that are similar to equatorial profiles discussed by many other investigators (e.g., Malkus 1958; Brummer 1978; Fitzjarrald and Garstang 1981; Nicholls and LeMone 1980; Firestone and Albrecht 1986; Kloesel and Albrecht 1989). As radial distance reduces to the radius of maximum winds, we encounter three atypical features that provide clues about the processes affecting the high-wind region of a tropical cyclone. Within 100–200 km of the circulation center, fresh supplies of inflow air may pass beneath air that has become essentially rotational and has been in and near the eyewall for some time. This can create a condition where the layer adjacent to the inflow has slightly higher θe than the inflow itself, temporarily reversing the vertical gradient of θe (Fig. 15). In such situations the layer with higher θe serves as an insulating layer promoting the rapid increase of energy near and under the eyewall as it inhibits the loss of energy through the top of the inflow via entrainment.

Along the outer edges of convective rainbands, adjacent to the eyewall, and sometimes in the hub cloud found within the eye, a MAUL may appear (Fig. 15). This sixth stability state, first identified by Bryan and Fritsch (2000), is a result of high moisture content, low instability, and strong mesoscale convergence. The presence of a MAUL is indicative of the decreasing conditional instability of the first kind as radial distance decreases to the RMW. At greater radial distances, increasing CAPE and decreasing mesoscale convergence will tend to suppress MAUL formation.

The frequency of occurrence of the prominent surface layer, where θe has a steep lapse rate through nearly 100-m depth, increases with increasing wind speed at 10-m altitude. Both higher moisture content and a superadiabatic lapse rate contribute to the deeper surface layer of θe. The prominent surface layer is supporting evidence for spray playing two roles. First, spray rapidly surrenders its sensible heat as it cools from sea surface temperature to air temperature (Andreas 1995), and second, it serves to water load the surface layer. This may keep a parcel in contact with the sea long enough to receive energy from a variety of sources—regular interfacial fluxes, spray sensible heat loss, and dissipative heating. Spray evaporation does not contribute to the steep lapse rate of θe, as it can only redistribute energy between sensible and latent energy, leaving θe unchanged. The poorly mixed-layer adjacent to the sea may be partially because of the presence of boundary layer rolls that would tend to keep air in contact with the sea for tens of seconds and allow the layer to achieve higher θe in its lower portion, resulting in the poorly mixed profiles. Both roll circulations and spray loading would promote a longer residence time for air parcels at the interface. In the future our understanding of the TC surface layer would greatly benefit from quantitative estimates of spray content.

Acknowledgments

This work would not have been possible without the generous support of the National Science Foundation via Grant ATM-0239648. The dedication and professionalism of NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division and the NOAA Aircraft Operations Center were vital in the collection of the dataset. Garpee Barleszi’s badgering led to an improved presentation.

REFERENCES

  • Andreas, E. L., 1992: Sea spray and the turbulent air-sea heat fluxes. J. Geophys. Res., 97 , 1142911441.

  • Andreas, E. L., 1995: The temperature of evaporating sea spray droplets. J. Atmos. Sci., 52 , 852862.

  • Andreas, E. L., , and K. A. Emanuel, 2001: Effects of spray on tropical cyclone intensity. J. Atmos. Sci., 58 , 37413751.

  • Augstein, E., , H. Schmidt, , and F. Ostapoff, 1974: The vertical structure of the atmospheric planetary boundary layer in undisturbed trade winds over the Atlantic Ocean. Bound.-Layer Meteor., 6 , 129150.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , and M. D. Powell, 1995: Evolution of the inflow boundary layer of Hurricane Gilbert (1988). Mon. Wea. Rev., 123 , 23482368.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , and P. Bogner, 2001: Comments on “Surface observations in the hurricane environment”. Mon. Wea. Rev., 129 , 12671269.

    • Search Google Scholar
    • Export Citation
  • Barnes, G. M., , G. D. Emmitt, , B. Brummer, , M. A. LeMone, , and S. Nicholls, 1980: The structure of a fair weather boundary layer based on the results of several measurement strategies. Mon. Wea. Rev., 108 , 349364.

    • Search Google Scholar
    • Export Citation
  • Bister, M., , and K. A. Emanuel, 1998: Dissipative heating and hurricane intensity. Meteor. Atmos. Phys., 65 , 233240.

  • Bogner, P. B., , G. M. Barnes, , and J. L. Franklin, 2000: Conditional instability and shear for six hurricanes over the Atlantic Ocean. Wea. Forecasting, 15 , 192207.

    • Search Google Scholar
    • Export Citation
  • Bolton, D., 1980: The computation of equivalent potential temperature. Mon. Wea. Rev., 108 , 10461053.

  • Brummer, B., 1978: Mass and energy budgets of a 1 km high atmospheric box over the GATE C-scale triangle during undisturbed and disturbed conditions. J. Atmos. Sci., 35 , 9971011.

    • Search Google Scholar
    • Export Citation
  • Bryan, G. H., , and J. M. Fritsch, 2000: Moist absolute instability: The sixth static stability state. Bull. Amer. Meteor. Soc., 81 , 12071230.

    • Search Google Scholar
    • Export Citation
  • Cione, J. J., , P. G. Black, , and S. H. Houston, 2000: Surface observations in the hurricane environment. Mon. Wea. Rev., 128 , 15501560.

    • Search Google Scholar
    • Export Citation
  • Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones, Part 1: Steady-state maintenance. J. Atmos. Sci., 43 , 585604.

    • Search Google Scholar
    • Export Citation
  • Fairall, C. W., , J. D. Kepert, , and G. J. Holland, 1994: The effect of sea spray on surface energy transports over the ocean. Global Atmos. Ocean Syst., 2 , 121142.

    • Search Google Scholar
    • Export Citation
  • Fairall, C. W., , J. E. Hare, , and A. A. Grachev, 2004: Sea spray droplet measurements in hurricanes Fabian and Isabel. Preprints, 26th Conf. on Hurricanes and Tropical Meteorology, Miami, FL, Amer. Meteor. Soc., CD-ROM, 3A.3.

  • Firestone, J. K., , and B. A. Albrecht, 1986: The structure of the atmospheric boundary layer in the central equatorial Pacific during January and February of FGGE. Mon. Wea. Rev., 114 , 22192231.

    • Search Google Scholar
    • Export Citation
  • Fitzjarrald, D. R., , and M. Garstang, 1981: Vertical structure of the tropical boundary layer. Mon. Wea. Rev., 109 , 15121526.

  • Foster, R. C., 2005: Why rolls are prevalent in the hurricane boundary layer. J. Atmos. Sci., 62 , 26472661.

  • Frank, W. M., 1977: The structure and energetics of the tropical cyclone. I. Storm structure. Mon. Wea. Rev., 105 , 11361150.

  • Franklin, J. L., , S. J. Lord, , and F. D. Marks Jr., 1988: Dropwindsonde and radar observations of the eye of Hurricane Gloria (1985). Mon. Wea. Rev., 116 , 12371244.

    • Search Google Scholar
    • Export Citation
  • Hawkins, H. F., , and S. M. Imbembo, 1976: The structure of a small, intense hurricane, Inez, 1966. Mon. Wea. Rev., 104 , 418442.

  • Hock, T. F., , and J. L. Franklin, 1999: The NCAR GPS dropwindsonde. Bull. Amer. Meteor. Soc., 80 , 407420.

  • Houze Jr., R. A., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Jordan, C. L., 1958: Mean soundings for the West Indies area. J. Atmos. Sci., 15 , 9197.

  • Jorgensen, D. P., 1984: Mesoscale and convective-scale characteristics of mature hurricanes. Part II: Inner core structure of Hurricane Allen (1980). J. Atmos. Sci., 41 , 12871311.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., , E. J. Zipser, , and M. A. LeMone, 1985: Vertical motions in intense hurricanes. J. Atmos. Sci., 42 , 839856.

  • Khalsa, S. J. S., , and G. Greenhut, 1985: Conditional sampling of updrafts and downdrafts in the marine atmospheric boundary layer. J. Atmos. Sci., 42 , 25502562.

    • Search Google Scholar
    • Export Citation
  • Kloesel, K. A., , and B. A. Albrecht, 1989: Low-level inversions over the tropical Pacific—Thermodynamic structure of the boundary layer and above-inversion moisture structure. Mon. Wea. Rev., 117 , 87101.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., , and W. T. Pennell, 1976: The relationship of trade wind cumulus distribution to subcloud layer fluxes and structure. Mon. Wea. Rev., 104 , 524539.

    • Search Google Scholar
    • Export Citation
  • LeMone, M. A., , G. M. Barnes, , E. J. Szoke, , and E. J. Zipser, 1984: The tilt of the leading edge of mesoscale tropical convective lines. Mon. Wea. Rev., 112 , 510519.

    • Search Google Scholar
    • Export Citation
  • Malkus, J. S., 1958: On the structure of the trade wind moist layer. Pap. Phys. Oceangr. Meteor., 13 , 2. 47. [NTIS AD121519/3ST.].

  • Malkus, J. S., , and H. Riehl, 1960: On the dynamics and energy transformation in steady-state hurricanes. Tellus, 12 , 120.

  • McCaul Jr., E. W., 1991: Buoyancy and shear characteristics within hurricane tornado environments. Mon. Wea. Rev., 119 , 19541978.

  • Molinari, J., , P. K. Moore, , and V. P. Idone, 1999: Convective structure of hurricanes as revealed by lightning locations. Mon. Wea. Rev., 127 , 520534.

    • Search Google Scholar
    • Export Citation
  • Morrison, I., , S. Businger, , F. D. Marks, , P. Dodge, , and J. A. Businger, 2005: An observational case for the prevalence of roll vortices in the hurricane boundary layer. J. Atmos. Sci., 62 , 26622673.

    • Search Google Scholar
    • Export Citation
  • Nicholls, S., , and M. A. LeMone, 1980: The fair weather boundary layer in GATE: The relationship of subcloud fluxes and structure to the distribution and enhancement of cumulus clouds. J. Atmos. Sci., 37 , 20512067.

    • Search Google Scholar
    • Export Citation
  • Pennell, W. T., , and M. A. LeMone, 1974: An experimental study of turbulence structure in the fair weather trade wind boundary layer. J. Atmos. Sci., 31 , 13081323.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., 1990: Boundary layer structure and dynamics in outer hurricane rainbands. Part I: Mesoscale rainfall and kinematic structure. Mon. Wea. Rev., 118 , 891917.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., , and S. H. Houston, 1996: Hurricane Andrew’s landfall in south Florida. Part II: Surface wind fields and potential real-time applications. Wea. Forecasting, 11 , 329349.

    • Search Google Scholar
    • Export Citation
  • Powell, M. D., , P. J. Vickery, , and T. A. Reinhold, 2003: Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422 , 279283.

    • Search Google Scholar
    • Export Citation
  • Reinking, R. F., , R. J. Doviak, , and R. O. Gilmer, 1981: Clear-air roll vortices and turbulent motions as detected with an airborne gust probe and dual-Doppler radar. J. Appl. Meteor., 20 , 678685.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., , and K. A. Emanuel, 1987: An air-sea interaction theory for tropical cyclones. Part II: Evolutionary study using a non-hydrostatic axisymmetric numerical model. J. Atmos. Sci., 44 , 542561.

    • Search Google Scholar
    • Export Citation
  • Schneider, R., , and G. M. Barnes, 2005: Low-level kinematic, thermodynamic, and reflectivity fields associated with Hurricane Bonnie (1998) at landfall. Mon. Wea. Rev., 133 , 32433259.

    • Search Google Scholar
    • Export Citation
  • Wang, Y., , J. D. Kepert, , and G. J. Holland, 2001: The effect of sea spray evaporation on tropical cyclone boundary layer structure and intensity. Mon. Wea. Rev., 129 , 24812500.

    • Search Google Scholar
    • Export Citation
  • Weckwerth, T. M., , J. W. Wilson, , and R. M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon. Wea. Rev., 124 , 769784.

    • Search Google Scholar
    • Export Citation
  • Wilczak, J. M., 1984: Large-scale eddies in the unstably stratified atmospheric surface layer. Part I: Velocity and temperature structure. J. Atmos. Sci., 41 , 35373550.

    • Search Google Scholar
    • Export Citation
  • Wroe, D. R., , and G. M. Barnes, 2003: Inflow layer energetics of Hurricane Bonnie (1998) near landfall. Mon. Wea. Rev., 131 , 16001612.

    • Search Google Scholar
    • Export Citation
  • Wurman, J., , and J. Winslow, 1998: Intense sub-kilometer boundary layer rolls in Hurricane Fran. Science, 280 , 555557.

  • Wyngaard, J. C., , W. T. Pennell, , D. H. Lenschow, , and M. A. LeMone, 1978: The temperature-humidity covariance budget in the convective boundary layer. J. Atmos. Sci., 35 , 4758.

    • Search Google Scholar
    • Export Citation
Fig. 1.
Fig. 1.

A schematic of the vertical profiles of θ (K) and θe (K) and the layers usually identified in a sounding over the tropical trade wind ocean region. Label Zi denotes the trade wind inversion at approximately 2-km altitude.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 2.
Fig. 2.

Three examples of vertical profiles of θe (K) as a function of height (m) obtained in Bonnie (1998) that exhibit ∂θe/∂z > 0 well below the usual location of the midtropospheric minimum. The base of the positive lapse rate begins at (a) 900, (b) 600, and (c) 500 m.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 3.
Fig. 3.

(a)–(c) Temperature (boldface solid) and dewpoint temperature (thin solid) skew T–logp diagrams that correspond to the profiles of θe shown in Figs. 2a–c, respectively. Pressure increases downward on the y axis. Temperature lines (°C, solid gray) slant from lower left to upper right. Mixing ratio lines (g kg−1, gray dashed) also slope from lower left to upper right. Adiabats (°C, thin gray solid lines) slope from lower right to upper left and pseudoadiabats (°C) are the curved gray dashed lines. Winds are shown at the right; a full barb is 5 m s−1, and a triangle is 25 m s−1.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 4.
Fig. 4.

Vertical profiles of θe (K) and the relative radial wind component (m s−1) as a function of height (m) for three soundings. Horizontal lines denote layer where ∂θe/∂z > 0. Negative values of the radial component (Vr) denote relative inflow.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 5.
Fig. 5.

(a) Skew T–logp diagram of a MAUL-like structure observed in Hurricane Bonnie (1998) that is believed to be sensor error. Plotting convention for the skew T–logp follows Fig. 3. (b) Vertical profile of θe (K) for the same sounding. The plotting convention follows Fig. 2.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 6.
Fig. 6.

Examples of two real MAULs (a) from 964 to 940 hPa and (b) from 960 to 925 hPa. Skew T–logp plotting convention follows that of Fig. 3.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 7.
Fig. 7.

Vertical profile of θe (K) with a MAUL denoted by the two horizontal lines.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 8.
Fig. 8.

Lower fuselage radar scan of the northwest portion of Bonnie at 1239 UTC 26 Aug 1998. Colors depicting reflectivity values (dBZ) are to the right of the picture. The picture is 240 km × 240 km. The aircraft is in the center of frame and is marked by a white plus sign. The arc of high reflectivity to the southwest of the aircraft is the outer eyewall.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 9.
Fig. 9.

Departure in specific humidity from well mixed as a function of wind speed (m s−1) at the nominal height of 10 m for five speed groups. The thin line with diamonds shows total number of soundings per category, the thicker solid line with squares shows number of unmixed soundings, and the solid line with open triangles shows the percent of unmixed soundings near the surface.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 10.
Fig. 10.

A schematic showing the possible effect that boundary layer rolls and spray may have on the vertical gradient of specific humidity. Main component of flow is into the page with thin lines with arrows depicting rolls. Specific humidity profiles are dark solid with a baseline q value shown as the dashed vertical line.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 11.
Fig. 11.

Schematic showing possible range of behavior for the temperature in the lowest 200 m; percentages show what was observed. Dashed line is a stable profile, solid line is dry adiabatic, and dotted is superadiabatic. Spray in layer is depicted.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 12.
Fig. 12.

Vertical profile of θ (K) that manifests slight cooling near the sea surface.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 13.
Fig. 13.

Skew T–logp diagrams showing examples of superadiabatic lapse rates adjacent to the sea. Plotting scheme follows Fig. 3.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 14.
Fig. 14.

(a) Temperature departure (°C) from adiabatic at the nominal height of 10 m as a function of wind speed (m s−1). (b) Schematic depicting median conditions for layers that have superadiabatic lapse rates in wind speeds exceeding 30 m s−1.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

Fig. 15.
Fig. 15.

Schematic summarizing the three atypical structures detected with the GPS sondes placed on the eastern side of a hurricane. Eye is on the far left with radial distance increasing to the right. The circle with an X within depicts predominantly tangential flow while boldface arrows below thin line depict layer with significant inflow. Vertical profiles of θe are shown by boldface lines, and the direction and magnitude of the flux of θe are shown by arrows. Numbers identify positive lapse rate of θe (1), MAUL (2), and steep decrease of θe adjacent to sea (3). Eyewall is shaded, heavy rain is thin clustered parallel lines, and spray is depicted by thin closed lines attached to ocean surface.

Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2033.1

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