Hurricane Research Division
FY-1999 Publications
(October 1, 1998-September 30, 1999)
(names of HRD authors appear in capital letters)

	[1]	ABERSON, S.D., and J.L. FRANKLIN.  Impact on hurricane track and intensity forecasts of GPS dropwindsonde observations from the first-season flights of the NOAA Gulfstream-IV jet aircraft.  Bulletin of the American Meteorological Society, 80(3):421-428 (1999).
In 1997, the Tropical Prediction Center (TPC) began operational Gulfstream-IV jet aircraft missions to improve the numerical guidance for hurricanes threatening the continental United States, Puerto Rico, and the Virgin Islands. During these missions, the new generation of Global Positioning System dropwindsondes were released from the aircraft at 150-200-km intervals along the flight track in the environment of the tropical cyclone to obtain profiles of wind, temperature, and humidity from flight level to the surface. The observations were ingested into the global model at the National Centers for Environmental Prediction, which subsequently serves as initial and boundary conditions to other numerical tropical cyclone models. Because of a lack of tropical cyclone activity in the Atlantic basin, only five such missions were conducted during the inaugural 1997 hurricane season.  Due to logistical constraints, sampling in all quadrants of the storm environment was accomplished in only one of the five cases during 1997. Nonetheless, the dropwindsonde observations improved mean track forecasts from the Geophysical Fluid Dynamics Laboratory hurricane model by as much as 32%, and the intensity forecasts by as much as 20% during the hurricane watch period (within 48 h of projected landfall). Forecasts from another dynamical tropical cyclone model (VICBAR) also showed modest improvements with the dropwindsonde observations. These improvements, if confirmed by a larger sample, represent a large step toward the forecast accuracy goals of TPC. The forecast track improvements are as large as those accumulated over the past 20-25 years, and those for forecast intensity provide further evidence that better synoptic-scale data can lead to more skillful dynamical tropical cyclone intensity forecasts.
	[2]	Atlas, D., C.W. Ulbrich, F.D. MARKS, E. Amitai, and C.R. Williams.  Systematic variation of drop size and radar-rainfall relations.  Journal of Geophysical Research, 104(D6):6155-6169 (1999).
Time histories of the characteristics of the drop size distribution of surface disdrometer measurements collected at Kapingamarangi Atoll were partitioned for several storms using rain rate, R, reflectivity factor Z, and median diameter of the distribution of water content D0.  This partitioning produced physically based systematic variations of the drop size distribution (DSD) and Z-R relations in accord with the precipitation types viewed simultaneously by a collocated radar wind profiler.  These variations encompass the complete range of scatter around the mean Z-R relations previously reported by Tokay and Short (1996) for convective and stratiform rain and demonstrate that the scatter is not random.  The systematic time or space variations are also consistent with the structure of mesoscale convective complexes with a sequence of convective, transition, and stratiform rain described by various authors.  There is a distinct inverse relation between the coefficient A and the exponent of the Z-R relations which has been obscured in prior work because of the lack of proper discrimination of the rain types.  Contrary to previous practice, it is evident that there is also a distinct difference in the DSD and the Z-R relations between the initial convective and the trailing transition zones.  The previously reported Z-R relation for convective rain is primarily representative of the transition rain that was included in the convective class. The failure of present algorithms to distinguish between the initial convective and the trailing transition rains causes an erroneous apportionment of the diabatic heating and cooling and defeats the primary intent of discriminating stratiform from convective rains.
	[3]	BLACK, R.A., and J. Hallett.  Electrification of the hurricane.  Journal of the Atmospheric Sciences, 56(12):2004-2028 (1999).
A survey of reports of electrical activity in hurricanes and typhoons from flight notes and personal  experience (18 years, >230 eyewall penetrations for R.A. Black; -20 years for J. Hallett, plus that of others at the Hurricane Research Division), and perusal of flight notes dating from 1980, show that lightning in and within 100 km or so of the eyewall is usually sparse. However, occasionally, significant electrical activity (greater than one flash per minute) occurs in or near the eyewall. National Oceanic and Atmospheric Administration WP-3D aircraft penetrations through a number of storms relate the lightning occurrence to strong vertical velocity (>10 m s-1) and the presence of supercooled liquid cloud droplets extending to temperatures below 20EC. Specific measurements of cloud properties during eyewall penetrations show that the supercooled cloud water content increases with upward velocities > -5.0 m s-1, as does the presence of large (>2 mm) supercooled drops. Measurements at temperatures >-13EC show that the transition of supercooled cloud water to ice along an outward radial in all systems is associated with local electric fields (occasionally >20 kV m-1) and negative charge above positive charge. In systems with stronger vertical velocity, there is a larger region of supercooled cloud extending to lower temperatures where charge separation may occur, as judged by the presence of regions containing graupel, small ice, and cloud droplets. The ratio of ice to supercooled water increases radially outward from the eyewall and depends upon altitude (temperature). The spatial distribution of charge is further influenced by the relation of vertical velocity to the radial flow, with the upper charge regions tending to be advected outward. In symmetrical, mature hurricanes, supercooled water usually occurs only in regions at temperatures above about -5EC.  The upward transport of supercooled cloud water is limited by a balance between water condensed in the eyewall updraft and its erosion by ice in downdrafts descending in the outward regions of the eyewall. This ice originates from both primary and secondary ice nucleation in the updraft. This is consistent with an exponential increase in ice concentration, as the rate at which the ice particle concentrations increase depends on the production of secondary particles by preexisting graupel, some of which ultimately grow into new graupel, and its outward transport in the anvil flow aloft. Penetrations at temperatures as low as -15EC show the presence of electric fields consistent with specific laboratory-derived criteria for charge separated during ice-graupel collisions, given that a liquid water-dependent sign reversal temperature may occur. Such a reversal may result from either a changing temperature in the vertical, a changing cloud liquid water content in the horizontal, or a combination of the two.  Since cloud-to-ground (CG) lightning can be observed with remote detection networks that provide the polarity and frequency of CG lightning, there is potential that hurricane evolution may be detected remotely and that lightning may be usable as an indicator of a change in the storm intensity and/or track.
	[4]	Bove, M.C., J.J. O’Brien, J.B. Elsner, C.W. LANDSEA, and X. Niu.  Effect of El Niño on U.S. landfalling hurricanes, revisited.  Bulletin of the American Meteorological Society, 79(11):2477-2482 (1998).
Changes in the frequency of U.S. landfalling hurricanes with respect to the El Niño–Southern Oscillation (ENSO) cycle are assessed. Ninety-eight years (1900-1997) of U.S. landfalling hurricanes are classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during an El Niño (anomalously warm tropical Pacific waters), La Niña (anomalously cold tropical Pacific waters), or neither (neutral).  The mean and variance of U.S. landfalling hurricanes are determined for each ENSO phase. Each grouping is then tested for Poisson distribution using a chi-squared test. Resampling using a “bootstrap” technique is then used to determine the 5% and 95% confidence limits of the results. Last, the frequency of major U.S. landfalling hurricanes (sustained winds of 96 kt or more) with respect to ENSO phase is assessed empirically.  The results indicated that El Niño events show a reduction in the probability of a U.S. landfalling hurricane, while La Niña shows an increase in the chance of a U.S. hurricane strike. Quantitatively, the probability of two or more landfalling U.S. hurricanes during an El Niño is 28%, of two or more landfalls during neutral conditions is 48%, and of two or more landfalls during La Niña is 66%. The frequencies of landfalling major hurricanes show similar results. The probability of one or more major hurricane landfall during El Niño is 23% but is 58% during neutral conditions and 63% during La Niña.
	[5]	DeMaria, M., and J. KAPLAN.  An updated Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic and eastern North Pacific basins. Weather and Forecasting, 14(3):326-337 (1999).
Updates to the Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic basin are described. SHIPS combines climatological, persistence, and synoptic predictors to forecast intensity changes using a multiple regression technique. The original version of the model was developed for the Atlantic basin and was run in near–real time at the Hurricane Research Division beginning in 1993. In 1996, the model was incorporated into the National Hurricane Center operational forecast cycle, and a version was developed for the eastern North Pacific basin. Analysis of the forecast errors for the period 1993–1996 shows that SHIPS had little skill relative to forecasts based upon climatology and persistence. However, SHIPS had significant skill in both the Atlantic and eastern Pacific basins during the 1997 hurricane season. The regression coefficients for SHIPS were rederived after each hurricane season since 1993 so that the previous season’s forecast cases were included in the sample. Modifications to the model itself were also made after each season. Prior to the 1997 season, the synoptic predictors were determined only from an analysis at the beginning of the forecast period. Thus, SHIPS could be considered a “statistical–synoptic” model. For the 1997 season, methods were developed to remove the tropical cyclone circulation from the global model analyses and to include synoptic predictors from forecast fields, so the current version of SHIPS is a “statistical-dynamical” model. It was only after the modifications for 1997 that the model showed significant intensity forecast skill.
	[6]	DODGE, P.P., R.W. Burpee, and F.D. MARKS.  The kinematic structure of a hurricane with sea-level pressure less than 900 mb.  Monthly Weather Review, 127(6):987-1004 (1999).
A National Oceanic and Atmospheric Administration aircraft recorded the first Doppler radar data in a tropical cyclone with a minimum sea level pressure (MSLP) <900 mb during a reconnaissance mission in Hurricane Gilbert on 14 September 1988, when its MSLP was -895 mb. A previous mission had found an MSLP of 888 mb, making Gilbert the most intense tropical cyclone yet observed in the Atlantic basin. Radar reflectivity identified the hurricane eye, inner and outer eyewalls, a stratiform region between the eyewalls, and an area outside the outer eyewall that contained a few rainbands but that had mostly stratiform rain. Pseudo-dual Doppler analyses depict the three-dimensional kinematic structure of the inner eyewall and a portion of the outer eyewall. The vertical profiles of tangential wind and reflectivity maxima in the inner eyewall are more erect than in weaker storms, and winds >50 m s-1 extended to 12 km, higher than has been reported in previous hurricanes. The inner eyewall contained weak inflow throughout most of its depth. In contrast, the portion of the outer eyewall described here had shallow inflow and a broad region of outflow.  The stratiform region between the two eyewalls had lower reflectivities and was the only region where the vertically incident Doppler radar data seemed to show downward motion below the freezing level. Gilbert’s structure is compared with other intense Atlantic and eastern North Pacific hurricanes with MSLP >900 mb. Storms with lower MSLP have higher wind speeds in both inner and outer eyewalls, and wind speeds >50 m s-1 extend higher in storms with lower MSLP. Hurricanes Gilbert and Gloria (1985), the strongest Atlantic hurricanes yet analyzed by the Hurricane Research Division, had different outer eyewall structures. Gloria’s outer eyewall had a deep region of inflow, while Gilbert’s inflow layer was shallow. This may explain differences in the subsequent evolution of the two storms.
	[7]	Donnelly, W.J., J.R. Carswell, R.E. McIntosh, P.S. Chang, J.C. Wilkerson, F.D. MARKS, and P.G. BLACK.  Revised ocean backscatter models at C and Ku-band under high wind conditions.  Journal of Geophysical Research, 104(C5):11,485-11,498 (1999).
A series of airborne scatterometer experiments designed to collect C and Ku-band ocean backscatter data in regions of high ocean surface winds has recently been completed. Over 100 hours of data were collected using the University of Massachusetts C and Ku-band  scatterometers, CSCAT and KUSCAT. These instruments measure the full azimuthal normalized radar cross section (NRCS) of a common surface area of the ocean simultaneously at four incidence angles. Our results demonstrate limitations of the current empirical models, CMOD4, SASSII and NSCAT1, that relate ocean backscatter to the near surface wind at high wind speeds. The discussion focuses on winds in excess of 15 m/sec in clear atmospheric conditions. The scatterometer data is collocated with measurements from ocean data buoys and GPS dropsondes, and a Fourier analysis is performed as a function of wind regime. A three-term Fourier series is fit to the backscatter data, and a revised set of coefficients is tabulated. These revised models, CMOD4HW and KUSCAT1, are the basis for a discussion of the NRCS at high wind speeds. Our scatterometer data show a clear over prediction of the derived NRCS response to high winds based on the CMOD4, SASSII and NSCAT1 models. Furthermore, saturation of the NRCS response begins to occur above 15 m/sec. Sensitivity of the upwind and crosswind response is discussed with implications towards high wind speed retrieval.
	[8]	Ellsberry, R.L., and F.D. MARKS.  The Hurricane Landfall Workshop summary.  Bulletin of the American Meteorological Society, 80(4):683-685 (1999).
No abstract.
	[9]	Hasler, A.F., K. Palaniappan, C. Kambhammetu, P.G. BLACK, E.W. UHLHORN, and D. Chesters.  High-resolution wind fields within the inner core and eye of a mature tropical cyclone from GOES 1-min images.  Bulletin of the American Meteorological Society, 79(11):2483-2496 (1998).
Mesoscale wind fields have been determined for a mature hurricane with high spatial and temporal resolution, continuity, and coherency. These wind fields, near the tropopause in the inner core and at low levels inside the eye, allow the evolution of mesoscale storm features to be observed. Previously, satellite-derived winds near hurricanes have been determined only at some distance from the eye over a typical time period of 1-2 h. Hurricane reconnaissance aircraft take 30 min to 1 h to complete an inner-core pattern. With the long observation periods of these previous methods, steady-state conditions must be assumed to give a complete description of the observed region.  With the advent of 1-min interval imagery, and fourfold improvement of image dynamic range from NOAA's current generation of GOES satellites, there is a new capability to measure inner-core tropical cyclone wind fields near the tropopause and within the eye, enabling mesoscale dynamical processes to be inferred. These measurements give insights into the general magnitude and structure of the hurricane vortex, along with very detailed measurements of the cloud-top wind's variations in response to convective outbursts.  This paper describes the new techniques used to take advantage of the GOES satellite improvements that, in turn, allowed the above innovations to occur.  The source of data for this study is a nearly continuous 12-h sequence of 1-min visible images from NOAA GOES-9 on 6 September 1995. These images are centered on Hurricane Luis with maximum winds of 120 kt (CAT4) when it was 250 km northeast of Puerto Rico. A uniform distribution of long-lived cirrus debris with detailed structure is observed in the central dense overcast (CDO), which has been tracked using the 1-min images. The derived wind field near the tropopause at approximately 15 km in the CDO region has a strong closed circulation with speeds up to 25 m s-1, which pulses in response to the convective outbursts in the eyewall. Cloud displacements are computed at every pixel in every image, resulting in a quarter-million u–v winds in each of 488 hurricane images observed at 1- to 4-min intervals over 12 h. For analysis and presentation, these ultradense wind fields are reduced to 8- or 16-km grids using a 7-min time base by smoothing displacement vectors in space and time.  Cloud structures were tracked automatically on a massively parallel processing computer, but with manual spot-checking. Manual tracking has been used to follow CDO structure over long time periods, up to 90 min for a small test sample. Cloud tracking for the wind fields presented here is accomplished using a Massively Parallel Semi-Fluid Motion Analysis (MPSMA) automatic technique. This robust deformable surface-matching algorithm has been implemented on the massively parallel Maspar supercomputer. MPSMA automatic tracking typically follows a feature for 7 min. For this time base the error of these winds is estimated to be 1.5 m s-1. However, systematic navigation and height assignment errors in the moderately sheared hurricane environment must still be considered. Spatial and temporal smoothing of the wind field have been performed to reduce systematic navigation errors and small-scale turbulent noise. The synthesis used here to compute the wind fields gives an order of magnitude reduction in the amount of data presented compared to the amount of data processed. Longer tracking could give higher accuracy but would smooth out the smaller-scale spatial and temporal features that appear dynamically significant.  The authors believe that the techniques described in this paper have great potential for further research on tropical cyclones and severe weather, as well as in operational use for nowcasting and forecasting. United States and foreign policymakers are urged to augment the GOES, GMS, FY2, and Meteosat geostationary satellite systems with dual imaging systems such that 1-min observations are routinely taken.
	[10]	Hock, T.F., and J.L. FRANKLIN.  The NCAR GPS dropwindsonde.  Bulletin of the American Meteorological Society, 80(3):407-420 (1999).
The National Center for Atmospheric Research (NCAR), in a joint effort with the National Oceanic and Atmospheric Administration (NOAA) and the German Aerospace Research Establishment, has developed a dropwindsonde based on the Global Positioning System (GPS) satellite navigation. The NCAR GPS dropwindsonde represents a major advance in both accuracy and resolution for atmospheric measurements over data-sparse oceanic areas of the globe, providing wind accuracies of 0.52.0 m s-1 with a vertical  resolution of ~5 m. One important advance over previous generations of sondes is the ability to measure surface (10 m) winds. The new dropwindsonde has already been used extensively in one major international research field experiment (Fronts and Atlantic Storm Track Experiment), in operational and research hurricane flights from NOAA's National Weather Service and Hurricane Research Division, during NCAR's SNOWBAND experiment, and in recent CALJET and NORPEX El NiZo experiments. The sonde has been deployed from a number of different aircraft, including NOAA's WP-3Ds and new Gulfstream IV jet, the Air Force C-130s, NCAR's Electra, and a leased Lear-36. This paper describes the characteristics of the new dropwindsonde and its associated aircraft data system, details the accuracy of its measurements, and presents examples from its initial applications.
	[11]	JONES, R.W., and M. DeMaria. Further studies of the optimization of a hurricane track prediction model using the adjoint equations. Monthly Weather Review, 127(7):1586-1598 (1999).
The method of model fitting, or adjoint method, is applied to a barotropic hurricane track forecast model described by DeMaria and Jones using a large sample of forecast cases. The sample includes all Atlantic tropical cyclones that reached hurricane intensity during the 1989-1993 hurricane seasons (141 72-h forecasts of 17 storms). The cases considered by DeMaria and Jones are a subset of the present sample. Model-fitting calculations using strong, weak, strong followed by weak, or weak followed by strong model constraints are discussed for data assimilation periods varying from 6 to 72 h. Generally, the best track forecasts occur for shorter assimilation periods and for weak constraints, although only the 12-h assimilation with the weak constraint has less track error than the control forecast without assimilation, and only for the 12-h forecast. The principle reason for this lack of improvement is that the fit of the model to the observed track is good at the middle of the assimilation period, but not very good at the end where the forecast begins. When a future track position at 6 h is included in the assimilation, in order to improve the track fit at the synoptic data time, the resulting track errors average about 10% smaller than the control forecast. The control forecast may also be improved in the same way. In that case, the best assimilation forecasts have 2.5% smaller track errors than the modified control forecasts.
	[12]	LANDSEA, C.W., R.A. Pielke, A.M. Mestas-Nuñez, and J.A. Knaff. Atlantic basin hurricanes: Indices of climatic changes.  Climatic Change, 42:89-129 (1999).
Accurate records of basin-wide Atlantic and U.S. landfalling hurricanes extend back to the mid 1940s and the turn of the century, respectively, as a result of aircraft reconnaissance and instrumented weather stations along the U.S. coasts. Such long-term records are not exceeded elsewhere in the tropics. The Atlantic hurricanes, U.S. landfalling hurricanes, and U.S. normalized damage time series are examined for interannual trends and multidecadal variability. It is found that only weak linear trends can be ascribed to the hurricane activity and that multidecadal variability is more characteristic of the region. Various environmental factors including Caribbean sea level pressures and 200 mb zonal winds, the stratospheric Quasi-Biennial Oscillation, the El Niño-Southern Oscillation, African West Sahel rainfall, and Atlantic sea surface temperatures, are analyzed for interannual links to the Atlantic hurricane activity. All show significant, concurrent relationships to the frequency, intensity, and duration of Atlantic hurricanes. Additionally, variations in the El Niño-Southern Oscillation are significantly linked to changes in U.S. tropical cyclone-caused damages. Finally, much of the multidecadal hurricane activity can be linked to the Atlantic Multidecadal Mode - an empirical orthogonal function pattern derived from a global sea surface temperature record.  Such linkages may allow for prediction of Atlantic hurricane activity on a multidecadal basis. These results are placed into the context of climate change and natural hazards policy.
	[13]	Pielke, R.A., and C.W. LANDSEA.  La Niña, El Niño, and Atlantic hurricane damages in the United States.  Bulletin of the American Meteorological Society, 80(10):2027-2034 (1999).
Hurricanes result in considerable damage in the United States. Previous work has shown that Atlantic hurricane landfalls in the United States have a strong relationship with the El Niño-Southern Oscillation phenomena. This paper compares the historical record of La Niña and El Niño events defined by eastern Pacific sea surface temperature with a data set of hurricane losses normalized to 1997 values. A significant relationship is found between the ENSO cycle and U.S. hurricane losses, with La Niña years exhibiting much more damage.  Used appropriately, this relationship is of potential value to decision makers who are able to manage risk based on probabilistic information.
	[14]	POWELL, M.D., and S.H. HOUSTON. Comments on “A multiscale numerical study of Hurricane Andrew (1992).  Part I: Explicit simulation and verification.  Monthly Weather Review, 127(7):1706-1710 (1999).
No abstract.
	[15]	POWELL, M.D., S.H. HOUSTON, L.R. AMAT, and N. MORISSEAU-LEROY.  The HRD real-time hurricane wind analysis system.  Journal of Wind Engineering and Industrial Aerodynamics, 77&78:53-64 (1998).
The HRD real-time wind analysis system is currently undergoing evaluation in the operational forecasting environment of the National Hurricane Center.  The system is an object-oriented, distributed, three-tiered client-server application that assimilates disparate observations and processes the data into a common framework for exposure, height, and averaging time. The data are then examined collectively or by type, quality controlled, and passed on to a scale-controlled objective analysis algorithm. Several products are derived from the analysis wind field and storm track, yielding effective tools for disaster assessment, emergency management, and recovery.
	[16]	Shapiro, L.J., and J.L. FRANKLIN.  Potential vorticity asymmetries and tropical cyclone motion.  Monthly Weather Review, 127(1):124-131 (1999).
A set of nine synoptic-flow cases, incorporating Omega dropwindsonde observations for six tropical storms and hurricanes, is used to deduce the three-dimensional distribution of potential vorticity (PV) that contributed to the deep-layer mean (DLM) wind that steered the cyclones. A piecewise inversion technique, the same as that previously applied by Shapiro to Hurricane Gloria of 1985, is used to derive the DLM wind induced by pieces of anomalous PV restricted to cylinders of different radii centered on each cyclone. The cylinder of PV that induces a DLM wind that best matches the observed DLM wind near the center of each cyclone is evaluated.  It is found that the results can be loosely placed into two categories describing the spatial scale of the PV anomalies that influenced the cyclone’s motion. Four of the cases, including Hurricane Gloria, had “local” control, with a good match (to within 40%) between the observed DLM wind near the cyclone center and the DLM wind attributable to a cylinder of PV with a given radius of 1500 km. Further decomposition of the PV anomaly into upper (400 mb and above) and lower levels (500 mb and below) indicates the dominance of upper-level features in steering two of the cyclones (Hurricanes Gloria of 1985 and Andrew of 1992), while Hurricane Debby of 1982 was steered by more barotropic features. These results supplement those found in other studies.  Five of the cases, by contrast, had “large-scale” control, with no cylinder of radius 2000 km having a good match between the induced and observed DLM wind. Hurricanes Emily of 1987 and 1993 fell into this category, as did Hurricane Josephine of 1984. Implications of the results for guiding in situ wind measurements to improve hurricane track forecasts are discussed.
	[17]	WILLOUGHBY, H.E.  Tropical cyclone eye thermodynamics.  Monthly Weather Review, 126(12):3053-3067 (1998).
In intense tropical cyclones, sea level pressures at the center are 50-100 hPa lower than outside the vortex, but only 10-30 hPa of the total pressure fall occurs inside the eye between the eyewall and the center. Warming by dry subsidence accounts for this fraction of the total hydrostatic pressure fall. Convection in the eyewall causes the warming by doing work on the eye to force the thermally indirect subsidence.  Soundings inside hurricane eyes show warm and dry air aloft, separated by an inversion from cloudy air below. Dewpoint depressions at the inversion level, typically 850-500 hPa, are 10-30 K rather than the 100 K that would occur if the air descended from the tropopause level without dilution by the surrounding cloud. The observed temperature and dewpoint distribution above the inversion can, however, be derived by 100 hPa of undilute dry subsidence from an initial sounding that is somewhat more stable than a moist adiabat. It is hypothesized that the air above the inversion has remained in the eye since it was enclosed when the eyewall formed and that it has subsided at most a few kilometers. The cause of the subsidence is the enclosed air’s being drawn downward toward the inversion level as the air below it flows outward into the eyewall. Shrinkage of the eye’s volume is more than adequate to supply the volume lost as dry air is incorporated into the eyewall or converted to moist air by turbulent mixing across the eye boundary.  The moist air below the inversion is in thermodynamic contact with the sea surface. Its moisture derives from evaporation of seawater inside the eye, frictional inflow of moist air under the eyewall, and from moist downdrafts induced as condensate mixes into the eye. The moist air’s residence time in the eye is much shorter than that of the dry air above the inversion. The height of the inversion is determined by the balance between evaporation, inflow, and inward mixing on one hand and loss to the eyewall updrafts on the other.