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    Official NHC intensity forecast errors (kt) between 1997 and 2006 for the Atlantic basin. The solid and dashed lines show the errors for a 12- and 36-h forecast, respectively. The dotted line represents errors for a 72-h forecast. (Data source: NHC)

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    NHC’s best track for Hurricane Bret between 1800 UTC 18 Aug and 0000 UTC 25 Aug 1999. The solid line with filled dots represents the MSLP (hPa), and the solid line with squares shows the maximum sustained wind speed (kt). Each dot and square indicates individual values for pressure and wind speed, respectively. The two solid vertical lines indicate the period of Bret’s rapid intensification. The best-track positions of Bret’s center are given in Fig. 7.

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    Forecasts of maximum sustained wind speed (kt) from (top) NHC and (bottom) SHIPS. The symbols indicate different forecasts between 0000 UTC 19 Aug and 0000 UTC 23 Aug 1999. The solid line with filled dots represents the best track. The first value for each SHIPS forecast corresponds to the best track. Each NHC forecast starts 9 h after it was issued.

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    AVN analysis of horizontal wind speed (m s−1) at 200 hPa at (a) 1200 UTC 21 Aug and (b) 1200 UTC 22 Aug 1999 between 15° and 40°N and 70° and 120°W. The hurricane symbol shows the position of Bret according to the best track. Circulation centers of upper-level high and low pressure systems are indicated with H and L, respectively. The black dashed line shows the trough axis associated with the upper-level low.

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    Composite of LF radar reflectivity (dBZ) observed in Hurricane Bret at (a) 2250 UTC 21 Aug, (b) 2118 UTC 22 Aug, and (c) 2340 UTC 22 Aug 1999. The black cross in the eye indicates the position of the NOAA aircraft at the time of the scan. The domain of the scan is 240 km × 240 km.

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    Average vertical velocity (m s−1) as a function of height (m) measured by GPS dropwindsondes deployed on NOAA flight missions on (a) 21 Aug and (b) 22 Aug 1999. The thick black solid line shows the averaged profile for all winds. The medium black solid line represents the averaged updrafts, while the thin black solid line indicates maximum updrafts. The medium and thin gray solid lines show averaged and maximum downdrafts, respectively.

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    Selected SST observations (°C) for AXBT locations on 21 Aug (squares) and 22 Aug 1999 (inverted triangles). The dashed line shows the best track. Best-track values for MSLP in hPa and maximum sustained winds (kt) are given every 6 h beginning at 1200 UTC 21 Aug 1999.

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    SHIPS forecasts for maximum sustained wind speed (kt) for the time between (top) 0600 UTC 21 Aug and 0000 UTC 23 Aug 1999 and (bottom) 0000 UTC 22 Aug and 0000 UTC 23 Aug 1999 for four different cases: CTRL (case 1), AVN28 (case 3), TEMP (case 6), and AVN28TEMP (case 7). Forecasts are given every 6 h. All forecasts are compared to the best track (solid line).

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Structural and Intensity Changes of Hurricane Bret (1999). Part I: Environmental Influences

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  • 1 Hurricane Research Division, NOAA/AOML, Miami, Florida
  • | 2 Department of Geography and Earth Sciences, University of North Carolina, Charlotte, North Carolina
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Abstract

Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These datasets are combined with gridded data from the National Centers for Environmental Prediction (NCEP) Global Model and the NCEP–National Center for Atmospheric Research (NCAR) reanalyses to document Bret’s atmospheric and oceanic environment as well as their relation to the observed structural and intensity changes. Bret’s RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a concurrent decrease in vertical wind shear. SSTs at the beginning of the storm’s RI were approximately 29°C and steadily increased to 30°C as it moved to the north. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20% higher at the beginning of the RI period than 6 h prior. The subsequent weakening was linked to the cooling of near-coastal shelf waters (to between 25° and 26°C) by prestorm mixing combined with an increase in vertical wind shear. The available observations suggest no intrusion of dry air into the circulation core contributed to the intensity evolution. Sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted to quantitatively describe the influence of environmental conditions on the intensity forecast. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between the four cases were relatively small because of the model design, but the greatest intensity changes resulted for much cooler prescribed SSTs. The results of this study underscore the importance of OHC and vertical wind shear as significant factors during RIs; however, internal dynamical processes appear to play a more critical role when a favorable environment is present.

* Current affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Corresponding author address: Alexander Lowag, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149. Email: alowag@rsmas.miami.edu

Abstract

Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These datasets are combined with gridded data from the National Centers for Environmental Prediction (NCEP) Global Model and the NCEP–National Center for Atmospheric Research (NCAR) reanalyses to document Bret’s atmospheric and oceanic environment as well as their relation to the observed structural and intensity changes. Bret’s RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a concurrent decrease in vertical wind shear. SSTs at the beginning of the storm’s RI were approximately 29°C and steadily increased to 30°C as it moved to the north. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20% higher at the beginning of the RI period than 6 h prior. The subsequent weakening was linked to the cooling of near-coastal shelf waters (to between 25° and 26°C) by prestorm mixing combined with an increase in vertical wind shear. The available observations suggest no intrusion of dry air into the circulation core contributed to the intensity evolution. Sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted to quantitatively describe the influence of environmental conditions on the intensity forecast. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between the four cases were relatively small because of the model design, but the greatest intensity changes resulted for much cooler prescribed SSTs. The results of this study underscore the importance of OHC and vertical wind shear as significant factors during RIs; however, internal dynamical processes appear to play a more critical role when a favorable environment is present.

* Current affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Corresponding author address: Alexander Lowag, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149. Email: alowag@rsmas.miami.edu

1. Introduction

Forecasting fluctuations in intensity of tropical cyclones (TCs) remains one of the major challenges for meteorologists. Even though forecast models have consistently become more sophisticated, intensity predictions have shown little improvement. While the National Hurricane Center (NHC) uses a combination of dynamical and statistical computer models, the average error of its Atlantic basin intensity forecasts between 1997 and 2006 has not significantly improved (Fig. 1).

Inaccurate intensity forecasts are partially a result of an incomplete understanding of the various physical mechanisms that control the cyclone’s intensity (Emanuel 1999). Such controls can be broadly grouped into three categories: environmental, oceanic, and internal. Environmental mechanisms include vertical wind shear (DeMaria 1996; Frank and Ritchie 2001; Zehr 2003), the interaction of the storm with synoptic upper-level features (Molinari and Vollaro 1989; DeMaria et al. 1993; Hanley et al. 2001), and the moisture content of midlevel air (Fink and Vincent 2003; Dunion and Velden 2004). Oceanic contributions to intensity variations arise through air–sea interactions (Chan et al. 2001) and thus knowledge of the thermal structure of the upper ocean is essential for their accurate prediction (Mainelli et al. 2008). Heat fluxes from the ocean to the atmosphere influence the internal dynamics of a TC. Hence, it is suggested that more precise locations of oceanic warm and cold pools in numerical models should be beneficial to TC forecasts (Walker et al. 2005), since heat fluxes strongly depend on the sea surface temperature (SST; Cione and Uhlhorn 2003). A study by Lin et al. (2005) showed that incorporation of additional eddy information from satellite-derived sea surface height anomalies (SHAs) resulted in an improved intensity forecast for Supertyphoon Maemi (2003) in the Coupled Hurricane Intensity Prediction Scheme (CHIPS) model. Finally, internal processes that contribute to intensity changes include eyewall replacement cycles (Willoughby et al. 1982; Willoughby 1990), vortex Rossby waves (Montgomery and Kallenbach 1997), mesovortices in the eye–eyewall region (Schubert et al. 1999; Kossin and Schubert 2001; Kossin and Eastin 2001; Eastin et al. 2005) and convective bursts (Heymsfield et al. 2001; Kelley et al. 2004). Intensity changes experienced by any given TC are likely a combination of these mechanisms working in concert.

Some of the larger errors in predicting the intensity of a TC are associated with periods of rapid intensification (RI) and weakening. The definition of RI is nonuniform. Holliday and Thompson (1979) related it to a decrease in minimum sea level pressure (MSLP) greater than 42 hPa over a 24-h period, whereas Brand (1973) associated it with an increase in wind speed of 50 kt day−1. A RI in the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model by DeMaria and Kaplan (1994a) is linked to an increase in wind speed of at least 30 kt within 24 h (Kaplan and DeMaria 2003), whereas rapid weakening is related to a decrease in wind speed of more than 20 kt day−1 (Brand 1973).

Even though only a small percentage of TCs undergo RI (DeMaria and Kaplan 1994a), the consequences of such events can be significant if RI occurs in close proximity to coastal regions, many of which have experienced near-exponential population growth in recent decades (Pielke and Pielke 1997). For example, Hurricane Andrew (1992) gained strength over the Bahamas as its 1-min maximum sustained wind speed increased from 110 to 150 kt within 18 h and made landfall on the South Florida coast as a category 5 hurricane. The storm killed 15 people directly and caused $25 billion in property damage (Mayfield et al. 1994; Rappaport 1994). More recently during the active 2004 and 2005 Atlantic Hurricane seasons, a total of six hurricanes (Charley, Dennis, Emily, Katrina, Rita, and Wilma) experienced RI events within 48 h of making landfall.

Although several environmental factors present prior to or during RI events have been identified (Holliday and Thompson 1979; Kaplan and DeMaria 2003), the physical mechanisms and interactions that cause sudden intensity changes in TCs are not well understood. One notable RI event that has received considerable study is Hurricane Opal. In October 1995, Opal intensified rapidly over the Gulf of Mexico where its MSLP decreased by 47 hPa within 16 h. During the first stage of intensification between 1800 UTC 3 October and 0600 UTC 4 October 1995, the storm was influenced by a synoptic trough that enhanced Opal’s outflow and increased the divergence in the upper troposphere. Bosart et al. (2000) showed that a consequence of this enhancement process was convective growth in the eyewall region and the onset of intensification. Low vertical wind shear also contributed to Opal’s initial strengthening (Shay et al. 2000). The second stage of intensification occurred when Opal moved over a warm core eddy shed by the Loop Current. During this passage the surface winds increased from 35 to over 60 m s−1. The effect of the warm core prevailed despite the cooling of the sea surface due to upwelling and vertical mixing in the upper ocean (Shay et al. 2000). The weakening of Opal was related to an eyewall replacement cycle that was completed shortly after the hurricane reached peak intensity (Lawrence et al. 1998). Bosart et al. (2000) also found that increased vertical wind shear and a shallower mixed layer depth also contributed to Opal’s weakening.

The number of detailed observational analyses of the environment and inner core of rapidly intensifying hurricanes is quite limited. This paper, which is Part I of a two-paper series, aims to reduce such deficiency by documenting the available observations from the atmospheric and oceanic environment of Hurricane Bret (1999) during a period of RI and weakening just prior to landfall. Inner-core observations directly relevant to the observed environmental changes are also discussed. The paper is organized as follows. Section 2 describes the history of Hurricane Bret, followed by a detailed overview of the available data in section 3. Section 4 discusses the evolution of external forcing and the storm’s response. Sensitivity studies with the SHIPS model are provided in section 5, and a summary of our findings is in section 6. In Part II, a detailed analysis of the inner-core observations during the RI event and the subsequent weakening prior to landfall will be presented.

2. Storm history

Hurricane Bret originated from a tropical wave that moved westward from Africa over the tropical Atlantic Ocean (Lawrence et al. 2001). The wave did not develop until the evening of 18 August 1999 when the system became a tropical depression situated over the Bay of Campeche. Twenty-four hours later, Bret was upgraded to a tropical storm after an upper-level trough causing high wind shear over the western Gulf of Mexico moved away from the cyclone. The storm then headed to the north and intensified progressively. Figure 2 shows the central pressure and wind speed of Bret according to NHC’s best track. The period of RI occurred between 1200 UTC 21 August and 1200 UTC 22 August. In the 6-h period beginning at 1800 UTC 21 August, the MSLP fell 21 hPa from 975 to 954 hPa, and over a 24-h period it dropped 35 hPa from 979 to 944 hPa. The deepening made Bret a category 4 hurricane with a sustained wind speed of 125 kt. A weak ridge over the northwestern Gulf of Mexico and a cyclonic circulation over the Rio Grande River valley in the midtroposphere caused the hurricane to track northwestward on 22 August. Despite forecasts that called for constant intensity, Bret weakened to a category 3 storm with a maximum sustained wind speed of 100 kt and a MSLP of 951 hPa at landfall along the south Texas coast around 0000 UTC 23 August. After moving over Texas and Mexico, the system dissipated on 25 August.

As for many other storms that rapidly deepened, the intensity forecasts for Bret did not capture the RI event well (Fig. 3). Both NHC and SHIPS forecasts are compared to the NHC best track that consists of 6-hourly estimates of the storm center location, 1-min sustained surface wind speeds, and MSLP. Note that Bret’s forecast intensity was underestimated in both cases. However, the difference between the NHC forecasts and the best track is larger than for SHIPS, especially during the period of RI. On 0300 UTC 19 August forecasters at NHC predicted a maximum sustained wind speed for 0000 UTC 22 August of 50 kt, while the SHIPS forecast from 0000 UTC 19 August was 78 kt for the same time. The actual intensity was 120 kt. The higher intensities in SHIPS are due to the fact that land effects were not included in the 1999 version of the model, while NHC accounted for Bret’s proximity to the western Gulf coast (L. A. Avila 2008, personal communication). Interestingly, Bret’s weakening is also not captured in the forecasts. A SHIPS forecast from 0600 UTC 20 August predicted maximum winds of 131 kt for 0000 UTC 23 August, while NHC at 0300 UTC 22 August predicted a constant wind speed of 120 kt for the 12-h period starting at 1200 UTC on the same day. The best track, however, indicates maximum winds of only 100 kt on 0000 UTC 23 August.

3. Data

Both the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and Aviation [(AVN; now Global Forecast System (GFS)] global model analyses were used to describe the large-scale atmospheric conditions surrounding Hurricane Bret. The AVN analyses have a spatial resolution of 1° in the horizontal and are obtained every 6 h for 28 vertical levels. NCEP–NCAR reanalyses are also based on the AVN model. The data are available for the same pressure levels as the AVN data, but have a coarser horizontal and temporal resolution of 2.5° and 12 h, respectively.

The SSTs and temperature characteristics of the oceanic upper layer are measured by airborne expendable bathythermographs (AXBTs). These instruments profile the water temperature to a depth of approximately 350 m. After the instrument has splashed onto the sea surface, it releases a temperature probe that descends at a constant rate of 1.5 m s−1. The water temperature is inferred from frequency fluctuations of the signal sent to the airplane (Boyd 1987). Consideration of SST alone is not sufficient to understand the ocean’s role on the intensification process of a tropical cyclone (Palmen 1948). Vertical mixing processes induced by minimal tropical storm force winds result in a cooling of the water at the sea surface (Kraus and Turner 1967). Consequently, knowledge of the conditions in the upper ocean is crucial to determine the oceanic influence on TC intensity variations. Besides SST, the oceanic heat content (OHC) is used to describe the oceanic contribution to Bret’s intensity changes. It is defined as the energy stored in the ocean that can be extracted by the storm through evaporation and conduction and is proportional to the temperature difference between the sea surface and 26°C isotherm, as well as the depth of the latter (Leipper and Volgenau 1972). The methodology used for estimation of the OHC is based on Shay et al. (2000). The ocean’s vertical structure is considered as a two-layer fluid in which the depth of the 20°C isotherm separates the two regimes. Estimation of the average upper-layer thickness and reduced gravity from climatological temperature and salinity fields in association with SHA data from altimeters, including the Ocean Topography Experiment (TOPEX)/Poseidon (T/P), allows for the determination of the upper-layer thickness. The study by Shay et al. (2000) also found that the depth of the 26°C isotherm is approximately half of the upper-layer thickness. Weekly averaged SSTs needed for the OHC estimation are obtained from the Advanced Very High Resolution Radiometer (AVHRR). A detailed description of the methodology is presented in Huber (2000).

Even though the T/P data have an accuracy of 2 cm on the mesoscale (Cheney et al. 1994), it should be noted that OHC values derived from this platform may have distinct discrepancies to the actual values. In the presence of a warm core eddy, the estimated SHA values from the T/P altimeter depend on the location of the eddy relative to the ground track of the platform (Shay et al. 2000). The larger the distance between the track and the center of the eddy, the higher the discrepancy between measured and real SHA values. Due to a 10-day sampling period of the T/P altimeter, actual SHA values can be underestimated by approximately 10%. An increased availability of in situ measurements is needed to determine the error associated with the estimation of the OHC and the depths of the isotherms (Huber 2000).

Observations from global positioning system (GPS) dropwindsondes released in the eyewall and National Oceanic and Atmospheric Administration’s (NOAA) WP-3D lower fuselage (LF) radar provided information about the inner-core structure of the storm. Radar reflectivity from the LF radar (see Jorgensen 1984a for radar details) were mapped into a storm-relative horizontal plane following Marks (1985). GPS dropwindsondes measure temperature, humidity, pressure, and winds between the flight level and the sea surface at half-second intervals (Hock and Franklin 1999). Each sonde moves approximately with the horizontal wind as it falls and is also influenced by updrafts and downdrafts during its descent. The horizontal wind speed and direction are determined from the GPS positions. Vertical motions are estimated from the difference between the actual fall rate of the sonde, which is determined hydrostatically from the thermodynamic data, and its theoretical fall velocity. The vertical resolution for all measured variables is ∼5 m, and the accuracy of the winds ranges between 0.5 and 2 m s−1. A total of 42 GPS dropwindsondes deployed by the NOAA aircraft in the eyewall region of the hurricane were used.

Our environmental analysis throughout the RI and subsequent weakening periods reveals significant changes in the vertical wind shear and underlying SST/OHC. To test the sensitivity of intensity forecasts during this period, we employ SHIPS and alter the initial environmental conditions. The SHIPS model is based on statistical relationships between climatological, persistence and synoptic predictors (DeMaria and Kaplan 1994a; DeMaria et al. 2005). The SHIPS version used in this study is slightly different from the operational one. The runs were dependent, which is a “best case” scenario. For the dependent runs the predictors are calculated along the observed storm track, and analyses are used to calculate the atmospheric predictors. In real time, the predictors are determined along the forecast track, and the atmospheric fields are obtained from GFS forecasts instead of the analyses. It is assumed that errors for dependent runs are approximately 20% lower than for real-time runs except for cases where the forecast track went over land, whereas the best track did not (M. DeMaria 2008, personal communication). Also, since it is the purpose to examine the SHIPS sensitivity to shear and SST, further complications by introducing track errors in operational runs were avoided.

4. Discussion

a. Atmosphere

To examine the external influences on Bret’s intensity variations, its large-scale atmospheric environment was analyzed. Frank and Ritchie (2001) have shown the influence of vertical wind shear on cyclone intensity. The definition of vertical shear and methods for its calculation vary. Shear calculations in Palmer and Barnes (2002) and Zehr (2003) were based on the difference between two pressure levels, usually 850 and 200 hPa or similar heights. Gallina (2002) used layer-averaged winds between 925 and 700 hPa and between 300 and 150 hPa. There are also variations in the horizontal domain relevant to shear calculation. Zehr (2003) considered the area between 200 and 800 km away from the storm center, while Gallina (2002) removed the vortex by discarding winds closer than 400 km to the storm center at upper levels and 800 km in lower levels. DeMaria and Huber (1998) and Paterson et al. (2005) averaged over a storm-centered area and consequently did not remove the vortex in order to cancel out the symmetric part of the primary circulation.

In this study, wind shear was obtained by calculating wind vector differences between two pressure levels at each grid point. The model grid point closest to the best-track position was defined as the storm center. Consequently, shear was not calculated over the same area for the AVN and NCEP–NCAR analyses, which may have contributed to the somewhat different shear values. Values for the 200–850 hPa vertical wind shear for the AVN and SHIPS analyses and NCEP–NCAR reanalyses for an annulus between 200 and 800 km away from Bret’s center are provided in Table 1. Even though the shear magnitudes for the three analyses differ from each other, they show a similar trend. Shear remained low within a 12-h period starting at 1200 UTC 21 August 1999, but increased significantly afterward. AVN shear at 0000 UTC 22 August indicated an increase from 7.8 to 14.0 kt within 6 h, while SHIPS values increased from 8.8 to 12.0 kt. Shear based on NCEP–NCAR reanalyses was more than 3 times higher than 12 h before. Within the following 24 h, the shear remained nearly constant or slightly increased (AVN). Because of the coarse spatial and temporal resolution, the NCEP shear analysis seemed less representative of the atmospheric conditions than the AVN analysis and was not included in the further discussion. As wind shear in Bret’s vicinity increased at 0000 UTC 22 August, the storm was still rapidly intensifying for 6 more hours. Similar results were found in a model study by Persing et al. (2002) who showed that wind shear imposed on Hurricane Opal (1995) did not weaken the storm immediately. Also, Frank and Ritchie (2001) found that it may take up to 36 h until the cyclone responds to increased wind shear. Hurricane Jimena (1991) did not weaken for 2 days even though it was exposed to shear ranging between 25 and 39 kt (Black et al. 2002). One possible mechanism for the time lag between the onset of wind shear and the vortex response to it was suggested by Reasor et al. (2004) who argued that shear-induced vortex Rossby waves reduce the tilt of the vortex and lead to a resiliency of the storm to vertical shearing.

AVN analyses revealed that Bret was predominantly influenced by upper-level wind shear. Within the 24-h period starting at 0000 UTC 22 August, the shear magnitude between 200 and 850 hPa ranged from 14.0 to 18.4 kt. For the same time period shear values for the 500–850-hPa levels (not shown) were between 1.7 and 6.2 kt indicating that strong upper-level winds were responsible for the high shear. AVN shear values for different domains (Table 2) show lower shear for smaller radii. Conditions for intensification were especially favorable for an area less than 400 km away from the circulation center at 1200 and 1800 UTC 21 August. This low shear was in best agreement with the RI of Bret and makes physical sense since Bret was a small storm. At its maximum intensity, hurricane-force winds only extended to 40 miles from Bret’s center.

The AVN horizontal wind fields at the 200-hPa level at 1200 UTC 21 and 22 August (Fig. 4) reveal a large anticyclone over Mexico. The high pressure system became stronger on 22 August and increased the wind shear in the vicinity of Bret (Table 1). At 0600 UTC 21 August the analysis (not shown) indicated a divergent flow pattern over and in proximity to Bret’s center, which lasted until landfall. Upper-level winds may have contributed to the intensification of the storm as was already discussed by Bosart et al. (2000) who suggested that increased divergence related to a trough in the upper troposphere enhanced the convection in the inner core of Hurricane Opal (1995). The analysis for 1200 UTC 21 August (Fig. 4a) also indicated a small upper-level low over the U.S.–Mexican border, which retrograded to the northwest within the next 24 h. Operational SHIPS forecasts were used to determine the momentum fluxes in Bret’s environment at the 200-hPa level within a 100–600-km annulus away from the circulation center. It was found that during the storm’s life cycle the fluxes never exceeded the threshold of 10 m s−1 day−1 as defined by DeMaria et al. (1993) for hurricane–trough interaction. It is assumed that the upper-level low and the trough related to it did not have an influence on the hurricane’s intensity. In addition, AVN analyses, water vapor satellite imagery, and dropwindsonde data showed that there was no intrusion of dry air into the storm.

During the RI and weakening of Bret, subtle changes in the storm structure were noticeable (Fig. 5). Radar reflectivity in the inner-core region showed a similar pattern between 2250 UTC 21 August and 2118 UTC 22 August. Note that Bret was rapidly deepening during the NOAA flight on 21 August, whereas it was already weakening during the mission 24 h later. The eyewall was characterized by a wavenumber 1 asymmetry, and spiral rainbands outside of the eyewall were present on both days. The radar reflectivity on 22 August was lower than on 21 August, and its structure changed significantly late on 22 August (Fig. 5c) as the storm appears to have become more asymmetric. While the area of high reflectivity in the eyewall (>40 dBZ) encompassed the western semicircle at 2118 UTC 22 August (Fig. 5b), it was significantly reduced 2.5 h later and only extended to the northwestern quadrant. Also note that the reflectivity in the rainband region was considerably lower indicating reduced convection and weakening of the storm.

The weakening was related to changes in vertical motions, which were apparent in the profiles of dropwindsonde vertical velocity observations in Fig. 6. Profiles were calculated using 50-m intervals in the vertical. Mean motions in a vertical bin were computed from all available dropwindsondes. The maximum and minimum values were the most extreme data points observed by any of the available sondes. The profiles were obtained from 22 sondes for 21 August and 20 sondes for 22 August. Mean up- and downdrafts were stronger on 21 than on 22 August. Updrafts on 21 August remained nearly constant at about 2 m s−1 up to 1000 m and gradually increased aloft to approximately 6 m s−1 at 3800 m. On 22 August they showed roughly the same magnitude of 2 m s−1 throughout the whole atmospheric layer below the flight level. Averaged downdrafts revealed a constant profile on both days, but were stronger on 21 August as are maximum values for up- and downdrafts. Also note that the mean profile of all drafts shows stronger vertical motions above 2500 m on 21 August, and was similar below on both days.

The higher vertical velocities on the first day were a consequence of stronger vertical overturning due to enhanced convection during Bret’s intensification. Downdrafts are typically located radially inside and outside from strong updrafts (Jorgensen 1984b; Black and Hallett 1999). The weaker convection on 22 August is in agreement with the lower radar reflectivity in the inner core (Fig. 5). The mean profiles on 21 August are similar to the findings of Black et al. (1996) who derived vertical motions in hurricanes from Doppler radar data. Values for mean up- and downdrafts were approximately 1 m s−1 weaker in Bret’s case. Also note that the negative values for the average of all drafts in Bret’s case were not observed in the aforementioned work.

The release of latent heat through condensation in the eyewall region leads to TC intensification. Therefore it seems plausible that because of the reduced convection in Bret’s eyewall region on 22 August, the transport of moist air from the boundary layer to higher altitudes (as described in Black et al. 1996) was diminished and could not maintain the strength of the cyclone. The size of Bret’s eye on late 21 August (Fig. 5a) was about 30 km in diameter and showed a slight contraction to approximately 28 km 24 h later (Fig. 5c). Even though eye contraction is indicative of storm strengthening (Willoughby et al. 1982; Willoughby 1990), Bret was weakening because of the less vigorous convection. The composites presented herein were only used to document gross features and evolution of Bret’s secondary circulation during the deepening and weakening period.

Finally, the southwesterly shear caused a wavenumber 1 asymmetry in the radar reflectivity images of the inner core (Fig. 5). It has been shown that the highest reflectivity is generally observed to the left of the shear vector (Marks and Gamache 1992; Franklin et al. 1993; Reasor et al. 2000; Black et al. 2002; Eastin et al. 2005). The highest reflectivity in the eyewall of Bret was in the northwest quadrant on both 21 and 22 August, consistent with the findings of these studies.

b. Ocean

Along with atmospheric interactions, Hurricane Bret’s intensity changes were associated with the oceanic conditions in the Gulf of Mexico. Figure 7 shows the SSTs measured by AXBTs deployed during the period of RI and subsequent weakening. At the beginning of the deepening at 1200 UTC 21 August, the hurricane moved over relatively warm waters of about 29°C. With wind shear values of less than 10 kt, the storm encountered very favorable conditions for RI (Kaplan and DeMaria 2003). Afterward, the SSTs were gradually increasing, and Bret encountered temperatures of approximately 30°C at 0600 UTC 22 August, 6 h before the storm reached its lowest MSLP. The high SSTs were related to two warm eddies that Bret passed to its right (Huber 2000). With air temperatures measured by GPS dropwindsondes of approximately 25°C close to the sea surface in the eyewall region, strong sensible and latent heat fluxes probably contributed to the strengthening of the cyclone. As it approached the Texas coast, the SSTs were significantly reduced. AXBT measurements on 22 August revealed relatively cool shelf waters between 25° and 26°C while the air temperature at the air–sea interface in the eyewall slightly dropped to 24°C. Cione and Uhlhorn (2003) found that small variations in SST in the inner core of a TC can tremendously modify air–sea fluxes in this region. For example, under high-wind conditions a 1-K reduction of inner-core SST may alter sensible and latent heat fluxes by as much as 40%. Reduced heat fluxes and consequently Bret’s weakening to a category 3 storm were attributed to the lower SSTs in the coastal region.

Cooler SSTs through hurricane-induced upwelling of colder water may influence the intensity of subsequent storms when passing over the cold wake (Brand 1971). Walker et al. (2005) also showed that cold SSTs induced by upwelling in the northern Gulf of Mexico caused a negative feedback to Hurricane Ivan (2004) resulting in a weakening of the storm. Consequently, the authors suggest that AXBT measurements of SST are crucial in order to improve the intensity forecast for landfalling TCs.

Since SST observations are necessary but not sufficient for TC intensity studies, the conditions of the upper ocean were also examined. Values for the OHC and the depth of the 26°C isotherm (h26) along Bret’s best track between 0000 UTC 21 August and 0000 UTC 23 August are presented in Table 3. Values for both variables remained nearly constant for the best-track positions at 0000 and 0600 UTC 21 August, and the 18-h period starting at 0600 UTC 22 August. The oceanic conditions changed markedly during the RI and Bret’s subsequent weakening phase. The OHC at Bret’s position at 1200 UTC 21 August, when the pressure drop began, was 90.4 kJ cm−2, and about 20% higher than where the storm center was 6 h before. The passage over this first warm pool contributed to Bret’s intensification. Similar results were found for Hurricane Opal (1995) by Hong et al. (2000) who showed in a numerical experiment that the hurricane’s MSLP while passing over a warm-core ring (WCR) in the Gulf of Mexico dropped more than twice as much as compared to when the WCR was removed. OHC values for the same position are 3.2% lower on 22 August, and 3.9% on 23 August. The lower heat contents were a consequence of reduced h26, since weekly averaged Reynolds SST used in this analysis method remained constant over the 3-day period. Note that due to the deep thermocline the upper ocean was not mixed and SSTs consequently remained high as indicated by Lin et al. (2005) who suggest that warm eddies insulate TCs from cold water by preventing entrainment at the bottom of the oceanic mixed layer (OML).

Six hours later at 1800 UTC 21 August, OHC values at Bret’s storm center were lower, and relative changes more pronounced with −14.3% and −17.1% on 22 August and 23 August, respectively. Lower OHCs were a result of stress-induced upwelling of cool water and heat fluxes from the ocean to the atmosphere (Kraus and Turner 1967) where both stress and heat fluxes increase with wind speed. Shear-induced entrainment of cooler water at the base of the OML (Pollard et al. 1973) also contributed to the decrease of the 26°C isotherm depth. The hurricane passed the second warm core on 1200 UTC 22 August. Even though the OHC in the inner core was 20% higher than 6 h before, the storm did not intensify. One plausible explanation is that the vertical wind shear reached its peak at the same time (Table 1 and Table 2) and outweighed the positive feedback of the ocean.

As Bret approached the Texas coast, it encountered waters with OHCs of about 26 kJ cm−2 due to the relatively shallow shelf and cooler SSTs. Three-day SST composites from AVHRR data showed warm SSTs (∼28°–29°C) along the Texas coast on 22 August. However, the composite for 24 August indicated SSTs of less than 27°C for the same area, which is in agreement with AXBT measurements. It is suggested that storm-induced mixing ahead of the storm was responsible for the lower SSTs. Moreover, the storm’s reduced translational speed of only 2.5 m s−1 near landfall enhanced the mixing of the upper-ocean layer (Huber 2000) and led to a negative feedback on the hurricane. Consequently, the convection in Bret’s inner-core region was reduced due to a decrease in surface heat fluxes.

Emanuel et al. (2004) showed in a modeling study that Hurricane Bret underwent a RI shortly before landfall due to the shoaling effect where the rising seafloor meets the base of the OML and thus prevents further ocean cooling. Flight-level observations from Air Force reconnaissance aircraft indicated a short period of reintensification for Bret. However, the authors suggest that this period was related to internal dynamics of the hurricane, as will be shown in Part II of this study, since the waters in the very shallow shelf along the Texas coast have already been well mixed prior to Bret’s approach and thus prevented the shoaling effect.

5. Sensitivity studies with the SHIPS model

As shown in Fig. 3, SHIPS intensity forecasts for Hurricane Bret were inaccurate. The quality of model forecasts partially relies on the accuracy of the initial conditions. Sensitivity studies with the SHIPS model were conducted for which the initial conditions for wind shear and SST were modified. Four different cases were studied. First, a control run (CTRL) using vertical wind shear from NCEP–NCAR reanalyses as the background field was performed. Second, vertical shear values were derived from AVN analyses within a domain between 200 and 800 km away from the storm center (AVN28, see also Table 1). Third, the vertical shear from the CTRL run was combined with modified SSTs to produce the TEMP run. In particular, the SST for 1200 UTC 21 August was set to 29.0°C and then increased to 29.5°C for the 18-h period beginning at 1800 UTC 21 August. Cooler SST of 26.5°C were imposed on 1800 UTC 22 August, and 25.5°C 6 h later, that represented the cool shelf waters along the Texas coast. Note that the modified SSTs were 0.2°C cooler on average within the first 24 h than the weekly averaged Reynolds SSTs along the best track used in CTRL. For the last 2 times, the modified SST were 2.4° and 3.3°C, respectively, lower. Finally, the AVN28TEMP run was a combination and used shear values from AVN28 and SSTs from TEMP in order to simultaneously study the effects of shear and SST on the intensity forecasts.

For all forecasts between 1800 UTC 18 August and 1800 UTC 22 August the average deviation from the CTRL was calculated for the other three cases. The control run showed the highest intensity forecast. The predicted maximum sustained winds for TEMP were 0.91 kt lower than for CTRL. The largest differences were obtained for AVN28 (1.47 kt) and AVN28TEMP (2.43 kt). The relatively small differences are due to the linear regression method that SHIPS is based on and is not indicative of the physical interactions for a particular storm. Each predictor only explains a small percentage of the total variation in the forecasts (DeMaria and Kaplan 1994a). The intensity forecasts for selected cases at 0600 UTC 21 August and 0000 UTC 22 August are shown in Fig. 8. Both dates were chosen to illustrate the sensitivity of the SHIPS model during the period of RI and subsequent weakening.

The forecasts from 0600 UTC 21 August for the examined cases differed only by 1 kt within the first 18 h of the forecast period. After that, the differences became a bit more distinct. Note that the cyclone was still intensifying at 0000 UTC 22 August, even though the shear in the AVN model (AVN28) nearly doubled from 6 h before. The shear in the control run increased by approximately one-third at the same time. TEMP showed lower wind speeds since the modified SST values were slightly cooler than in CTRL. Bret also had a lower intensity in AVN28 than in CTRL. This was a consequence of an averaged 5.7-kt-higher shear in the AVN model. The lowest wind speeds were predicted in AVN28TEMP, which was a result of the higher shear and lower SST values compared to the original initial conditions.

For the forecasts from 0000 UTC 22 August, the intensity for TEMP was lower than in CTRL, but higher than in AVN28 because SHIPS is less sensitive to SST changes when the storm is closer to its maximum potential intensity (DeMaria and Kaplan 1994b). Also, the influence of shear in the SHIPS model is increasing with storm intensity. Consequently, the influence of cooler SSTs in TEMP on Bret’s weakening was smaller than the impact of strong shear in the AVN model in AVN28. Due to warmer SST and lowest shear values, CTRL predicted the highest intensity for Bret. It should also be pointed out that the modeled intensities were higher than the observed ones. Nevertheless, the discrepancies between the forecasts and the best track during the weakening stage were significantly smaller than during the RI.

Even though the differences between the three cases and the control run were small, this sensitivity study shows that SHIPS and, therefore intensity forecasts, are very sensitive to SST and OHC. Since SST is directly related to OHC beneath an intense hurricane, it implies that OHC is a very important parameter. This is consistent with the results of observational studies of Bosart et al. (2000), Shay et al. (2000), and Cione and Uhlhorn (2003) and the theoretical work by Schade and Emanuel (1999). The general lack of sensitivity further underscores the importance of the internal processes during an RI event, which are completely ignored in SHIPS. It also indicates that initial conditions play an important role in model predictions of cyclone intensities. Rhome et al. (2006) have done a similar study with the SHIPS model in which they correlated vertical shear estimates to changes in intensity. They found for the 2005 hurricane season that the 850–500 hPa shear over an annulus with inner and outer radius of 200 km and 800 km, respectively, was most highly correlated with intensity changes.

Finally, it should be noted that SHIPS is based on a regression technique that uses climatological, persistence, and synoptic predictors. Since each predictor only explains a small fraction of the intensity variation (DeMaria and Kaplan 1994a), extreme events such as RI tend to be underpredicted.

6. Summary

Hurricane Bret underwent an RI and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 in the Gulf of Mexico before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. Both the National Hurricane Center and SHIPS did not forecast the observed intensity changes very well for this time period, underestimating both the amount of strengthening and weakening.

AVN analyses of Bret’s large-scale environment on 21 August revealed favorable atmospheric conditions at the beginning of the RI with vertical wind shear relaxing below 10 kt. The simultaneous passage over a warm eddy with increased OHC and SSTs above 29°C supported the strengthening of the storm. Increasing shear due to an anticyclone in the upper troposphere in association with reduced OHC values and cooler SSTs of ∼26°C along the Texas coast contributed to Bret’s weakening on 22 August.

Shear values based on the AVN and NCEP model output were calculated for different radii and compared to the SHIPS wind shear. Differences in the magnitude between the two models were shown, but both AVN and NCEP shear had a similar trend that was also represented in SHIPS. AVN shear values were lower for smaller radii and indicated the most favorable conditions for storm intensification. This is in agreement with observations, since Bret was a relatively small storm. The shear influence was noticeable in the inner-core structure of the hurricane. Reflectivity data from lower fuselage radar showed pronounced wavenumber asymmetries left of the shear vector on both days.

Significant structural changes were observed within 2 h on late 22 August, as the storm appeared to be more asymmetric. The area of high reflectivity in the rainband region became considerably smaller and indicated weaker convection in the inner core. Profiles of vertical motions in the eyewall region obtained from GPS dropwindsonde measurements revealed average updrafts of up to 6 m s−1, while they reached only 2 m s−1 on the next day. Averaged downdrafts reveal a constant profile on both days, but are stronger on 21 August as are maximum values for up- and downdrafts.

Sensitivity studies with the SHIPS model were conducted. Four different cases were studied for which modified shear and SST values were used for model initialization. The results indicate that SHIPS and therefore intensity forecasts are very sensitive to SST and OHC and stress the importance of the OHC in forecasting tropical cyclone intensities consistent with the results of observational studies and theoretical work. The general lack of sensitivity underscores the importance of internal processes, which are not incorporated into SHIPS, during an RI event.

While the first part of the paper solely described the environmental influences and the temporal changes thereof on Bret’s intensity, Part II will discuss the dramatic evolution of the inner core in detail.

Acknowledgments

The authors thank Dr. Mark DeMaria and John Kaplan for rerunning the SHIPS model and their assistance with data interpretation. Dr. Lynn K. Shay and Jodi Brewster of the University of Miami are kindly acknowledged for providing data about the oceanic conditions and helpful comments. Discussions with Dr. Sim Aberson of the Hurricane Research Division (HRD) significantly improved the quality of the paper, and HRD’s Ms. Sonia Otero is thanked for her editorial help on an earlier draft of the paper.

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Fig. 1.
Fig. 1.

Official NHC intensity forecast errors (kt) between 1997 and 2006 for the Atlantic basin. The solid and dashed lines show the errors for a 12- and 36-h forecast, respectively. The dotted line represents errors for a 72-h forecast. (Data source: NHC)

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 2.
Fig. 2.

NHC’s best track for Hurricane Bret between 1800 UTC 18 Aug and 0000 UTC 25 Aug 1999. The solid line with filled dots represents the MSLP (hPa), and the solid line with squares shows the maximum sustained wind speed (kt). Each dot and square indicates individual values for pressure and wind speed, respectively. The two solid vertical lines indicate the period of Bret’s rapid intensification. The best-track positions of Bret’s center are given in Fig. 7.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 3.
Fig. 3.

Forecasts of maximum sustained wind speed (kt) from (top) NHC and (bottom) SHIPS. The symbols indicate different forecasts between 0000 UTC 19 Aug and 0000 UTC 23 Aug 1999. The solid line with filled dots represents the best track. The first value for each SHIPS forecast corresponds to the best track. Each NHC forecast starts 9 h after it was issued.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 4.
Fig. 4.

AVN analysis of horizontal wind speed (m s−1) at 200 hPa at (a) 1200 UTC 21 Aug and (b) 1200 UTC 22 Aug 1999 between 15° and 40°N and 70° and 120°W. The hurricane symbol shows the position of Bret according to the best track. Circulation centers of upper-level high and low pressure systems are indicated with H and L, respectively. The black dashed line shows the trough axis associated with the upper-level low.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 5.
Fig. 5.

Composite of LF radar reflectivity (dBZ) observed in Hurricane Bret at (a) 2250 UTC 21 Aug, (b) 2118 UTC 22 Aug, and (c) 2340 UTC 22 Aug 1999. The black cross in the eye indicates the position of the NOAA aircraft at the time of the scan. The domain of the scan is 240 km × 240 km.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 6.
Fig. 6.

Average vertical velocity (m s−1) as a function of height (m) measured by GPS dropwindsondes deployed on NOAA flight missions on (a) 21 Aug and (b) 22 Aug 1999. The thick black solid line shows the averaged profile for all winds. The medium black solid line represents the averaged updrafts, while the thin black solid line indicates maximum updrafts. The medium and thin gray solid lines show averaged and maximum downdrafts, respectively.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 7.
Fig. 7.

Selected SST observations (°C) for AXBT locations on 21 Aug (squares) and 22 Aug 1999 (inverted triangles). The dashed line shows the best track. Best-track values for MSLP in hPa and maximum sustained winds (kt) are given every 6 h beginning at 1200 UTC 21 Aug 1999.

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Fig. 8.
Fig. 8.

SHIPS forecasts for maximum sustained wind speed (kt) for the time between (top) 0600 UTC 21 Aug and 0000 UTC 23 Aug 1999 and (bottom) 0000 UTC 22 Aug and 0000 UTC 23 Aug 1999 for four different cases: CTRL (case 1), AVN28 (case 3), TEMP (case 6), and AVN28TEMP (case 7). Forecasts are given every 6 h. All forecasts are compared to the best track (solid line).

Citation: Monthly Weather Review 136, 11; 10.1175/2008MWR2438.1

Table 1.

Values for vertical wind shear between 200- and 850-hPa pressure levels within a domain 200 and 800 km away from the storm center. Values are provided for three different models: AVN, SHIPS, and NCEP. Shear values for AVN and SHIPS are available every 6 h between 0600 UTC 21 Aug and 0000 UTC 23 Aug 1999, whereas NCEP values are presented every 12 h. The magnitude is given in knots and the direction in degrees. Note that a direction of 90° indicates pure westerly shear.

Table 1.
Table 2.

As in Table 1, but only for AVN model for three different domains: 200–400, 0–400, and 0–800 km.

Table 2.
Table 3.

Values for the OHC (kJ cm−2), and the depth of the 26°C isotherm (h26; m) for model grid points closest to the best-track positions for Hurricane Bret every 6 h between 0000 UTC 21 Aug and 0000 UTC 23 Aug 1999. The model has a spatial resolution of 0.5°. Hence, discrepancies between model grid points and best-track positions are limited to a few tenths of a degree.

Table 3.
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