A21C-0632
New Challenges in High-Resolution Modeling and Data Assimilation of Hurricanes
The extreme active Atlantic hurricane seasons in recent years have highlighted the urgent need for a better understanding of the factors that contribute to hurricane intensity and for development of the corresponding advanced hurricane prediction models to improve intensity forecasts. The lack of skill in present forecasts of hurricane structure and intensity may be attributed in part to deficiencies in the current prediction models: insufficient grid resolution, inadequate surface and boundary layer formulations, and the lack of full coupling to a dynamic ocean. The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in hurricanes push the surface-exchange parameters for temperature, water vapor, and momentum into untested regimes. The recent modeling effort is to develop and test a fully coupled atmosphere-wave-ocean modeling system that is capable of resolving the eye and eyewall in a hurricane at ~1 km grid resolution. The new challenges for these very high resolution models are the corresponding physical representations at 1-km scale, including microphysics, sub-grid turbulence parameterization, atmospheric boundary layer, physical processes at the air-sea interface with surface waves among others. The lack of accurate initial conditions for high-resolution hurricane modeling presents another major challenge. Improvements in initial conditions rest on the use of more airborne and remotely sensed observations in high-resolution data assimilation systems and on the application of advanced assimilation schemes to hurricanes. This study aimed to provide an overview of these new challenges using high-resolution model simulations of Hurricanes Isabel (2003), Frances (2004), Katrina and Rita (2005) that were observed extensively by two recent field programs, namely, the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-Hurricane in 2003-2004 and the Hurricane Rainbands and Intensity Change Experiment (RAINEX) in 2005. Data assimilation experiments with satellite scatterometer data using 4DVar in a number of hurricanes will also be discussed.
A21C-0633
Intensity and Development Forecasts of Tropical Cyclones by the JMA High-Resolution Global NWP Model: Impacts of Resolution Enhancement
It is widely considered that a spatial resolution of numerical weather prediction (NWP) model plays an important role for forecasting severe weather events such as tropical cyclones (TCs) and heavy rainfall. Under the KAKUSHIN project (funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology), the Japan Meteorological Agency (JMA) has developed a new Global Spectral Model (GSM) with a high horizontal resolution of about 20km and 60 vertical layers (hereafter called g20km GSMh), which is utilized to evaluate severe weather events in future climate. The 20km GSM will be operational in November 2007 replacing the current GSM with a horizontal resolution of about 60km and 40 vertical layers (hereafter called g60km GSMh). In the present study, we investigate how a model resolution impacts on TC forecasts because this resolution enhancement aims to improve the modelfs ability to forecast severe weather. Due to the more realistic model topography in higher horizontal resolution, the 20km GSM can give more accurate forecasts of orographic precipitation than the 60km GSM, especially over the area range of heavy precipitation. According to the statistically verified results, the enhancement of horizontal and vertical resolution appears to fairly improve the accuracy of TC intensity forecasts. However, for TC track forecasts, it may be more important to accurately represent large-scale environmental contexts surrounding the TC than to resolve the TC structure itself. In order to clarify resolution impacts on the TC intensity prediction, we categorize the TC intensity forecasts into three stages (development stage, maturation stage and dissipation stage). The results show that the effectiveness of the resolution enhancement is bigger in the development stage and relatively small in the maturation and dissipation stages. For the maturation and dissipation stages, improvement of physical processes seems to be more important than the resolution enhancement to provide better accuracy of TC intensity forecasts by the 20km GSM. For very strong TCs in the development stage, however, it may be hard to provide enough accuracy in the intensity forecasts even by the 20km GSM. It is also suggested in the study that initial analyzed fields and model physics are more essential rather than the resolution enhancement for TC genesis forecasting.
A21C-0634
The impacts of resolution and boundary layer parameterization on the structure of the wind field in high resolution simulations of Hurricane Isabel (2003)
We present detailed analyses of high resolution simulations of Hurricane Isabel (2003) initialized from GFDL model initial conditions at 00Z on Sept. 12th, 2003. The effects of horizontal resolution, vertical resolution, nested grid size, and boundary layer parameterization on the accuracy of the simulation are illustrated. In particular, the vertical structures of the inner-core wind fields are evaluated by direct comparison to the wind fields observed with pseudo-dual-Doppler radar analyses, and the boundary layer winds are evaluated by comparisons with dropsondes and stepped descent observations from CBLAST flights into the storm. Simulations with 4 km resolution and 1.33 km resolution are considered. On the 12th and 13th of September, the simulated boundary layer structures compare remarkably well with those observed, with the YSU PBL scheme using the Donelan et al. correction for the drag coefficient giving the best results. Increasing the resolution in the boundary layer does not give improved results. The azimuthally averaged middle and upper-level wind fields also match quite well. However, comparison with the observed wind fields shows a consistent bias in all the simulations, that the maximum winds at the top of the boundary layer are much larger than observed. In fact, even though the simulated surface winds are lower than those reported in the best track data set, the peak winds aloft are far larger than what was observed by dropsondes. This bias appears to be common to many hurricane simulations. We investigate modifications to the parameterizations that might improve these defects.
A21C-0635
Cycling Variational Assimilation of Remotely Sensed Observations for Prediction of Hurricane Katrina
The impact of assimilating observations from conventional instruments, Radar, and satellites on Hurricane Katrina (2005) was assessed using high-resolution model simulations. Four nested domains were configured with grid spacings of 54 km, 18 km, 4.5 km, and 1.5 km for domains 1 to 4, respectively. Three sets of experiments, EXP1-3, with various initial times and data cycling periods were conducted. The data cycling periods were 1800 UTC August 25 to 0000 UTC August 26 (6 h), 0000 UTC to 0600 UTC August 26 (6 h), and 1800 UTC August 25 to 0600 UTC August 26 (12 h) for EXP1-3, respectively. The first two sets (i.e. EXP1 and EXP2) were to evaluate the impact of assimilating different observations on Katrina simulations, while the last one (EXP3) was to examine the influence of different assimilation periods for radar data. Thirty-six hour model integrations were then performed after the data cycling on all four domains. The results from EXP1 shows that the assimilation of radar and conventional data had a positive impact on simulated storm's intensities during the first 24 hours, while the influence of satellite data was also positive, though less significant, and the influence was able to extend the whole 36-h simulation since the coverage of satellite data was much larger than that of radar. However, none of EXP1 experiments with the assimilation of observations had reduced track errors, which increased with time and reached about 200-250 km after a 36-h integration. The forecasts from EXP2 show great improvement in simulated storm's intensity and track after the use of observations, in particular for the first 24 h of the forecasts. The track error was close to 50 km during the whole simulation period. The results from EXP3, which started the data cycling at 1800 UTC August 25 as in EXP1, confirmed that the observations from 0000 UTC to 0600 UTC August 26 used in this study played a key role in improving simulated tracks. Moreover, the results further concluded that the impact on the simulated track was contributed from GTS data and/or satellite data. Doppler radar data assimilation mainly contributed to the improvement of the hurricane intensity forecast. Finally, all three sets of experiments indicated that the assimilation of radar data in an interval of every three hours for a 6-h time period is an optimal setting.
A21C-0636
High-resolution simulation of tropical cyclone Debby (2006) and validation with NAMMA data
Tropical cyclone Debby (2006) formed from an African wave only a short distance off the west African coast. Because it formed during the African Monsoon Multidisciplinary Analyses (AMMA) program, there are more data than usual for model initialization over the African continent. The NASA downstream addition to AMMA (NAMMA) provides a unique opportunity to validate these simulations with a detailed description of the structure of Debby with data from the DC-8 aircraft flight of August 23. This dataset is particularly valuable because it includes clear evidence of the wind, cloud, precipitation, and aerosol distribution as a function of height, never before obtained for a storm in the east Atlantic. A nested-grid WRF model at resolution of 36, 12, 4, and 1.33 km is initialized with NCEP global forecast system final analysis about 36 hours before the tropical storm formation. A total of 5 days of simulation is conducted to depict the structure and evolution of tropical cyclone Debby. Numerical simulation results are compared with the observations obtained during NAMMA. Among the questions to be addressed are the following: (1) The actual storm is warm core, but with the strongest winds are at 700-600 hPa; does the simulation have similar structure? (2) The actual storm has an eyewall extending to only about 500 hPa, with the cyclone center at 400-300 hPa displaced almost 100 km to the southeast; does the simulation have a similar structure? (3) The actual storm has dry, aerosol-laden air just outside the rain area in most quadrants; does the simulation agree with these observations? (4) How sensitive are the numerical simulation to the cloud microphysics and boundary layer parameterizations? To the choice of 4 km or 1.33 km for the finest grid?
A21C-0637
Interaction of Environmental Moisture, Rainbands, and Inner-Core Dynamics in Hurricanes Katrina and Rita
The interaction of the environmental water vapor distribution around a tropical cyclone (TC), rainbands, and inner- core dynamics can affect hurricane structure and intensity change, which is not well understood. Although previous studies have addressed various aspects of this problem, a full three way interaction and its implications for hurricane intensity change has not been documented. Using data collected during the Hurricane Rainband and Intensity Experiment (RAINEX) in Hurricanes Katrina and Rita, the three way interaction of the environment moisture, rainbands, and inner-core dynamics can be evaluated. The TRMM TMI total precipitable water (PW) data with 1/4 degree horizontal resolution, TRMM TMI rainrate data with a 4 km horizontal resolution and the GPS dropsondes with a ½ second temporal resolution are used to characterize the environmental moisture. The high resolution model output from the real-time MM5 forecasts of Hurricanes Katrina and Rita are used to investigate the complex interactions in both storms. The model forecasts were made using a vortex-following nested grid with horizontal resolutions of 15, 5, and 1.67km, respectively. There were 28 vertical sigma levels. The Goddard microphysics scheme was used. The TRMM PW and the GPS dropsonde data show strong moisture gradients in the outer rainband region in Rita with a dry outer environment, which may contribute to the development of outer rainbands with a high circularity. It created a secondary ring of potential vorticity (PV). In addition, the vortex Rossby waves (VRW) propagating radialy outward from the eyewall were unable to propagate beyond the secondary ring of PV. The combination of these VRW and the environmental water vapor distribution may play a role in enhancing the rainbands that developed into a secondary eyewall, which leads to a temporary weakening of the hurricane. In contrast, Katrina had a relative weak moisture gradient surrounding the storm. There were not persistent outer rainbands with high circularity, which may explain the different evolution in Katrina compared with Rita.
A21C-0638
Continuous Estimates of Precipitable Water Vapor from Caribbean GPS Stations During the 2007 Atlantic Hurricane Season
Hurricanes derive their strength through water vapor that is both evaporated from warm ocean surfaces and the existing moisture in the surrounding atmospheric environment. Observationally, there are relatively few instruments that can accurately measure water vapor in the presence of clouds and rain. Retrievals of precipitable water vapor (PW) using Global Positioning System (GPS) stations may be the most reliable way to continuously monitor column integrated water vapor. We present PW estimates collected during the 2007 Atlantic hurricane season derived from a network of GPS stations recently installed in the Caribbean. This network produced 30-minute estimates of PW from more than 20 stations in the region. Preliminary results indicate a wet bias in the Global Forecast System (GFS) analysis fields of approximately 1 mm in PW, with root mean square differences greater than 3 mm. Both of these statistical comparisons are significantly larger than those derived from analysis fields over the continental United States, which suggests that there is significant room for improvement in the initial conditions used for numerical weather prediction forecasts. We present results in the temporal and spatial changes in PW as hurricanes Dean and Felix moved through the Caribbean. We also will show what impact these observations have on the Weather Research and Forecasting (WRF) model forecasts using various data assimilation strategies and methods. http://www.suominet.ucar.edu
A21C-0639
Using COAMPS Microphysics To Model Satellite and Aircraft Radar Data: An Evaluation During Hurricane Dennis
The field phase of the Tropical Cloud Systems and Processes (TCSP) experiment took place between 1-27 July 2005, based out of San Jose, Costa Rica. Although the focus area for TCSP was planned to be the Eastern Pacific, the unusually early genesis of tropical disturbances in the Eastern Caribbean prompted missions dedicated to the formation and evolution of Hurricane Dennis. The high-altitude (20-km) NASA ER-2 flew 12 missions, including three dates dedicated to Hurricane Dennis. The flights on July 5-6 captured Dennis as it was transitioning from a tropical storm to a hurricane, and July 9 as it was entering a period of rapid intensification. The ER-2 deployed three microwave sensors, the Advanced Microwave Precipitation Radiometer (AMPR), the High Altitude MMIC Sounding Radiometer (HAMSR) and the the ER-2 Doppler Radar (EDOP). The AMPR is the aircraft "simulator" of the TRMM sensor; from 20-km altitude its 85 GHz imagery has an on-Earth resolution of 700-m (2.8-km at 10 GHz) at nadir, nearly 20 times finer than typical TRMM or SSMI satellite imagery. EDOP is an 10 GHz Doppler radar capable of resolving the fine scale vertical reflectivity. Since microwave observations respond to the presence of water vapor, liquid and ice hydrometeors, they are useful for evaluating the capabilities and deficiencies of mesoscale prediction models in representing the spatial evolution of the underlying cloud structure. In this presentation, the microphysical outputs from simulation of Hurricane Dennis using the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS) were used to forward model observed TRMM and SSMI satellite data, and the AMPR, HAMSR and EDOP data from the ER-2 aircraft overpasses. This allows model-vs- observation diagnostics to be performed in observational space, avoiding usage of retrieved satellite quantities. Statistical intercomparisons show model overpredictions of the reflectivities at upper levels, and an overprediction of the coldest 85 GHz brightness temperatures reflecting excessive graupel. Although the results are limited to a single case, the methodology is potentially adaptable to routine COAMPS model runs for analyzing modifications to microphysical parameterization schemes. http://camex.nsstc.nasa.gov/tcsp/
A21C-0640
Prototype of an Integrated Hurricane Information System for Research: Description and Illustration of its Use in Evaluating WRF Model Simulations
In spite of recent improvements in hurricane track forecast accuracy, currently there are still many unanswered questions about the physical processes that determine hurricane genesis, intensity, track and impact on large- scale environment. Furthermore, a significant amount of work remains to be done in validating hurricane forecast models, understanding their sensitivities and improving their parameterizations. None of this can be accomplished without a comprehensive set of multiparameter observations that are relevant to both the large- scale and the storm-scale processes in the atmosphere and in the ocean. To address this need, we have developed a prototype of a comprehensive hurricane information system of high- resolution satellite, airborne and in-situ observations and model outputs pertaining to: i) the thermodynamic and microphysical structure of the storms; ii) the air-sea interaction processes; iii) the larger-scale environment as depicted by the SST, ocean heat content and the aerosol loading of the environment. Our goal was to create a one-stop place to provide the researchers with an extensive set of observed hurricane data, and their graphical representation, together with large-scale and convection-resolving model output, all organized in an easy way to determine when coincident observations from multiple instruments are available. Analysis tools will be developed in the next step. The analysis tools will be used to determine spatial, temporal and multiparameter covariances that are needed to evaluate model performance, provide information for data assimilation and characterize and compare observations from different platforms. We envision that the developed hurricane information system will help in the validation of the hurricane models, in the systematic understanding of their sensitivities and in the improvement of the physical parameterizations employed by the models. Furthermore, it will help in studying the physical processes that affect hurricane development and impact on large-scale environment. This talk will describe the developed prototype of the hurricane information systems. Furthermore, we will use a set of WRF hurricane simulations and compare simulated to observed structures to illustrate how the information system can be used to discriminate between simulations that employ different physical parameterizations. The work described here was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics ans Space Administration.
A21C-0641
High-Resolution Radiosonde Measurements from Cape Verde: Details of Easterly Wave Passage
The NASA African Monsoon Multi-disciplinary Analyses (NAMMA) mission took place from Cape Verde, approximately 600 km west of Dakar, Senegal between August 15 and September 14, 2006. Aircraft and ground- based measurements were staged from the island of Sal while a large number of radiosondes were released from Praia, on the island of Sao Tiago 150 km south of Sal. High-resolution GPS equipped radiosondes provided measurements of temperature, relative humidity, and wind at a vertical resolution of 4-6 meters every 4 hours from Cape Verde Islands. The high vertical- and temporal- resolution of the measurements effectively captured significant features of convection and easterly wave processes. West African-originating easterly waves were observed to develop to the south of Cape Verde as they move westward over the southern Atlantic Ocean. Time series of temperature, water vapor, and wind available during the 28-day mission are discussed. Five waves were observed during their passage; two depressions developed into tropical cyclones and three did not. Differences in their development are presented. Examples are provided that discuss methods used to detect wave passage, the effect of the Saharan Air Layer, and daily energy distribution. Data were distributed to all NAMMA participants and are available at http://msfc.nasa.gov.
A21C-0642
Distinct Structure and Intensity in Hurricanes Katrina and Ophelia (2005)
Although many previous studies have shown that a large area of warm water provides a favorable condition for intensification of hurricanes, storm development and intensity over the warm ocean varies in a broad range. Various factors contribute to the storm intensity including the atmospheric environment and internal dynamic of each individual storm. In the 2005 hurricane season, Hurricanes Ophelia and Katrina both developed over the warm water near the east coast of South Florida. However, they evolved differently with distinct structure and intensity. Katrina became one of the most intense Category 5 hurricanes in the Gulf of Mexico, whereas Ophelia remained a relatively weak Category 1 hurricane over several days near the Gulf Stream. In this study, we aim to investigate the relationship between the hurricanes and their environment and examine the structures of the hurricanes developed over the warm water. We focus on analyzing the structures of these two hurricanes to shed light on why they both move over the warm water, one intensified quickly and the other did not. To best resolve the storm structure, we use the high resolution, non-hydrostatic Weather Research and Forecasting (WRF) modeling system, with 1.33 km grid horizontal spacing on the finest nested mesh. The model includes the vortex-following nested grids similar to that developed at the University of Miami and tested in various hurricane studies. There are 4 nested domains for the simulation of Hurricane Katrina and 3 for Hurricane Ophelia. The initial and lateral boundary conditions for Katrina are the output from NCAR's ensemble Kalman filter (EnKF) data assimilation, and the conditions for Ophelia are interpolated from the 1°2National Centers for Environmental Prediction (NCEP) global analysis fields at 6-h intervals. The extensive observations from the Hurricane Rainbands and Intensity Change Experiment (RAINEX) are used to evaluate and validate the model simulations. Multiple airborne radar reflectivity and 3D Doppler wind observations during both hurricanes provided unprecedented data sets for this high-resolution modeling study. A comprehensive analysis of model output in comparison with RAINEX observations will help to understand the physical and dynamic processes that contributed to the distinct evolution of the two hurricanes.
A21C-0643
Evaluation of Real-time Hurricane Forecasts Using the Advanced Hurricane WRF Model for the 2007 Atlantic Hurricane Season.
Real-time forecasts have been conducted with the Advanced Hurricane WRF Model (AHW) for named storms of the 2007 Atlantic hurricane season. Taking advantage of increased computational power over previous years, 5- day forecasts are conducted daily using three domains; two nests of 4km and 1.3km grid-spacing track the vortex within a fixed parent domain of 12km grid-spacing. In this presentation, forecast accuracy in terms of track and intensity will be presented. The quality of the forecast storm intensity can vary dramatically between storms, and sometimes between successive forecasts of a given storm. This variability in model performance is explored by analyzing the statistics of the observed and model storm intensities for the 2007 hurricane season. Conditions under which the model performs poorly are identified and a series of sensitivity simulations highlight aspects of the modeling system to which the forecast intensity is most sensitive.
A21C-0644
A Numerical Investigation of Feedback Processes in the Droplet Evaporation Layer for Tropical Cyclones
Numerical experiments using simple, idealized models indicate that the ratio of enthalpy bulk transfer coefficient to surface drag coefficient must be in the range of 0.75 to 1.25 in order for simulated hurricanes to be of realistic intensity. However, within the range in which reliable direct flux measurements exist, the observed value of this ratio over the sea is 0.70 at low winds speeds, but decreases significantly below 0.70 with increasing wind speed. One possibility for this discrepancy is the failure of the traditional bulk transfer coefficients to take into account the effect of sea-spray. High winds generate large re-entrant sea-spray droplets which tend to increase the sea-air enthalpy transfer and, thus, have positive feedback to the intensification of hurricanes. On the other hand, sea spray is generated at the expense of the momentum, which in effect causes the turbulent momentum flux to decrease. The parameterization of the feedback effect of sea spray to the overall air-sea momentum and enthalpy fluxes is still a subject of research. In this study we use the explicit sea-spray model of Kepert et al. (1999) coupled with the 1-D Mellor-Yamada turbulence mixing model to investigate the feedback effect. The spray model is capable of simulating the evaporation and dispersion of saline water droplets of various sizes. There is full coupling among the spray- droplet microphysics, turbulence mixing, and droplet transport. The Fairall-Banner parameterization is used in the sea spray droplet source term (which predicts the size spectrum of sea spray produced by the ocean in terms of wind speed, surface stress, and wave properties) along with direct dynamic and thermal feedback. The characteristics of the way in which evaporating droplets of various sizes modifies the turbulence mixing near the surface, which in turn affects further droplet evaporation, are summarized and discussed.
A21C-0645
The Microphysical and Optical Properties of Saharan Dust as Observed During NAMMA
Recent analysis of satellite observations showed strong links between the variabilities of North Atlantic tropical cyclone (TC) activities and dust present in Saharan air layers (SALs). SALs are typically characterized by a layer of dry and warm air with dust. It has been hypothesized that that SALs play a significant role in suppressing the development of tropical cyclones. To better understand the interactions between SAL and TC development, NASA Africa Monsoon Multidisciplinary Analysis (NAMMA) conducted airborne observations of SAL and Saharan dust during missions flown from Cape Verde during the summer of 2006. A summary will be presented of in-situ observations of Saharan dust microphysical and optical properties as well as the vertical distributions of the dust layers. A total of ~60 individual dust layers were observed during the NAMMA airborne field campaign. The observed average dust volume loading ranged from 10 to 50 μm3 cm-3, with a typical volume mean diameter of 2 μm. The observed scattering coefficient at 550 nm varied from 50 to 200 Mm-1. Estimated mass scattering efficiencies ranged from 0.7 and 1.0 m2 g-1. The observed dust absorption was typically very weak, with SSA values at 550 nm around 0.99. A closure assessment suggests that ~ 85% of the observed scattering can be explained by the predictions derived from observed size distributions. Also presented are the evolution processes of dust microphysical and optical properties as being transported across the Atlantic. Finally, model simulations will be discussed in terms of the Saharan dust effect on the cloud droplet size distributions.
A21C-0646
Vortex Rossby Waves in Hurricanes Katrina and Rita (2005)
Radar observations in hurricanes reveal inner spiraling rainbands emanating from the eyewall and propagating outward. Theoretical analysis indicated that these inner bands are azimuthally and radially propagating vortex Rossby waves (VRW). The outward propagating waves convey PV from the inner core to outer regions and thus lead to PV redistribution within a hurricane. It has been hypothesized that the outward propogating VRWs may play a role in interacting with an existing secondary PV ring in the outer region of a hurricane, which could lead to a development of concentric eyewalls. However, the lack of simultaneous observations over the inner-core and rainband regions is a major difficulty in our understanding of the complex interaction. The importance of VRWs in hurricane intensity change remains to be a question. This study aims to address the question using high- resolution model (MM5) forecasts of Hurricanes Katrina and Rita during the Hurricane Rainbands and Intensity Change Experiment (RAINEX) in 2005. The two major hurricanes went through a similar rapid intensification over the Gulf of Mexico. Both RAINEX observations and model forecast fields showed that Rita developed a secondary eyewall and went through an eyewall replacement before landfall, whereas Katrina did not. We analyze the model output at 1.67 km grid-resolution with 12-min time intervals. Azimuthally and radially propagating VRWs were found in the PV, rainrate, and vertical velocity fields in both storms. In the case of Katrina, no secondary PV maximum exists due to the lack of highly circular rainbands. Thus the VRWs propagate outward smoothly over a relatively long distance. No VRW activity has been found beyond 80-100 km radius in Katrina. This result indicates that interaction between the VRWs and outer PV disturbance must take place within this region, otherwise no effect concerning the importance of VRW would occur. The stagnation radius depends on the background PV- gradient which itself can be changed by wave-redistributed PV. It is also a function of the azimuthal wavenumber. Higher wavenumbers generally propagate farther and are thus more likely to interact with outer PV disturbance in the first place. In contrast, Rita developed a PV ring in the outer rainband region. Detailed analysis of Rita is underway. The comparison between the two hurricanes may shed some lights on the interaction of VRWs and rainbands as well as its implication on hurricane intensity change.
A21C-0647
Visualizing and Quantifying Transport in Hurricanes.
Flows surrounding hurricane storms are manifestly complex. The chaotic nature of individual particle trajectories forestalls attempts to understand material transport by appealing to the Eulerian velocity field or particle trajectories alone. However, even in these complex flows, we demonstrate the existence of time-dependent coherent structures that govern the pathways for transport. These structures, called Lagrangian Coherent Structures (LCS), are moving barriers to transport in the flow and define the boundaries of almost invariant regions. They also act as separatrices in that they separate flow regions with different dynamical behavior. Given the velocity field, we extract the LCS numerically through computation of the finite time Lyapunov exponent which is a measure of local stretching in the flow. Once computed, the LCS allow us to visualize and understand the essential mechanisms underlying transport in the flow. We illustrate these results through several examples in both atmospheric and oceanic flows with special emphasis on the insights gained from transport structures in hurricane flows. In particular, we see that transport in the flow surrounding hurricanes and tropical storms occurs via the mechanism of lobe dynamics, a result that is not immediately obvious from inspection of the flow field.
A21C-0648
The Utility of a Geostationary Doppler radar applied to the hurricane analysis and prediction problem: A Report on the 1st Nexrad in Space Workshop
Last April the first Nexrad in Space (NIS) workshop was held in Miami, Florida to discuss the value and requirements for a possible satellite mission featuring a Doppler radar in geostationary orbit capable of measuring the internal structure of tropical cyclones over a circular scan area 50 degrees latitude in diameter. The proposed NIS technology, based on the PR2 radar design developed at JPL and an innovative deployable antenna design developed at UCLA would be capable of 3D volume sampling with 12 km horizontal and 300 m vertical resolution and 1 hour scan period. The workshop participants consisted of the JPL and UCLA design teams and cross section of tropical cyclone forecasters, researchers and modelers who could potentially benefit from this technology. The consensus of the workshop included: (a) the NIS technology would provide observations to benefit hurricane forecasters, real time weather prediction models and model researchers, (b) the most important feature of NIS was its high frequency coverage together with its 3D observation capability. These features were found to fill a data gap, now developing within cloud resolving analysis and prediction systems for which there is no other proposed solution, particularly over the oceans where TCs form. Closing this data gap is important to the improvement of TC intensity prediction. A complete description of the potential benefits and recommended goals for this technology concluded by the workshop participants will be given at the oral presentation. http://nis.ssec.wisc.edu/index.html
A21C-0649
Influence of Loop Current on Hurricane Katrina in a High-Resolution Coupled Model
Recent observational and numerical modeling studies have shown that coupled atmosphere-wave-ocean models may lead to substantial improvement in prediction of hurricane intensity. However, forecasting hurricane structure and intensity change is still a real challenge, especially when storm moving over the complex ocean features, such as the Gulf Stream and warm ocean eddies, which can result in a rapid intensification in some hurricanes under a favorable atmospheric environment. Uncertainty remains to be high from storm to storm. A lack of understanding of the mechanisms of how these ocean features influence hurricane structure could be one of the reasons. To address this issue, a coupled atmospheric-wave-ocean model is adopted in this study to simulate the evolution of Hurricane Katrina (2005), which shows the rapid intensification when moving into the Gulf of Mexico. The coupled model is consist of the 5th generation Pennsylvania State University / National Center for Atmospheric Research mesoscale model (MM5), WAVEWATCH III (WW3), and the 3D Price-Weller-Pinkel (3DPWP) upper ocean model. To examine the influence of the Loop Current and warm eddy on Katrina, coupled modeling experiments were conducted with both a simplified and a more realistic upper-ocean initial condition. The latter is based on the full physics Hybrid Coordinate Ocean Model (HYCOM) assimilation fields at 0000UTC 26 August 2005. Furthermore, experiments with and without WW3 are also compared to highlight the influence of the ocean surface waves on hurricane structure and intensity. Preliminary results show that both the surface waves and a better oceanic initial condition improved significantly the storm intensity forecasts compared to the observations. The inclusion of surface waves seems to produce a better wind-pressure relationship. To fully understand the physical processes that affect the hurricane intensity by oceanic initial condition and the surface waves, a comprehensive analysis of the storm structure, such as the eyewall evolution and rain band distribution, in the coupled model experiments is underway.
A21C-0650
Improving Hurricane Heat Content Estimates From Satellite Altimeter Data
Hurricanes are amongst the most destructive natural disasters known to mankind. The primary energy source driving these storms is the latent heat release due to the condensation of water vapor, which ultimately comes from the ocean. While the Sea Surface Temperature (SST) has a direct correlation with wind speeds, the oceanic heat content is dependent on the upper ocean vertical structure. Understanding the impact of these factors in the mutual interaction of hurricane-ocean is critical to more accurately forecasting intensity change in land-falling hurricanes. Use of hurricane heat content derived from the satellite radar altimeter measurements of sea surface height has been shown to improve intensity prediction. The general approach of estimating ocean heat content uses a two-layer model representing the ocean with its anomalies derived from altimeter data. Although these estimates compare reasonably well with in-situ measurements, they are generally about 10% under-biased. Additionally, recent studies show that the comparisons are less than satisfactory in the Western North Pacific. Therefore, our objective is to develop a methodology to more accurately represent the upper ocean structure using in-situ data. As part of a NOAA/ USWRP sponsored research, upper ocean observations were acquired in the Gulf of Mexico during the summers of 1999 and 2000. Overall, 260 expendable profilers (XCTD, XBT and XCP) acquired vertical temperature structure in the high heat content regions corresponding to the Loop Current and Warm Core Eddies. Using the temperature and salinity data from the XCTDs, first the Temperature-Salinity relationships in the Loop Current Water and Gulf Common water are derived based on the depth of the 26° C isotherm. These derived T-S relationships compare well with those inferred from climatology. By means of these relationships, estimated salinity values corresponding to the XBT and XCP temperature measurements are calculated, and used to derive continuous profiles of density. Ocean heat content is then estimated from these profiles, and compared to that derived from altimeter data, showing - as mentioned earlier - a consistent bias. Using a procedure that conserves density in the vertical, these density profiles are discretized into five isopycnic layers representative of the upper ocean in the Gulf of Mexico. Statistical correlations are then derived between the altimetric sea surface height anomalies and the thickness of these layers in the region. Using these correlations, a higher resolution upper ocean structure is derived from the altimeter data. Withholding observations from one snapshot of data in the correlations, and comparing the estimated ocean heat content with in-situ values, will allow us to quantify errors in this approach. This methodology will then be extended to the Western Pacific using Argo data, and results will be presented.
A21C-0651
High Resolution Hurricane Simulations Using NRL's Mesoscale Modeling System
Development of mesoscale models and advances in tropical cyclone (TC) observation systems have allowed numerical simulations and investigations at high resolution (several kilometers) of dynamic and thermodynamic structures of TCs, which, in turn, help improve TC intensity and structure forecasts. Used as the operational numerical weather prediction model by the US Navy and other communities for a wide range of weather phenomena, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS@) is under continuous improvements in its physics and numerics. Recent improvements to the COAMPS include an improved microphysical parameterization, modifications to the surface flux parameterization over the ocean incorporating new observations at high winds, a new radiation scheme that enhances interactions between clouds and radiation, and the representation of dissipative heating in the model. The objective of this study is to assess the COAMPS capability of TC predictions at high resolution (1-3 km). Results will be shown that describe inner core structure of hurricanes during various development stage at various resolutions. Additionally, impact of the one- way and two-way interactive boundary conditions for the moving nests under high resolution will be evaluated.
A21C-0652
Tropical Cyclone Formation in 30-day Simulation Using Cloud-System-Resolving Global Nonhydrostatic Model (NICAM)
We are developing an icosahedral-grid non-hydrostatic AGCM, which can explicitly represent cumulus or meso-scale convection over the entire globe. We named the model NICAM (Nonhydrostatic ICosahedral Atmospheric Model). On 2005, we have performed a simulations with horizontal grid intervals of 14, 7 and 3.5 km using realistic topography and sea surface temperature in April 2004 (Miura et al., 2007; GRL). It simulated a typhoon Sudal that actually developed over the Northwestern Pacific in 2004. In the present study, the NICAM model with the horizontal grid interval of 14 km was used for perpetual July experiment with 30 forecasting days. In this simulation, several tropical cyclones formed over the wesetern and eastern North Pacific, althought the formation over the western North Pacific occured a little further north to the actually observed region. The mature tropical cyclones with intense wind speed had a structure of a cloud-free eye and eye wall. We have found that the enviromental parameters associated with the tropical cyclone genesis explain well the simulated region of tropical cyclone generation. Over the North Atlantic and eastern North Pacific, westward-moving disturbances like African wave are simulated, which seems to be related to the cyclone formation over the eastern North Pacific. On the other hand, the simulated tropical cyclones over the western North Pacifis seem to form by different factors as has been suggested by the previous studies based on observation. Although the model still has some problems and is under continuous improvement, we can discuss what dynamics is to be represented using a global high-resolution model.
A21C-0653
An Improved Bulk Microphysical Scheme for Studying Precipitation Processes: Comparisons with Other Schemes
Cloud microphysical processes play an important role in non-hydrostatic high-resolution simulations. Over the past decade both research and operational numerical weather prediction models have started using more complex cloud microphysical schemes that were originally developed for high-resolution cloud-resolving models. An improved bulk microphysical parameterization (adopted from the Goddard microphysics schemes) has recently implemented into the Weather Research and Forecasting (WRF) model. This bulk microphysical scheme has three different options --- 2ICE (cloud ice & snow), 3ICE-graupel (cloud ice, snow & graupel) and 3ICE-hail (cloud ice, snow & hail). High-resolution model simulations are conducted to examine the impact of microphysical schemes on two different weather events (a midlatitude linear convective system and an Atlantic hurricane). In addition, this bulk microphysical parameterization is compared with WRF's&pthree other bulk microphysical schemes. The results suggest that microphysics has a major impact on the organization and precipitation processes associated with a summer midlatitude convective system. The 3ICE scheme with a cloud ice-snow-hail configuration led to a better simulation of the summer midlatitude convective line system than the other schemes since the scheme includes dense ice precipitating (hail) particle with very fast fall speed (over 10 m/s). For an Atlantic hurricane case, varying the microphysical schemes had no significant impact on the track forecast but did affect the intensity (important for air-sea interaction) and the vertical distribution of cloud species (important for satellite retrieval). Results also suggest that different configuration of ice schemes are required to simulate the mid-latitude Mesoscale Convective System (MCS) and the hurricane.