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- Author or Editor: Sethu Raman x
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Abstract
Midlatitude cyclones develop off the Carolinas during winters and move north producing gale-force winds, ice, and heavy snow. It is believed that boundary-layer and air-sea interaction processes are very important during the development stages of these East Coast storms. The marine boundary layer (MBL) off the mid- Atlantic coastline is highly baroclinic due to the proximity of the Gulf Stream just offshore.
Typical horizontal distances between the Wilmington coastline and the western edge of the Gulf Stream vary between 90 and 250 km annually, and this distance can deviate by over 30 km within a single week. While similar weekly Gulf Stream position standard deviations also exist at Cape Hatteras, the average annual distance to the Gulf Stream frontal zone is much smaller off Cape Hatteras, normally ranging between 30 and 100 km.
This research investigates the low-level baroclinic conditions present prior to observed storm events. The examination of nine years of data on the Gulf Stream position and East Coast winter storms seems to indicate that the degree of low-level baroclinicity and modification existing prior to a cyclonic event may significantly affect the rate of cyclonic deepening off the mid-Atlantic coastline. Statistical analyses linking the observed surface-pressure decrease with both the Gulf Stream frontal location and the prestorm coastal baroclinic conditions are presented. These results quantitatively indicate that Gulf Stream-induced wintertime baroclinicity may significantly affect the regional intensification of East Coast winter cyclones.
Abstract
Midlatitude cyclones develop off the Carolinas during winters and move north producing gale-force winds, ice, and heavy snow. It is believed that boundary-layer and air-sea interaction processes are very important during the development stages of these East Coast storms. The marine boundary layer (MBL) off the mid- Atlantic coastline is highly baroclinic due to the proximity of the Gulf Stream just offshore.
Typical horizontal distances between the Wilmington coastline and the western edge of the Gulf Stream vary between 90 and 250 km annually, and this distance can deviate by over 30 km within a single week. While similar weekly Gulf Stream position standard deviations also exist at Cape Hatteras, the average annual distance to the Gulf Stream frontal zone is much smaller off Cape Hatteras, normally ranging between 30 and 100 km.
This research investigates the low-level baroclinic conditions present prior to observed storm events. The examination of nine years of data on the Gulf Stream position and East Coast winter storms seems to indicate that the degree of low-level baroclinicity and modification existing prior to a cyclonic event may significantly affect the rate of cyclonic deepening off the mid-Atlantic coastline. Statistical analyses linking the observed surface-pressure decrease with both the Gulf Stream frontal location and the prestorm coastal baroclinic conditions are presented. These results quantitatively indicate that Gulf Stream-induced wintertime baroclinicity may significantly affect the regional intensification of East Coast winter cyclones.
Abstract
Current land surface schemes used for mesoscale weather forecast models use the Jarvis-type stomatal resistance formulations for representing the vegetation transpiration processes. The Jarvis scheme, however, despite its robustness, needs significant tuning of the hypothetical minimum-stomatal resistance term to simulate surface energy balances. In this study, the authors show that the Jarvis-type stomatal resistance/transpiration model can be efficiently replaced in a coupled land–atmosphere model with a photosynthesis-based scheme and still achieve dynamically consistent results. To demonstrate this transformative potential, the authors developed and coupled a photosynthesis, gas exchange–based surface evapotranspiration model (GEM) as a land surface scheme for mesoscale weather forecasting model applications. The GEM was dynamically coupled with a prognostic soil moisture–soil temperature model and an atmospheric boundary layer (ABL) model. This coupled system was then validated over different natural surfaces including temperate C4 vegetation (prairie grass and corn field) and C3 vegetation (soybean, fallow, and hardwood forest) under contrasting surface conditions (such as different soil moisture and leaf area index). Results indicated that the coupled model was able to realistically simulate the surface fluxes and the boundary layer characteristics over different landscapes. The surface energy fluxes, particularly for latent heat, are typically within 10%–20% of the observations without any tuning of the biophysical–vegetation characteristics, and the response to the changes in the surface characteristics is consistent with observations and theory. This result shows that photosynthesis-based transpiration/stomatal resistance models such as GEM, despite various complexities, can be applied for mesoscale weather forecasting applications. Future efforts for understanding the different scaling parameterizations and for correcting errors for low soil moisture and/or wilting vegetation conditions are necessary to improve model performance. Results from this study suggest that the GEM approach using the photosynthesis-based soil vegetation atmosphere transfer (SVAT) scheme is thus superior to the Jarvis-based approaches. Currently GEM is being implemented within the Noah land surface model for the community Weather Research and Forecasting (WRF) Advanced Research Version Modeling System (ARW) and the NCAR high-resolution land data assimilation system (HRLDAS), and validation is under way.
Abstract
Current land surface schemes used for mesoscale weather forecast models use the Jarvis-type stomatal resistance formulations for representing the vegetation transpiration processes. The Jarvis scheme, however, despite its robustness, needs significant tuning of the hypothetical minimum-stomatal resistance term to simulate surface energy balances. In this study, the authors show that the Jarvis-type stomatal resistance/transpiration model can be efficiently replaced in a coupled land–atmosphere model with a photosynthesis-based scheme and still achieve dynamically consistent results. To demonstrate this transformative potential, the authors developed and coupled a photosynthesis, gas exchange–based surface evapotranspiration model (GEM) as a land surface scheme for mesoscale weather forecasting model applications. The GEM was dynamically coupled with a prognostic soil moisture–soil temperature model and an atmospheric boundary layer (ABL) model. This coupled system was then validated over different natural surfaces including temperate C4 vegetation (prairie grass and corn field) and C3 vegetation (soybean, fallow, and hardwood forest) under contrasting surface conditions (such as different soil moisture and leaf area index). Results indicated that the coupled model was able to realistically simulate the surface fluxes and the boundary layer characteristics over different landscapes. The surface energy fluxes, particularly for latent heat, are typically within 10%–20% of the observations without any tuning of the biophysical–vegetation characteristics, and the response to the changes in the surface characteristics is consistent with observations and theory. This result shows that photosynthesis-based transpiration/stomatal resistance models such as GEM, despite various complexities, can be applied for mesoscale weather forecasting applications. Future efforts for understanding the different scaling parameterizations and for correcting errors for low soil moisture and/or wilting vegetation conditions are necessary to improve model performance. Results from this study suggest that the GEM approach using the photosynthesis-based soil vegetation atmosphere transfer (SVAT) scheme is thus superior to the Jarvis-based approaches. Currently GEM is being implemented within the Noah land surface model for the community Weather Research and Forecasting (WRF) Advanced Research Version Modeling System (ARW) and the NCAR high-resolution land data assimilation system (HRLDAS), and validation is under way.
Abstract
A satellite–model coupled procedure for assimilating geostationary satellite sounder data was adapted to a mesoscale analysis and forecast system jointly developed by the Naval Research Laboratory and the Air Force Research Laboratory. The coupled procedure involves the use of the model output fields as the first guess for the thermodynamic retrievals. Atmospheric thermodynamic profiles and ground temperatures were retrieved from observed radiances of the VISSR Atmospheric Sounder (VAS) on board the Geostationary Operational Environmental Satellite. The successive corrections objective analysis scheme in the mesoscale analysis and forecast system was modified to consider the horizontal spatial correlation of the satellite data. The procedure was tested using a wintertime case from the 1986 Genesis of Atlantic Lows Experiment project. The retrievals generated by the coupled method were modestly improved relative to independent stand-alone retrievals. Coupled analyses and forecasts of temperature and moisture fields compared favorably to forecasts from a control run without the VAS assimilation.
Abstract
A satellite–model coupled procedure for assimilating geostationary satellite sounder data was adapted to a mesoscale analysis and forecast system jointly developed by the Naval Research Laboratory and the Air Force Research Laboratory. The coupled procedure involves the use of the model output fields as the first guess for the thermodynamic retrievals. Atmospheric thermodynamic profiles and ground temperatures were retrieved from observed radiances of the VISSR Atmospheric Sounder (VAS) on board the Geostationary Operational Environmental Satellite. The successive corrections objective analysis scheme in the mesoscale analysis and forecast system was modified to consider the horizontal spatial correlation of the satellite data. The procedure was tested using a wintertime case from the 1986 Genesis of Atlantic Lows Experiment project. The retrievals generated by the coupled method were modestly improved relative to independent stand-alone retrievals. Coupled analyses and forecasts of temperature and moisture fields compared favorably to forecasts from a control run without the VAS assimilation.
Abstract
The successive correction scheme of Bratseth, which converges to optimum interpolation, is applied for the numerical analysis of data collected during the Genesis of Atlantic Lows Experiment. A first guess for the analysis is provided by a 12-h forecast produced by integrating a limited-area model from a prior coarse operational analysis. Initially, univariate analyses of the mass and wind fields are produced. To achieve the coupling of the mass and wind fields, additional iterations on the geopotential are performed by extrapolating the geopotential to grid points, using improving estimates of the geostrophic wind. This improved geostrophic wind is then used to update the geostrophic component of the initial univariate wind analysis. Use of a background forecast produces much improved mesoscale structures in the analysis. Enhanced gradients of the geopotential and larger wind shears are the result of the coupling of the mass and wind fields, particularly in regions of lower data density. Application of the vertical mode initialization scheme of Bourke and McGregor is used to diagnose the divergent component of the mesoscale circulations produced with the analysis scheme.
Abstract
The successive correction scheme of Bratseth, which converges to optimum interpolation, is applied for the numerical analysis of data collected during the Genesis of Atlantic Lows Experiment. A first guess for the analysis is provided by a 12-h forecast produced by integrating a limited-area model from a prior coarse operational analysis. Initially, univariate analyses of the mass and wind fields are produced. To achieve the coupling of the mass and wind fields, additional iterations on the geopotential are performed by extrapolating the geopotential to grid points, using improving estimates of the geostrophic wind. This improved geostrophic wind is then used to update the geostrophic component of the initial univariate wind analysis. Use of a background forecast produces much improved mesoscale structures in the analysis. Enhanced gradients of the geopotential and larger wind shears are the result of the coupling of the mass and wind fields, particularly in regions of lower data density. Application of the vertical mode initialization scheme of Bourke and McGregor is used to diagnose the divergent component of the mesoscale circulations produced with the analysis scheme.
Abstract
A soil–vegetation–atmospheric boundary layer model was developed to study the performance of two local-closure and two nonlocal-closure boundary layer mixing schemes for use in meteorological and air quality simulation models. Full interaction between the surface and atmosphere is achieved by representing surface characteristics and associated processes using a prognostic soil–vegetation scheme and atmospheric boundary layer schemes. There are 30 layers in the lowest 3 km of the model with a high resolution near the surface. The four boundary layer schemes are tested by simulating atmospheric boundary layer structures over densely and sparsely vegetated regions using the observational data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) and from Wangara.
Simulation results indicate that the near-surface turbulent fluxes predicted by the four boundary layer schemes differ from each other, even though the formulation used to represent the surface-layer processes is the same. These differences arise from the differing ways of representing subgrid-scale vertical mixing processes. Results also indicate that the vertical profiles of predicted parameters (i.e., temperature, mixing ratio, and horizontal winds) from the four mixed-layer schemes differ from each other, particularly during the daytime growth of the mixed layer. During the evening hours, after the mixed layer has reached its maximum depth, the differences among these respective predicted variables are found to be insignificant.
There were some general features that were associated with each of the schemes in all of the simulations. Compared with observations, in all of the cases the simulated maximum depths of the boundary layer for each scheme were consistently either lower or higher, superadiabatic lapse rates were consistently either stronger or weaker, and the intensity of the vertical mixing was either stronger or weaker. Also, throughout the simulation period in all case studies, most of the differences in the predicted parameters are present in the surface layer and near the top of the mixed layer.
Abstract
A soil–vegetation–atmospheric boundary layer model was developed to study the performance of two local-closure and two nonlocal-closure boundary layer mixing schemes for use in meteorological and air quality simulation models. Full interaction between the surface and atmosphere is achieved by representing surface characteristics and associated processes using a prognostic soil–vegetation scheme and atmospheric boundary layer schemes. There are 30 layers in the lowest 3 km of the model with a high resolution near the surface. The four boundary layer schemes are tested by simulating atmospheric boundary layer structures over densely and sparsely vegetated regions using the observational data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) and from Wangara.
Simulation results indicate that the near-surface turbulent fluxes predicted by the four boundary layer schemes differ from each other, even though the formulation used to represent the surface-layer processes is the same. These differences arise from the differing ways of representing subgrid-scale vertical mixing processes. Results also indicate that the vertical profiles of predicted parameters (i.e., temperature, mixing ratio, and horizontal winds) from the four mixed-layer schemes differ from each other, particularly during the daytime growth of the mixed layer. During the evening hours, after the mixed layer has reached its maximum depth, the differences among these respective predicted variables are found to be insignificant.
There were some general features that were associated with each of the schemes in all of the simulations. Compared with observations, in all of the cases the simulated maximum depths of the boundary layer for each scheme were consistently either lower or higher, superadiabatic lapse rates were consistently either stronger or weaker, and the intensity of the vertical mixing was either stronger or weaker. Also, throughout the simulation period in all case studies, most of the differences in the predicted parameters are present in the surface layer and near the top of the mixed layer.
Abstract
Hurricane Opal (1995) experienced a rapid, unexpected intensification in the Gulf of Mexico that coincided with its encounter with a warm core ring (WCR). The relative positions of Opal and the WCR and the timing of the intensification indicate strong air–sea interactions between the tropical cyclone and the ocean. To study the mutual response of Opal and the Gulf of Mexico, a coupled model is used consisting of a nonhydrostatic atmospheric component of the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), and the hydrostatic Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model version 2 (MOM 2).
The coupling between the ocean and the atmosphere components of the model are accomplished by conservation of heat, salt, momentum, as well as the sensible and latent heat fluxes at the air–sea interface. The atmospheric model has two nests with spatial resolutions of 0.6° and 0.2°. The ocean model has a uniform resolution of 0.2°. The oceanic model domain covers the Gulf of Mexico basin and coincides with a fine-mesh atmospheric domain of the COAMPS. The initial condition for the atmospheric component of COAMPS is the archived Navy Operational Global Atmospheric Prediction System operational global analysis, enhanced with observations. The initial ocean condition for the oceanic component is obtained from a 2-yr MOM 2 simulation with climatological forcing and fixed mass inflow into the Gulf. The initial state in the Gulf of Mexico consists of a realistic Loop Current and a shed WCR.
The 72-h simulation of the coupled system starting from 1200 UTC 2 October 1995 reproduces the observed storm intensity with a minimum sea level pressure (MSLP) of 918 hPa, occurring at 1800 UTC 4 October, a 6-h delay compared to the observation. The rapid intensification to the maximum intensity and the subsequent weakening are not as dramatic as the observed. The simulated track is located slightly to the east of the observed track, placing it directly over the simulated WCR, where the sea surface temperature (SST) cooling is approximately 0.5°C, consistent with buoy measurements acquired within the WCR. This cooling is significantly less over the WCR than over the common Gulf water due to the deeper and warmer layers in the WCR. Wind-induced currents of 150 cm s−1 are similar to those in earlier idealized simulations, and the forced current field in Opal’s wake is characterized by near-inertial oscillations superimposed on the anticyclonic circulation around the WCR.
Several numerical experiments are conducted to isolate the effects of the WCR and the ocean–atmosphere coupling. The major findings of these numerical experiments are summarized as follows.
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Opal intensifies an additional 17 hPa between the times when Opal’s center enters and exits the outer edge of the WCR. Without the WCR, Opal only intensifies another 7 hPa in the same period.
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The maximum surface sensible and latent heat flux amounts to 2842 W m−2. This occurs when Opal’s surface circulation brings northwesterly flow over the SST gradient in the northwestern quadrant of the WCR.
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Opal extracts 40% of the available heat capacity (temperature greater than 26°C) from the WCR.
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While the WCR enhances the tropical cyclone and ocean coupling as indicated by strong interfacial fluxes, it reduces the negative feedback. The negative feedback of the induced SST cooling to Hurricane Opal is 5 hPa. This small feedback is due to the relatively large heat content of the WCR, and the negative feedback is stronger in the absence of the WCR, producing a difference of 8 hPa in the MSLP of Opal.
Abstract
Hurricane Opal (1995) experienced a rapid, unexpected intensification in the Gulf of Mexico that coincided with its encounter with a warm core ring (WCR). The relative positions of Opal and the WCR and the timing of the intensification indicate strong air–sea interactions between the tropical cyclone and the ocean. To study the mutual response of Opal and the Gulf of Mexico, a coupled model is used consisting of a nonhydrostatic atmospheric component of the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), and the hydrostatic Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model version 2 (MOM 2).
The coupling between the ocean and the atmosphere components of the model are accomplished by conservation of heat, salt, momentum, as well as the sensible and latent heat fluxes at the air–sea interface. The atmospheric model has two nests with spatial resolutions of 0.6° and 0.2°. The ocean model has a uniform resolution of 0.2°. The oceanic model domain covers the Gulf of Mexico basin and coincides with a fine-mesh atmospheric domain of the COAMPS. The initial condition for the atmospheric component of COAMPS is the archived Navy Operational Global Atmospheric Prediction System operational global analysis, enhanced with observations. The initial ocean condition for the oceanic component is obtained from a 2-yr MOM 2 simulation with climatological forcing and fixed mass inflow into the Gulf. The initial state in the Gulf of Mexico consists of a realistic Loop Current and a shed WCR.
The 72-h simulation of the coupled system starting from 1200 UTC 2 October 1995 reproduces the observed storm intensity with a minimum sea level pressure (MSLP) of 918 hPa, occurring at 1800 UTC 4 October, a 6-h delay compared to the observation. The rapid intensification to the maximum intensity and the subsequent weakening are not as dramatic as the observed. The simulated track is located slightly to the east of the observed track, placing it directly over the simulated WCR, where the sea surface temperature (SST) cooling is approximately 0.5°C, consistent with buoy measurements acquired within the WCR. This cooling is significantly less over the WCR than over the common Gulf water due to the deeper and warmer layers in the WCR. Wind-induced currents of 150 cm s−1 are similar to those in earlier idealized simulations, and the forced current field in Opal’s wake is characterized by near-inertial oscillations superimposed on the anticyclonic circulation around the WCR.
Several numerical experiments are conducted to isolate the effects of the WCR and the ocean–atmosphere coupling. The major findings of these numerical experiments are summarized as follows.
-
Opal intensifies an additional 17 hPa between the times when Opal’s center enters and exits the outer edge of the WCR. Without the WCR, Opal only intensifies another 7 hPa in the same period.
-
The maximum surface sensible and latent heat flux amounts to 2842 W m−2. This occurs when Opal’s surface circulation brings northwesterly flow over the SST gradient in the northwestern quadrant of the WCR.
-
Opal extracts 40% of the available heat capacity (temperature greater than 26°C) from the WCR.
-
While the WCR enhances the tropical cyclone and ocean coupling as indicated by strong interfacial fluxes, it reduces the negative feedback. The negative feedback of the induced SST cooling to Hurricane Opal is 5 hPa. This small feedback is due to the relatively large heat content of the WCR, and the negative feedback is stronger in the absence of the WCR, producing a difference of 8 hPa in the MSLP of Opal.
Abstract
Data from the Special Sensor Microwave/Imager (SSM/I) on board a Defense Meteorological Satellite Program satellite are used to study the precipitation patterns and wind fields associated with Hurricane Florence (1988). SSM/I estimates indicate that the intensification of Florence was coincident with the increase in total latent beat release. Additionally, an increase in the concentration and areal coverage of heavier rain rates near the center is observed. SSM/I marine surface winds of Florence are examined and compared to in situ data, and to an enhanced objective isotach analysis over the Gulf of Mexico. Results indicate that the SSM/I winds are weaker than those depicted in the enhanced objective analysis and slightly stronger than in situ observations. Finally, center positions of Florence are estimated using the 85-GHz brightness temperature imagery. Much improved estimates are achieved using this imagery compared to using GOES infrared imagery. These results concur with previous studies that applications of SSM/I data could be valuable in augmenting current methods of tropical cyclone analysis.
Abstract
Data from the Special Sensor Microwave/Imager (SSM/I) on board a Defense Meteorological Satellite Program satellite are used to study the precipitation patterns and wind fields associated with Hurricane Florence (1988). SSM/I estimates indicate that the intensification of Florence was coincident with the increase in total latent beat release. Additionally, an increase in the concentration and areal coverage of heavier rain rates near the center is observed. SSM/I marine surface winds of Florence are examined and compared to in situ data, and to an enhanced objective isotach analysis over the Gulf of Mexico. Results indicate that the SSM/I winds are weaker than those depicted in the enhanced objective analysis and slightly stronger than in situ observations. Finally, center positions of Florence are estimated using the 85-GHz brightness temperature imagery. Much improved estimates are achieved using this imagery compared to using GOES infrared imagery. These results concur with previous studies that applications of SSM/I data could be valuable in augmenting current methods of tropical cyclone analysis.
Abstract
A system for the frequent intermittent assimilation of surface observations into a mesoscale model is described. The assimilation begins by transforming the surface observations to model coordinates. Next, the lowest-level model fields of potential temperature, relative humidity, u and v component winds, and surface pressure are updated by an objective analysis using the successive correction approach. The deviations of the analysis from the first guess at the lowest model layer are then used to adjust the other model layers within the planetary boundary layer. The PBL adjustment is carried out by using the model's values of eddy diffusivity, which are nudged to reflect the updated conditions, to determine the influence of the lowest-layer deviations on the other model layers. Results from a case study indicate that the frequent intermittent assimilation of surface data can provide superior mososcale analyses and forecasts compared to assimilation of synoptic data only. The inclusion of the PBL adjustment procedure is an important part of generating the better forecasts. Extrapolation of the results here suggests that two-dimensional data can be successfully assimilated into a model provided there is a mechanism to smoothly blend the data into the third dimension.
Abstract
A system for the frequent intermittent assimilation of surface observations into a mesoscale model is described. The assimilation begins by transforming the surface observations to model coordinates. Next, the lowest-level model fields of potential temperature, relative humidity, u and v component winds, and surface pressure are updated by an objective analysis using the successive correction approach. The deviations of the analysis from the first guess at the lowest model layer are then used to adjust the other model layers within the planetary boundary layer. The PBL adjustment is carried out by using the model's values of eddy diffusivity, which are nudged to reflect the updated conditions, to determine the influence of the lowest-layer deviations on the other model layers. Results from a case study indicate that the frequent intermittent assimilation of surface data can provide superior mososcale analyses and forecasts compared to assimilation of synoptic data only. The inclusion of the PBL adjustment procedure is an important part of generating the better forecasts. Extrapolation of the results here suggests that two-dimensional data can be successfully assimilated into a model provided there is a mechanism to smoothly blend the data into the third dimension.
Abstract
Diabatic forcing has been incorporated into a nonlinear normal-mode initialization scheme to provide more realistic initial conditions and to alleviate the problem of the spinup time of the Naval Research Laboratory Limited-Area Numerical Weather Prediction Model. Latent heating profiles are computed from the observed rainfall and from the model-generated convective rainfall at locations where there were no observations. The latent heating is distributed in the vertical according to the cumulus convective parameterization scheme (Kuo scheme) of the model. The results of a case study from the Genesis of Atlantic Lows Experiment indicated that model spinup of forecast rainfall can be reduced when diabatic initialization with merging of heat and/or rain is used.
Abstract
Diabatic forcing has been incorporated into a nonlinear normal-mode initialization scheme to provide more realistic initial conditions and to alleviate the problem of the spinup time of the Naval Research Laboratory Limited-Area Numerical Weather Prediction Model. Latent heating profiles are computed from the observed rainfall and from the model-generated convective rainfall at locations where there were no observations. The latent heating is distributed in the vertical according to the cumulus convective parameterization scheme (Kuo scheme) of the model. The results of a case study from the Genesis of Atlantic Lows Experiment indicated that model spinup of forecast rainfall can be reduced when diabatic initialization with merging of heat and/or rain is used.
Abstract
Fields of rainfall rates, integrated water vapor (IWV), and marine surface wind speeds retrieved by the Special Sensor Microwave/Imager (SSM/I) during the intensive observational period 4 on 4 January 1989 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) were analyzed. Subjectively analyzed and model-simulated frontal structures were used to examine the spatial relationship of the SSM/I observed fields to the rapidly intensifying storm and the associated fronts. Qualitative and quantitative comparisons of SSM/I retrievals with GOES imagery, conventional observations, and results produced from the Naval Research Laboratory's (NRL) limited-area numerical model were also made.
SSM/I rainfall was found along the cold and warm fronts, with heavy precipitation within frontal bands. The spatial pattern and characteristics of SSM/I precipitation closely resembled those simulated by the model. Both the warm and the cold front were found to be located near the area of the strongest gradient in IWV. In the warm sector, areas of IWV greater than 40 mm were found, an amount supported by model simulations. Both SSM/I rain rate and IWV distribution were found to be useful in locating the cold and warm fronts. There was good agreement on the relationship of frontal locations to the precipitation patterns and IWV gradients. Most of the high-wind area near the storm center was obscured by clouds for marine surface wind retrieval. SSM/I-retrieved marine surface winds outside the cloud shield (flag 0) were compared to ship- and buoy-reported winds. It was found that the retrieved wind estimates were within 0–3 m s−1 of in situ observation over areas of slow wind shifts. The errors became larger in regions of rapid wind shifts.
Abstract
Fields of rainfall rates, integrated water vapor (IWV), and marine surface wind speeds retrieved by the Special Sensor Microwave/Imager (SSM/I) during the intensive observational period 4 on 4 January 1989 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) were analyzed. Subjectively analyzed and model-simulated frontal structures were used to examine the spatial relationship of the SSM/I observed fields to the rapidly intensifying storm and the associated fronts. Qualitative and quantitative comparisons of SSM/I retrievals with GOES imagery, conventional observations, and results produced from the Naval Research Laboratory's (NRL) limited-area numerical model were also made.
SSM/I rainfall was found along the cold and warm fronts, with heavy precipitation within frontal bands. The spatial pattern and characteristics of SSM/I precipitation closely resembled those simulated by the model. Both the warm and the cold front were found to be located near the area of the strongest gradient in IWV. In the warm sector, areas of IWV greater than 40 mm were found, an amount supported by model simulations. Both SSM/I rain rate and IWV distribution were found to be useful in locating the cold and warm fronts. There was good agreement on the relationship of frontal locations to the precipitation patterns and IWV gradients. Most of the high-wind area near the storm center was obscured by clouds for marine surface wind retrieval. SSM/I-retrieved marine surface winds outside the cloud shield (flag 0) were compared to ship- and buoy-reported winds. It was found that the retrieved wind estimates were within 0–3 m s−1 of in situ observation over areas of slow wind shifts. The errors became larger in regions of rapid wind shifts.
