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- Author or Editor: Mercedes N. Lakhtakia x
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Abstract
Alternative treatments of the hydrologic and thermodynamic processes at the earth's surface within a mesoscale model are discussed in this study. Specifically, the question of under what circumstances it is necessary to use a complex surface parameterization scheme as opposed to simpler ones is addressed.
Three versions of a one-dimensional planetary boundary layer model were employed, where the primary differences among them are in their surface modules. One uses a simple treatment of the surface characteristics (time independent). In another, the surface processes are represented by a complex surface physics-soil hydrology scheme, while the third one is similar to the first one but the moisture-availability parameter has a specified temporal variation during and after a precipitation event.
Several numerical simulations were performed. They showed that the models’ solutions differ the most when the vegetation cover and the surface net radiative flux are large, and the soil-water content cannot satisfy the evapotranspiration demand. When a precipitation event is present during the simulation period, the largest differences are apparent when the preprecipitation surface evapotranspiration is restricted and the precipitation event occurs in the morning. The simulations also showed that the upgraded simple scheme can sometimes represent a satisfactory substitute for the simple scheme when a precipitation event is present during the simulation period.
Abstract
Alternative treatments of the hydrologic and thermodynamic processes at the earth's surface within a mesoscale model are discussed in this study. Specifically, the question of under what circumstances it is necessary to use a complex surface parameterization scheme as opposed to simpler ones is addressed.
Three versions of a one-dimensional planetary boundary layer model were employed, where the primary differences among them are in their surface modules. One uses a simple treatment of the surface characteristics (time independent). In another, the surface processes are represented by a complex surface physics-soil hydrology scheme, while the third one is similar to the first one but the moisture-availability parameter has a specified temporal variation during and after a precipitation event.
Several numerical simulations were performed. They showed that the models’ solutions differ the most when the vegetation cover and the surface net radiative flux are large, and the soil-water content cannot satisfy the evapotranspiration demand. When a precipitation event is present during the simulation period, the largest differences are apparent when the preprecipitation surface evapotranspiration is restricted and the precipitation event occurs in the morning. The simulations also showed that the upgraded simple scheme can sometimes represent a satisfactory substitute for the simple scheme when a precipitation event is present during the simulation period.
Abstract
An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9–10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML).
The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the precipitation, during this case. Compared to the precipitation amounts produced by the complete model during the 9-h period of heavy precipitation (2100–0600 GMT), only 3% was produced after the elimination of the surface differential heating associated with the cloud-cover and soil moisture-availability gradients. The elimination of the latent-heating feedback in the model atmosphere caused a decrease in the 18-h precipitation amounts of ∼50%. Finally, the strong gradient in the low-level mixing ratio along the edge of the EML had a surprisingly important direct dynamic influence on the underrunning, and consequently on the precipitation.
Abstract
An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9–10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML).
The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the precipitation, during this case. Compared to the precipitation amounts produced by the complete model during the 9-h period of heavy precipitation (2100–0600 GMT), only 3% was produced after the elimination of the surface differential heating associated with the cloud-cover and soil moisture-availability gradients. The elimination of the latent-heating feedback in the model atmosphere caused a decrease in the 18-h precipitation amounts of ∼50%. Finally, the strong gradient in the low-level mixing ratio along the edge of the EML had a surprisingly important direct dynamic influence on the underrunning, and consequently on the precipitation.
The purpose of this study is to demonstrate the feasibility of determining the soil-water content fields required as initial conditions for land surface components within atmospheric prediction models. This is done using a model of the hydrologic balance and conventional meteorological observations, land cover, and soils information.
A discussion is presented of the subgrid-scale effects, the integration time, and the choice of vegetation type on the soil-water content patterns. Finally, comparisons are made between two The Pennsylvania State University/National Center for Atmospheric Research mesoscale model simulations, one using climatological fields and the other one using the soil-moisture fields produced by this new method.
The purpose of this study is to demonstrate the feasibility of determining the soil-water content fields required as initial conditions for land surface components within atmospheric prediction models. This is done using a model of the hydrologic balance and conventional meteorological observations, land cover, and soils information.
A discussion is presented of the subgrid-scale effects, the integration time, and the choice of vegetation type on the soil-water content patterns. Finally, comparisons are made between two The Pennsylvania State University/National Center for Atmospheric Research mesoscale model simulations, one using climatological fields and the other one using the soil-moisture fields produced by this new method.
Abstract
The Penn State/NCAR mesoscale model is initialized with calm winds, a barotropic temperature pattern, and a uniform surface pressure in studies of the response of the marine atmospheric boundary layer (MABL) to realistic differential fluxes of heal and moisture at the sea surface in the vicinity of the Gulf Stream. A maritime sounding from the GALE data region during Intensive Observation Period 2 (IOP 2) is used to define the initial vertical structure of the temperature and humidity fields. The sensitivity of the MABL to two sea surface temperature (SST) patterns is tested. One is a relatively smooth analysis that is typical of those used by research and operational models applied on the synoptic scale and mesoscale. The other is based on the experimental 14 km high-resolution analysis of NOAA. In addition, other simulations are used to determine the sensitivity of the MABL response to physical factors such as surface moisture fluxes, latent heating, and the sea-surface roughness. These studies have two purposes: one is to provide a better understanding of the three- dimensional MABL response to a realistic SST pattern; the other is to isolate the mesoscale circulations produced by this differential thermal forcing so that their interaction with other processes, such as cyclogenesis, can be inferred in real-data simulations.
The results of simulations using the two SST analyses are quite different. For example, the MABL front that develops near the north wall of the Gulf Stream is much stronger with the high-resolution analysis. Horizontal temperature gradients below 950 mb are 2–3 times larger, horizontal velocities near the surface are in excess of 7 m s−1 instead of ∼2 m s−1, and the vertical velocity patterns showed significantly different spatial characteristics and amplitudes. In both simulations, responses to the surface forcing extended upward to about 800 mb. In the experiment with the high-resolution SST analysis, a moderately strong mesoscale circulation was produced in the MABL within 12 h. Additional factors found to be important contributors to the MABL response are latent heat release in the lower atmosphere and sea-surface fluxes of moisture. The enhancement of the heat and moisture fluxes associated with the higher winds in the vicinity of the MABL front also significantly contributes to the amplitude of the circulation.
Abstract
The Penn State/NCAR mesoscale model is initialized with calm winds, a barotropic temperature pattern, and a uniform surface pressure in studies of the response of the marine atmospheric boundary layer (MABL) to realistic differential fluxes of heal and moisture at the sea surface in the vicinity of the Gulf Stream. A maritime sounding from the GALE data region during Intensive Observation Period 2 (IOP 2) is used to define the initial vertical structure of the temperature and humidity fields. The sensitivity of the MABL to two sea surface temperature (SST) patterns is tested. One is a relatively smooth analysis that is typical of those used by research and operational models applied on the synoptic scale and mesoscale. The other is based on the experimental 14 km high-resolution analysis of NOAA. In addition, other simulations are used to determine the sensitivity of the MABL response to physical factors such as surface moisture fluxes, latent heating, and the sea-surface roughness. These studies have two purposes: one is to provide a better understanding of the three- dimensional MABL response to a realistic SST pattern; the other is to isolate the mesoscale circulations produced by this differential thermal forcing so that their interaction with other processes, such as cyclogenesis, can be inferred in real-data simulations.
The results of simulations using the two SST analyses are quite different. For example, the MABL front that develops near the north wall of the Gulf Stream is much stronger with the high-resolution analysis. Horizontal temperature gradients below 950 mb are 2–3 times larger, horizontal velocities near the surface are in excess of 7 m s−1 instead of ∼2 m s−1, and the vertical velocity patterns showed significantly different spatial characteristics and amplitudes. In both simulations, responses to the surface forcing extended upward to about 800 mb. In the experiment with the high-resolution SST analysis, a moderately strong mesoscale circulation was produced in the MABL within 12 h. Additional factors found to be important contributors to the MABL response are latent heat release in the lower atmosphere and sea-surface fluxes of moisture. The enhancement of the heat and moisture fluxes associated with the higher winds in the vicinity of the MABL front also significantly contributes to the amplitude of the circulation.
