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- Author or Editor: S. G. Gopalakrishnan x
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Abstract
A systematic analysis of the impacts of heat patches and topographical features on the dispersion of passive materials in a shear-free convective boundary layer (CBL) was performed. Large eddy simulations and a Lagrangian particle dispersion model were used for that purpose. Over a homogeneous, flat terrain, the dispersion statistics produced by the model are in agreement with convection tank data and other model results. The horizontal pressure gradients created by surface heat flux heterogeneities generate atmospheric circulations, which impede vertical mixing and, as a result, have a remarkable influence on particle dispersion in the CBL. For a near-surface release, the particles are advected horizontally rather than “lifted-off,” maintaining a high concentration near the ground surface. Particles released at higher elevations reach the ground surface more slowly than when released above a flat, homogeneous domain. In a shear-free CBL, hilly terrain has little impact on lift-off, dimensionless crosswind-integrated concentration, mean particle height, particle spread, and near-ground-level concentration of particles released near the ground surface. This is true even with hills as high as 25% of the height of the CBL. However, it has a noticeable effect on the dispersion statistics of particles released from higher elevations. In particular, the locus of the maximum in crosswind-integrated concentration of particles released from a source located about 25% of the height of the CBL descends to the surface of an even moderate hill noticeably slower than above a flat, homogeneous domain.
Abstract
A systematic analysis of the impacts of heat patches and topographical features on the dispersion of passive materials in a shear-free convective boundary layer (CBL) was performed. Large eddy simulations and a Lagrangian particle dispersion model were used for that purpose. Over a homogeneous, flat terrain, the dispersion statistics produced by the model are in agreement with convection tank data and other model results. The horizontal pressure gradients created by surface heat flux heterogeneities generate atmospheric circulations, which impede vertical mixing and, as a result, have a remarkable influence on particle dispersion in the CBL. For a near-surface release, the particles are advected horizontally rather than “lifted-off,” maintaining a high concentration near the ground surface. Particles released at higher elevations reach the ground surface more slowly than when released above a flat, homogeneous domain. In a shear-free CBL, hilly terrain has little impact on lift-off, dimensionless crosswind-integrated concentration, mean particle height, particle spread, and near-ground-level concentration of particles released near the ground surface. This is true even with hills as high as 25% of the height of the CBL. However, it has a noticeable effect on the dispersion statistics of particles released from higher elevations. In particular, the locus of the maximum in crosswind-integrated concentration of particles released from a source located about 25% of the height of the CBL descends to the surface of an even moderate hill noticeably slower than above a flat, homogeneous domain.
Abstract
Five local K-closure formulations and a TKE closure were incorporated in a one-dimensional version of the Pielke’s model, and a comparative evaluation of the closure schemes was made for strong and weak wind stable boundary layer (SBL). The Cabauw (Netherlands) and EPRI-Kincaid site (United States) observations were used for this purpose. The results indicate that for the strong wind case study, the profiles of turbulent diffusivities in terms of shape, depth of significant mixing, and the height above the surface where diffusion reaches a maximum are more or less the same for the various closure schemes. Only the magnitudes of mixing produced by various closure schemes are different. This difference produced by various closure formulations causes minor but noticeable changes in the mean wind field and thermodynamic structure of the model SBL. However, although the profiles of turbulent diffusivities become weak, variable, and poorly defined under weak wind conditions, the mean profiles become insensitive to the differences in the diffusion that arise due to various parameterization schemes. Apart from the TKE closure scheme, Estournel and Guedalia simple local closure scheme is able to produce the essential features of the SBL quite well.
Abstract
Five local K-closure formulations and a TKE closure were incorporated in a one-dimensional version of the Pielke’s model, and a comparative evaluation of the closure schemes was made for strong and weak wind stable boundary layer (SBL). The Cabauw (Netherlands) and EPRI-Kincaid site (United States) observations were used for this purpose. The results indicate that for the strong wind case study, the profiles of turbulent diffusivities in terms of shape, depth of significant mixing, and the height above the surface where diffusion reaches a maximum are more or less the same for the various closure schemes. Only the magnitudes of mixing produced by various closure schemes are different. This difference produced by various closure formulations causes minor but noticeable changes in the mean wind field and thermodynamic structure of the model SBL. However, although the profiles of turbulent diffusivities become weak, variable, and poorly defined under weak wind conditions, the mean profiles become insensitive to the differences in the diffusion that arise due to various parameterization schemes. Apart from the TKE closure scheme, Estournel and Guedalia simple local closure scheme is able to produce the essential features of the SBL quite well.
Abstract
Extensive contrasts of land surface heterogeneities have a pivotal role in modulating boundary layer processes and consequently, the regional-scale dispersion of air pollutants. The Weather Research and Forecasting (WRF) modeling system has been used to analyze the regional-scale boundary layer features over northern India. Two cases, 9–11 December 2004 and 20–22 May 2005, representing the winter and summer season, respectively, are chosen for the simulations. The model results have been compared with the observations from the India Meteorological Department (IMD) and Wyoming Weather Web data archive over three cities: Delhi, Ahmedabad, and Jodhpur. The simulations show that the thermal stratifications and the associated wind pattern are very well supported by land surface characteristics over the region. The results signify that the underlying land surface along with the prevailing hemispheric-scale meteorological processes (synoptic conditions) is the driver of the simulated patterns. The study implies that thermally driven regional circulations play a major role in the transport of particulate matter from the Thar Desert to Delhi and its neighboring regions during summer.
Abstract
Extensive contrasts of land surface heterogeneities have a pivotal role in modulating boundary layer processes and consequently, the regional-scale dispersion of air pollutants. The Weather Research and Forecasting (WRF) modeling system has been used to analyze the regional-scale boundary layer features over northern India. Two cases, 9–11 December 2004 and 20–22 May 2005, representing the winter and summer season, respectively, are chosen for the simulations. The model results have been compared with the observations from the India Meteorological Department (IMD) and Wyoming Weather Web data archive over three cities: Delhi, Ahmedabad, and Jodhpur. The simulations show that the thermal stratifications and the associated wind pattern are very well supported by land surface characteristics over the region. The results signify that the underlying land surface along with the prevailing hemispheric-scale meteorological processes (synoptic conditions) is the driver of the simulated patterns. The study implies that thermally driven regional circulations play a major role in the transport of particulate matter from the Thar Desert to Delhi and its neighboring regions during summer.
Abstract
Turbulence in stable conditions is local, that is, it is locally defined by small eddies. A local formulation for σ w based on a level 2 approximation of Mellor and Yamada (1974) is proposed. The proposed formulation is able to describe the nondimensional profile of (σ w /U∗)2 against Z/H consistently when compared with the Minnesota observations, where H is the height of the turbulent stable boundary layer.
Abstract
Turbulence in stable conditions is local, that is, it is locally defined by small eddies. A local formulation for σ w based on a level 2 approximation of Mellor and Yamada (1974) is proposed. The proposed formulation is able to describe the nondimensional profile of (σ w /U∗)2 against Z/H consistently when compared with the Minnesota observations, where H is the height of the turbulent stable boundary layer.
Abstract
The major objective of this study was to evaluate at which scale topography starts to significantly affect the mean characteristics and structure of turbulence in the convective boundary layer (CBL). The large eddy simulation option of the Regional Atmospheric Modeling System developed at Colorado State University was used for that purpose. It is found that turbulence is nonlinearly dependent on the scale of the topographical features. At a horizontal length scale of less than about 5 km, topography has very little impact on the mean properties of the CBL, even with hills as high as 30% of the height of the CBL. However, it has a significant impact on the organization of the eddies. At larger horizontal scales, topographical features as small as about 10% of the height of the CBL have some effect on the mean characteristics of the CBL. In particular, a pronounced impact on the “dispersion” statistics (i.e., horizontal and vertical velocity variances and higher moments) is noticed. Furthermore, the mean turbulence kinetic energy profile depicts two maxima, one near the ground surface and one near the top of the CBL, corresponding to the strong horizontal flow that develops near the ground surface and the return flow at the top of the CBL resulting from the organization of eddies into rolls. The larger the sensible heat flux fueling the CBL at the ground surface, the less important this impact is. It is concluded that in a very irregular terrain, where topography presents a vertical scale of at least 200–400 m, and a horizontal scale larger than about 5 km, CBL parameterizations of turbulence currently employed in mesoscale and large-scale atmospheric models (e.g., general circulation models), as well as in dispersion models, need to be improved.
Abstract
The major objective of this study was to evaluate at which scale topography starts to significantly affect the mean characteristics and structure of turbulence in the convective boundary layer (CBL). The large eddy simulation option of the Regional Atmospheric Modeling System developed at Colorado State University was used for that purpose. It is found that turbulence is nonlinearly dependent on the scale of the topographical features. At a horizontal length scale of less than about 5 km, topography has very little impact on the mean properties of the CBL, even with hills as high as 30% of the height of the CBL. However, it has a significant impact on the organization of the eddies. At larger horizontal scales, topographical features as small as about 10% of the height of the CBL have some effect on the mean characteristics of the CBL. In particular, a pronounced impact on the “dispersion” statistics (i.e., horizontal and vertical velocity variances and higher moments) is noticed. Furthermore, the mean turbulence kinetic energy profile depicts two maxima, one near the ground surface and one near the top of the CBL, corresponding to the strong horizontal flow that develops near the ground surface and the return flow at the top of the CBL resulting from the organization of eddies into rolls. The larger the sensible heat flux fueling the CBL at the ground surface, the less important this impact is. It is concluded that in a very irregular terrain, where topography presents a vertical scale of at least 200–400 m, and a horizontal scale larger than about 5 km, CBL parameterizations of turbulence currently employed in mesoscale and large-scale atmospheric models (e.g., general circulation models), as well as in dispersion models, need to be improved.
Abstract
Idealized coupled tropical cyclone (TC) simulations are conducted to isolate ocean impacts on intensity forecasts. A one-dimensional ocean model is embedded into the Hurricane Weather Research and Forecasting (HWRF) mesoscale atmospheric forecast model. By inserting an initial vortex into a horizontally uniform atmosphere above a horizontally uniform ocean, the SST cooling rate becomes the dominant large-scale process controlling intensity evolution. Westward storm translation is introduced by bodily advecting ocean fields toward the east. The ocean model produces a realistic cold wake structure allowing the sensitivity of quasi-equilibrium intensity to storm (translation speed, size) and ocean (heat potential) parameters to be quantified. The atmosphere provides feedback through adjustments in 10-m temperature and humidity that reduce SST cooling impact on quasi-equilibrium intensity by up to 40%. When storms encounter an oceanic region with different heat potential, enthalpy flux adjustment is governed primarily by changes in air–sea temperature and humidity differences that respond within 2–4 h in the inner-core region, and secondarily by wind speed changes occurring over a time interval up to 18 h after the transition. Atmospheric feedback always acts to limit the change in enthalpy flux and intensity through adjustments in 10-m temperature and humidity. Intensity change is asymmetric, with a substantially smaller increase for storms encountering larger heat potential compared to the decrease for storms encountering smaller potential. The smaller increase results initially from the smaller wind speed present at the transition time plus stronger limiting atmospheric feedback. The smaller wind speed increase resulting from these two factors further enhances the asymmetry.
Abstract
Idealized coupled tropical cyclone (TC) simulations are conducted to isolate ocean impacts on intensity forecasts. A one-dimensional ocean model is embedded into the Hurricane Weather Research and Forecasting (HWRF) mesoscale atmospheric forecast model. By inserting an initial vortex into a horizontally uniform atmosphere above a horizontally uniform ocean, the SST cooling rate becomes the dominant large-scale process controlling intensity evolution. Westward storm translation is introduced by bodily advecting ocean fields toward the east. The ocean model produces a realistic cold wake structure allowing the sensitivity of quasi-equilibrium intensity to storm (translation speed, size) and ocean (heat potential) parameters to be quantified. The atmosphere provides feedback through adjustments in 10-m temperature and humidity that reduce SST cooling impact on quasi-equilibrium intensity by up to 40%. When storms encounter an oceanic region with different heat potential, enthalpy flux adjustment is governed primarily by changes in air–sea temperature and humidity differences that respond within 2–4 h in the inner-core region, and secondarily by wind speed changes occurring over a time interval up to 18 h after the transition. Atmospheric feedback always acts to limit the change in enthalpy flux and intensity through adjustments in 10-m temperature and humidity. Intensity change is asymmetric, with a substantially smaller increase for storms encountering larger heat potential compared to the decrease for storms encountering smaller potential. The smaller increase results initially from the smaller wind speed present at the transition time plus stronger limiting atmospheric feedback. The smaller wind speed increase resulting from these two factors further enhances the asymmetry.
Abstract
The major objective of this study was to analyze the mean structure and evolution of the nocturnal boundary layer (NBL) under strong and weak wind conditions. Meteorological data collected during the plume-validation experiment conducted by the Electric Power Research Institute (EPRI) over a flat homogeneous terrain at Kincaid, Illinois (39°35′N, 89°25′W), were utilized. A one-dimensional meteorological boundary layer model originally developed by R. A. Pielke, modified with turbulent kinetic energy mixing-length closure, a layer-by-layer emissivity-based radiation scheme, and nonlinear nondimensional temperature and wind profiles in the surface layer, was used. In the four cases that were considered, ranging from strong to weak geostrophic forcing, the model reproduced the observed mean profiles, their evolutions in the NBL, and the inertial oscillations reasonably well. The NBL developed into three layers wherein 1) very close to the surface, radiative cooling dominated over turbulence cooling; 2) a layer above, turbulent cooling was the dominant mechanism; and 3) near the top of the turbulent layer and above, clear-air radiative cooling was the dominating mechanism. However, depending on the geostrophic wind, the structure of these layers varied from one situation to another. The wind maximum, which was at least above 200 m of altitude under windy conditions, was located at an altitude of less than 100 m for the weak-wind case, probably because of weaker diffusion in the boundary layer during transition.
Abstract
The major objective of this study was to analyze the mean structure and evolution of the nocturnal boundary layer (NBL) under strong and weak wind conditions. Meteorological data collected during the plume-validation experiment conducted by the Electric Power Research Institute (EPRI) over a flat homogeneous terrain at Kincaid, Illinois (39°35′N, 89°25′W), were utilized. A one-dimensional meteorological boundary layer model originally developed by R. A. Pielke, modified with turbulent kinetic energy mixing-length closure, a layer-by-layer emissivity-based radiation scheme, and nonlinear nondimensional temperature and wind profiles in the surface layer, was used. In the four cases that were considered, ranging from strong to weak geostrophic forcing, the model reproduced the observed mean profiles, their evolutions in the NBL, and the inertial oscillations reasonably well. The NBL developed into three layers wherein 1) very close to the surface, radiative cooling dominated over turbulence cooling; 2) a layer above, turbulent cooling was the dominant mechanism; and 3) near the top of the turbulent layer and above, clear-air radiative cooling was the dominating mechanism. However, depending on the geostrophic wind, the structure of these layers varied from one situation to another. The wind maximum, which was at least above 200 m of altitude under windy conditions, was located at an altitude of less than 100 m for the weak-wind case, probably because of weaker diffusion in the boundary layer during transition.
Abstract
A series of idealized experiments with the NOAA Experimental Hurricane Weather Research and Forecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer (BL), subgrid convection, cloud microphysics, and radiation. Results from all the experiments are compared in terms of the maximum surface 10-m wind (VMAX) and minimum sea level pressure (PMIN)—operational metrics of TC intensity—as well as the azimuthally averaged temporal and spatial structure of the tangential wind and its material acceleration.
The conventional metrics of TC intensity (VMAX and PMIN) are found to be insufficient to reveal the sensitivity of the simulated TC to variations in model physics. Comparisons of the sensitivity runs indicate that (i) different boundary layer physics parameterization schemes for vertical subgrid turbulence mixing lead to differences not only in the intensity evolution in terms of VMAX and PMIN, but also in the structural characteristics of the simulated tropical cyclone; (ii) the surface drag coefficient is a key parameter that controls the VMAX–PMIN relationship near the surface; and (iii) different microphysics and subgrid convection parameterization schemes, because of their different realizations of diabatic heating distribution, lead to significant variations in the vortex structure.
The quantitative aspects of these results indicate that the current uncertainties in the BL mixing, surface drag, and microphysics parameterization schemes have comparable impacts on the intensity and structure of simulated TCs. The results also indicate that there is a need to include structural parameters in the HWRFX evaluation.
Abstract
A series of idealized experiments with the NOAA Experimental Hurricane Weather Research and Forecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer (BL), subgrid convection, cloud microphysics, and radiation. Results from all the experiments are compared in terms of the maximum surface 10-m wind (VMAX) and minimum sea level pressure (PMIN)—operational metrics of TC intensity—as well as the azimuthally averaged temporal and spatial structure of the tangential wind and its material acceleration.
The conventional metrics of TC intensity (VMAX and PMIN) are found to be insufficient to reveal the sensitivity of the simulated TC to variations in model physics. Comparisons of the sensitivity runs indicate that (i) different boundary layer physics parameterization schemes for vertical subgrid turbulence mixing lead to differences not only in the intensity evolution in terms of VMAX and PMIN, but also in the structural characteristics of the simulated tropical cyclone; (ii) the surface drag coefficient is a key parameter that controls the VMAX–PMIN relationship near the surface; and (iii) different microphysics and subgrid convection parameterization schemes, because of their different realizations of diabatic heating distribution, lead to significant variations in the vortex structure.
The quantitative aspects of these results indicate that the current uncertainties in the BL mixing, surface drag, and microphysics parameterization schemes have comparable impacts on the intensity and structure of simulated TCs. The results also indicate that there is a need to include structural parameters in the HWRFX evaluation.
Abstract
A three-dimensional mesoscale model was used to understand the meteorological conditions and the influence of the terrain on the local flow pattern during the Bhopal methyl isocyanate (MIC) gas leak. The study reveals that under the prevailing conditions of weak wind and strong stability the lakes in Bhopal influenced the local circulation significantly and caused northwesterly flow near the surface. The modified flow pattern resulted in the transport of MIC into the city area of Bhopal. However, with the increase in the ambient synoptic wind, the role of the lakes was found to diminish. Further, the other topographical features such as the hillocks in and around the city and the gently rolling terrain toward the southeastern sector of the city seem to have played a secondary role in influencing the meteorological conditions on the episodic night.
Abstract
A three-dimensional mesoscale model was used to understand the meteorological conditions and the influence of the terrain on the local flow pattern during the Bhopal methyl isocyanate (MIC) gas leak. The study reveals that under the prevailing conditions of weak wind and strong stability the lakes in Bhopal influenced the local circulation significantly and caused northwesterly flow near the surface. The modified flow pattern resulted in the transport of MIC into the city area of Bhopal. However, with the increase in the ambient synoptic wind, the role of the lakes was found to diminish. Further, the other topographical features such as the hillocks in and around the city and the gently rolling terrain toward the southeastern sector of the city seem to have played a secondary role in influencing the meteorological conditions on the episodic night.
Abstract
The role of radiation and turbulence was studied in a weak wind nocturnal inversion layer using a one-dimensional model. In contrast to a strong wind stable boundary layer where cooling within the surface inversion layer is dominated by turbulence, radiative cooling becomes larger than turbulent cooling under weak wind conditions. Further, the surface inversion layer was found to grow all through the night under weak wind conditions, whereas it attained a near equilibrium in the case of strong wind.
Abstract
The role of radiation and turbulence was studied in a weak wind nocturnal inversion layer using a one-dimensional model. In contrast to a strong wind stable boundary layer where cooling within the surface inversion layer is dominated by turbulence, radiative cooling becomes larger than turbulent cooling under weak wind conditions. Further, the surface inversion layer was found to grow all through the night under weak wind conditions, whereas it attained a near equilibrium in the case of strong wind.
