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Abstract
When the western coast of India lies in the path of the low-level west-southwest wind crossing the Arabian Sea during the summer monsoon season, deep convection frequently develops over the ocean off the coast. In such a situation, the maximum rainfall occurs near the coast, not over the Western Ghats. In order to study the physics underlying orographic-convective precipitation over this area, a two-dimensional compressible moist cloud model is applied. The model is written in terrain-following coordinates and includes the Coriolis force and a planetary boundary layer parameterization. The initial fields of thermodynamic variables are specified using observed data gathered upstream of the offshore precipitating systems over the Arabian Sea. Two wind profiles are considered: vertically uniform and nonuniform flows. The latter profile represents a monsoonal westerly jet at low levels and easterlies in the layer above 5 km. Three cases are considered for each wind profile by including or omitting moisture in the atmosphere and heat and moisture fluxes from the ocean.
Among six cases considered, results from the moist and nonuniform wind profile case with heat and moisture fluxes from the ocean are found to be the most consistent with observations of precipitation rate, preferred location of rainfall, and lack of high-level clouds in the downwind side of the mountain. When fluxes from the ocean are excluded, the predicted rainfall accumulation is about the same. However, the maximum rainfall rate occurs over the mountain peak area, in disagreement with the observation. When fluxes from the ocean are included, but with the vertically uniform basic flow, the predicted maximum rainfall occurs at the coast. However, its rate is about half that observed. It is thus concluded that, in order to account for the observed features of rainfall over the Arabian Sea and the Ghat Mountains during the summer monsoon season, two factors, the strongly sheared environment and fluxes of latent and sensible heat from the ocean, are essential. These factors were not considered by Smith and Lin nor Grossman and Durran.
Abstract
When the western coast of India lies in the path of the low-level west-southwest wind crossing the Arabian Sea during the summer monsoon season, deep convection frequently develops over the ocean off the coast. In such a situation, the maximum rainfall occurs near the coast, not over the Western Ghats. In order to study the physics underlying orographic-convective precipitation over this area, a two-dimensional compressible moist cloud model is applied. The model is written in terrain-following coordinates and includes the Coriolis force and a planetary boundary layer parameterization. The initial fields of thermodynamic variables are specified using observed data gathered upstream of the offshore precipitating systems over the Arabian Sea. Two wind profiles are considered: vertically uniform and nonuniform flows. The latter profile represents a monsoonal westerly jet at low levels and easterlies in the layer above 5 km. Three cases are considered for each wind profile by including or omitting moisture in the atmosphere and heat and moisture fluxes from the ocean.
Among six cases considered, results from the moist and nonuniform wind profile case with heat and moisture fluxes from the ocean are found to be the most consistent with observations of precipitation rate, preferred location of rainfall, and lack of high-level clouds in the downwind side of the mountain. When fluxes from the ocean are excluded, the predicted rainfall accumulation is about the same. However, the maximum rainfall rate occurs over the mountain peak area, in disagreement with the observation. When fluxes from the ocean are included, but with the vertically uniform basic flow, the predicted maximum rainfall occurs at the coast. However, its rate is about half that observed. It is thus concluded that, in order to account for the observed features of rainfall over the Arabian Sea and the Ghat Mountains during the summer monsoon season, two factors, the strongly sheared environment and fluxes of latent and sensible heat from the ocean, are essential. These factors were not considered by Smith and Lin nor Grossman and Durran.
Abstract
The Big Thompson storm occurred on 31 July–1 August 1976 over Big Thompson Canyon, Colorado, when a secondary cold frontal surge was accelerated and reached the foothills of the Front Range. Two- and three-dimensional moist compressible cloud models developed by Ogura and Yoshizaki are applied to this storm event. Adopting highly simplified terrain shapes, this study addresses two aspects of the storm. One is the distinct characteristics of the storm structure, as schematically depicted by Carasena et al.; the other is that heavy precipitation occurred in the basin area rather than over the mountain peak area.
When the model was initialized in such a way that moisture-rich, low-level, strong easterlies impinged upon the orography, the model predicted the development of a storm that not only caused heavy precipitation at the right location relative to the mountain peak, but also reproduced the observed storm in many aspects, both in two- and three-dimensional (2D and 3D) simulations. The major qualitative differences between 2D and 3D simulators is that the model storm in two dimensions is highly transient and exhibits the distinct multicellular structure, whereas the model storm in three dimensions tends to be quasi-stationary. This difference was attributed to the weakness of the induced low pressure inside the storm in three dimensions. Qualitatively, the precipitation accumulation in three dimensions is found to be substantially larger than the 2D counterpart In three dimensions, the low-level easterlies ahead of the mountainous area are deflected as they approach the valley to flow nearly parallel to the elevation contours in each side of the valley and these two airflows eventually converge along the valley to produce heavy rainfall. An interesting finding in the model is the creation of a cold air pool beneath the storm in the situation where the cloud base height is lower than the maximum terrain. The model storm slants severely downstream (particularly in 2D simulations) and precipitating particles fall through the cloud layer, thusthus enhancing evaporating cooling.
When the initial distribution of moisture is assumed to be uniform horizontally in the model, the first deep convection (and consequently heavy preciptation) occurs only at or near the mountain peak, in disagreement with the observations.
Abstract
The Big Thompson storm occurred on 31 July–1 August 1976 over Big Thompson Canyon, Colorado, when a secondary cold frontal surge was accelerated and reached the foothills of the Front Range. Two- and three-dimensional moist compressible cloud models developed by Ogura and Yoshizaki are applied to this storm event. Adopting highly simplified terrain shapes, this study addresses two aspects of the storm. One is the distinct characteristics of the storm structure, as schematically depicted by Carasena et al.; the other is that heavy precipitation occurred in the basin area rather than over the mountain peak area.
When the model was initialized in such a way that moisture-rich, low-level, strong easterlies impinged upon the orography, the model predicted the development of a storm that not only caused heavy precipitation at the right location relative to the mountain peak, but also reproduced the observed storm in many aspects, both in two- and three-dimensional (2D and 3D) simulations. The major qualitative differences between 2D and 3D simulators is that the model storm in two dimensions is highly transient and exhibits the distinct multicellular structure, whereas the model storm in three dimensions tends to be quasi-stationary. This difference was attributed to the weakness of the induced low pressure inside the storm in three dimensions. Qualitatively, the precipitation accumulation in three dimensions is found to be substantially larger than the 2D counterpart In three dimensions, the low-level easterlies ahead of the mountainous area are deflected as they approach the valley to flow nearly parallel to the elevation contours in each side of the valley and these two airflows eventually converge along the valley to produce heavy rainfall. An interesting finding in the model is the creation of a cold air pool beneath the storm in the situation where the cloud base height is lower than the maximum terrain. The model storm slants severely downstream (particularly in 2D simulations) and precipitating particles fall through the cloud layer, thusthus enhancing evaporating cooling.
When the initial distribution of moisture is assumed to be uniform horizontally in the model, the first deep convection (and consequently heavy preciptation) occurs only at or near the mountain peak, in disagreement with the observations.
Abstract
An exceptionally heavy rainstorm hit the coastal area of the western part of Japan on 23 July 1983. The 6-h rainfall accumulation exceeded 300 mm locally. At its peak period, the hourly precipitation rate was as high as 90 mm. Yet the area of heavy precipitation was limited in its extent so that the Maddox criteria for midlatitude Mesoscale Convection Complex was not met in terms of size, when viewed in satellite data. The precipitation occurred in the warm, moist southwesterly sector of a weak, eastward propagating medium scale cyclone that developed along the Baiu front. Prior to the onset of heavy precipitation, the atmosphere was very moist up to the 600 mb level, and became convectively very unstable.
The main topographic feature in the area where heavy precipitation occurred is a mountain ridge that runs approximately parallel to the coastline, with modest elevations of generally less than 1000 m except for mountain peaks. An analysis of raingage records over land clearly indicates that the rainfall accumulation was maximized in the coastal area rather than in the mountainous area. A detailed analysis of PPI radar data reveals that, during the heavy precipitation period, convective cells formed in succession over the sea about 50 km off the coast. As they moved eastward and approached the coastline, they developed rapidly and organized into a band structure. They then weakened on the downwind side of the mountains.
A one-layer model developed by Danard is applied to investigate the topographic effect on the surface flow in the situations under study. The model result indicates that the surface flow over land is deflected mainly by the effect of the topographic barrier and partly by the increased surface friction over land. A convergence zone forms over the coastal strip and the adjacent sea between this deflected flow and the relatively undeflected flow over water. A local maximum of convergence is located just over the area of the maximum rainfall accumulation. This feature accounts for the enhancement of traveling convective cells over the coastal strip and suggests that even a mountain ridge of modest height could enhance precipitation significantly.
Abstract
An exceptionally heavy rainstorm hit the coastal area of the western part of Japan on 23 July 1983. The 6-h rainfall accumulation exceeded 300 mm locally. At its peak period, the hourly precipitation rate was as high as 90 mm. Yet the area of heavy precipitation was limited in its extent so that the Maddox criteria for midlatitude Mesoscale Convection Complex was not met in terms of size, when viewed in satellite data. The precipitation occurred in the warm, moist southwesterly sector of a weak, eastward propagating medium scale cyclone that developed along the Baiu front. Prior to the onset of heavy precipitation, the atmosphere was very moist up to the 600 mb level, and became convectively very unstable.
The main topographic feature in the area where heavy precipitation occurred is a mountain ridge that runs approximately parallel to the coastline, with modest elevations of generally less than 1000 m except for mountain peaks. An analysis of raingage records over land clearly indicates that the rainfall accumulation was maximized in the coastal area rather than in the mountainous area. A detailed analysis of PPI radar data reveals that, during the heavy precipitation period, convective cells formed in succession over the sea about 50 km off the coast. As they moved eastward and approached the coastline, they developed rapidly and organized into a band structure. They then weakened on the downwind side of the mountains.
A one-layer model developed by Danard is applied to investigate the topographic effect on the surface flow in the situations under study. The model result indicates that the surface flow over land is deflected mainly by the effect of the topographic barrier and partly by the increased surface friction over land. A convergence zone forms over the coastal strip and the adjacent sea between this deflected flow and the relatively undeflected flow over water. A local maximum of convergence is located just over the area of the maximum rainfall accumulation. This feature accounts for the enhancement of traveling convective cells over the coastal strip and suggests that even a mountain ridge of modest height could enhance precipitation significantly.
Abstract
A cold front which passed through the dense network of the SESAME-AVE (Severe Environmental Storms and Mesoscale Experiment–Atmospheric Variability Experiment) on 25–26 April 1979 was investigated. Rawinsonde data collected from 23 special stations and 19 National Weather Service stations at three-hour intervals for a 24-hour period were used along with hourly surface data, radar summary charts and GOES-East satellite images. Severe storms formed along the surface front during this period. The analysis focused on the vertical circulation across the frontal surface at low levels.
The major features of the cold frontal system that emerged from an analysis of this unique data set include a familiar direct vertical circulation, with moist warm air ascending just above the surface front. However, the upgliding motion was intercepted by a secondary circulation at middle levels. The analysis result was compared with model predictions of Hoskins and Bretherton (1972) as calculated by Blumen (1980). Several features of the observed front were found to agree qualitatively well with the model prediction. These include: a) Both the horizontal temperature gradient and the vertical component of vorticity have their maxima near the ground surface; b) The horizontal gradient of potential temperature is smaller in the warm air region than in the cold air region; c) The temperature inversion layer representing the frontal surface is located behind and below the axis of the maximum cyclonic relative vorticity. However, the model is found to be less successful in predicting the low-level convergence field; the observed surface convergence and cyclonic vorticity are of the same order of magnitude and concentrated in zones of approximately the same width of 300 km. The observed maximum ascending motion is located at low levels, rather than in middle levels as predicted. The subsidence in the cold air region is also much stronger than the model prediction.
Abstract
A cold front which passed through the dense network of the SESAME-AVE (Severe Environmental Storms and Mesoscale Experiment–Atmospheric Variability Experiment) on 25–26 April 1979 was investigated. Rawinsonde data collected from 23 special stations and 19 National Weather Service stations at three-hour intervals for a 24-hour period were used along with hourly surface data, radar summary charts and GOES-East satellite images. Severe storms formed along the surface front during this period. The analysis focused on the vertical circulation across the frontal surface at low levels.
The major features of the cold frontal system that emerged from an analysis of this unique data set include a familiar direct vertical circulation, with moist warm air ascending just above the surface front. However, the upgliding motion was intercepted by a secondary circulation at middle levels. The analysis result was compared with model predictions of Hoskins and Bretherton (1972) as calculated by Blumen (1980). Several features of the observed front were found to agree qualitatively well with the model prediction. These include: a) Both the horizontal temperature gradient and the vertical component of vorticity have their maxima near the ground surface; b) The horizontal gradient of potential temperature is smaller in the warm air region than in the cold air region; c) The temperature inversion layer representing the frontal surface is located behind and below the axis of the maximum cyclonic relative vorticity. However, the model is found to be less successful in predicting the low-level convergence field; the observed surface convergence and cyclonic vorticity are of the same order of magnitude and concentrated in zones of approximately the same width of 300 km. The observed maximum ascending motion is located at low levels, rather than in middle levels as predicted. The subsidence in the cold air region is also much stronger than the model prediction.
Abstract
This paper presents results of simulations for a case of heavy precipitation that occurred on 23 July 1982 over western Japan. Special emphasis is placed on synoptic- and subsynoptic-scale processes that led to heavy precipitation and also its linkage with the low-level jet (LLJ).
The model result recaptures the major observed features of this event reasonably well. The sequence of events revealed by the model starts with the formation of a localized surface warm front caused by the deformation field that is associated with an eastward traveling, nondeepening meso-αscale low. It is followed by the initiation of both concentrated convective precipitation at the surface front and stratiform grid-scale precipitation along the sloping frontal surface. The simulations with different model physics reveal significant roles that diabatic heating processes play in the linkage between heavy precipitation and the LLJ. While condensation heating produces a cyclonic circulation with failing pressure manifested as a mesoscale trough over the front, evaporative cooling from stratiform raindrops generates a marked frontogenetic forcing and creates a cold pool beneath the sloping frontal surface. An anticyclonic outflow from the cold pool is accompanied by convergence on its southwestern flank, which further enhances and concentrates the convective activity and the mesoscale trough. The supergeostrophic LLJ is formed in this situation, where an air parcel crosses height contours into low pressure with large angles due to a combination of an alongfront flow in the southwestern part of the anticyclonic outflow anomaly induced by the evaporative cooling and a cross-front flow in the upper branch of the direct secondary circulation associated with the warm-frontogenetical processes.
Isentropic and isobaric ageostrophic motion diagnoses show that the inertial advective component, mainly arising from the horizontal displacement, is the dominant part in the ageostrophic wind in the entrance region of the LLJ, supporting the conclusion that the rapid parcel acceleration itself occurs almost adiabatically through the horizontal displacement crossing into low pressure.
Abstract
This paper presents results of simulations for a case of heavy precipitation that occurred on 23 July 1982 over western Japan. Special emphasis is placed on synoptic- and subsynoptic-scale processes that led to heavy precipitation and also its linkage with the low-level jet (LLJ).
The model result recaptures the major observed features of this event reasonably well. The sequence of events revealed by the model starts with the formation of a localized surface warm front caused by the deformation field that is associated with an eastward traveling, nondeepening meso-αscale low. It is followed by the initiation of both concentrated convective precipitation at the surface front and stratiform grid-scale precipitation along the sloping frontal surface. The simulations with different model physics reveal significant roles that diabatic heating processes play in the linkage between heavy precipitation and the LLJ. While condensation heating produces a cyclonic circulation with failing pressure manifested as a mesoscale trough over the front, evaporative cooling from stratiform raindrops generates a marked frontogenetic forcing and creates a cold pool beneath the sloping frontal surface. An anticyclonic outflow from the cold pool is accompanied by convergence on its southwestern flank, which further enhances and concentrates the convective activity and the mesoscale trough. The supergeostrophic LLJ is formed in this situation, where an air parcel crosses height contours into low pressure with large angles due to a combination of an alongfront flow in the southwestern part of the anticyclonic outflow anomaly induced by the evaporative cooling and a cross-front flow in the upper branch of the direct secondary circulation associated with the warm-frontogenetical processes.
Isentropic and isobaric ageostrophic motion diagnoses show that the inertial advective component, mainly arising from the horizontal displacement, is the dominant part in the ageostrophic wind in the entrance region of the LLJ, supporting the conclusion that the rapid parcel acceleration itself occurs almost adiabatically through the horizontal displacement crossing into low pressure.
Abstract
A cumulus parameterization scheme developed by Arakawa and Schubert was tested through a semiprognostic approach using two different datasets: one for a tropical cloud band, the other for tropical composite easterly wave disturbances. Both were observed in GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment using a computational algorithm different from that of Lord. Also, an efficient software package from the International Mathematics and Statistics Library was used in determining the cloud mass flux at the cloud base level.
The semiprognostic results indicate that the cloud heating and drying effects predicted by the Arakawa-Schubert scheme in both cases agree rather well with the observations. Also, the predicted cloud population in terms of cloud-base mass flux shows the prominent features as already revealed by other previous diagnostic studies in the tropical area. The Arakawa-Schubert scheme underestimates both condensation and evaporation rates substantially when compared with the cumulus ensemble model results for the cloud band case by Soong and Tao and another subsequent case by Tao. An inclusion of the downdraft effects associated with the evaporation of rainfall appears to alleviate this deficiency.
Abstract
A cumulus parameterization scheme developed by Arakawa and Schubert was tested through a semiprognostic approach using two different datasets: one for a tropical cloud band, the other for tropical composite easterly wave disturbances. Both were observed in GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment using a computational algorithm different from that of Lord. Also, an efficient software package from the International Mathematics and Statistics Library was used in determining the cloud mass flux at the cloud base level.
The semiprognostic results indicate that the cloud heating and drying effects predicted by the Arakawa-Schubert scheme in both cases agree rather well with the observations. Also, the predicted cloud population in terms of cloud-base mass flux shows the prominent features as already revealed by other previous diagnostic studies in the tropical area. The Arakawa-Schubert scheme underestimates both condensation and evaporation rates substantially when compared with the cumulus ensemble model results for the cloud band case by Soong and Tao and another subsequent case by Tao. An inclusion of the downdraft effects associated with the evaporation of rainfall appears to alleviate this deficiency.
Abstract
As an extension of the work presented in an accompanying paper of Kao and Ogura, the Arakawa-Schubert cumulus parameterization scheme is examined prognostically in a modeling study of the evolution of a convectively driven tropical mesoscale rainband that developed on 12 August 1974 over the eastern Atlantic. A two-dimensional hydrostatic model is used with a mixed layer parameterization. Observed soundings were used as initial conditions and 24-h integrations of the model are made.
In the control experiment, a prescribed time-independent large scale forcing is imposed in a limited area at low levels. Many aspects of the observed evolution of the rainband are well simulated by the model, including the shift of height of the area-averaged maximum upward motion from low levels to upper levels and the development of downward motion at low levels while upward motion of a significant magnitude is still present at upper levels. The predicted rainfall rate, in the mature stage, is also found to agree with the observation. A secondary development of deep convection away from the primary convective area is generated at later times due to the adiabatic subsidence warming at upper levels. The predicted cloud population is bimodal at the developing stage of the mesoscale circulation, whereas it is dominated by deep convection at the mature stage, consistent with earlier diagnostic studies. Quantitatively, however, some discrepancies are observed between the predicted and observed evolution of the mesoscale circulation.
Several sensitivity tests of the model are made. Removing the large-scale forcing at the time when the convective system reaches its peak intensity immediately results in the decay of the system. Including the vertical wind shear does not have much effect on the life cycle of the system except that the convective activities shift slightly to the downwind area. Permitting cumulus downdraft effects in the Arakawa-Schubert scheme reduces the intensity of circulation due to the cooling of subcloud layer introduced by the detrainment of the downdrafts.
Abstract
As an extension of the work presented in an accompanying paper of Kao and Ogura, the Arakawa-Schubert cumulus parameterization scheme is examined prognostically in a modeling study of the evolution of a convectively driven tropical mesoscale rainband that developed on 12 August 1974 over the eastern Atlantic. A two-dimensional hydrostatic model is used with a mixed layer parameterization. Observed soundings were used as initial conditions and 24-h integrations of the model are made.
In the control experiment, a prescribed time-independent large scale forcing is imposed in a limited area at low levels. Many aspects of the observed evolution of the rainband are well simulated by the model, including the shift of height of the area-averaged maximum upward motion from low levels to upper levels and the development of downward motion at low levels while upward motion of a significant magnitude is still present at upper levels. The predicted rainfall rate, in the mature stage, is also found to agree with the observation. A secondary development of deep convection away from the primary convective area is generated at later times due to the adiabatic subsidence warming at upper levels. The predicted cloud population is bimodal at the developing stage of the mesoscale circulation, whereas it is dominated by deep convection at the mature stage, consistent with earlier diagnostic studies. Quantitatively, however, some discrepancies are observed between the predicted and observed evolution of the mesoscale circulation.
Several sensitivity tests of the model are made. Removing the large-scale forcing at the time when the convective system reaches its peak intensity immediately results in the decay of the system. Including the vertical wind shear does not have much effect on the life cycle of the system except that the convective activities shift slightly to the downwind area. Permitting cumulus downdraft effects in the Arakawa-Schubert scheme reduces the intensity of circulation due to the cooling of subcloud layer introduced by the detrainment of the downdrafts.
Abstract
FGGE revel III-b data provided by the European Centre for Medium-Range Weather Forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large-scale meteorological fields during the Northern Hemisphere summer (May–September) of 1979 over the eastern Pacific. Aside from the summer monsoon area over southern and eastern Asia, the eastern Pacific is the area where strong, deep convection frequently develops in the northern summer. This is also one of the tropical ocean areas that have been least explored meteorologically.
A power spectral analysis using the Maximum Entropy Method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from May through September. The waves with a period of 4–6 days are not only stronger in July and August than those of other months, but active in the regions of 100°–130°W in the eastern Pacific and 130°–160°E in the western Pacific within the zone of 5°–15°N. These waves possess a wavelength of 3000–3500 km and travel westward with a speed of 5–7 m s−1. Deep convection is found to occur at or slightly behind the wave trough axis.
The structure of easterly waves obtained by a composite technique is similar to those of African waves observed in GATE Phase III and easterly waves in the western Pacific determined by Reed and Recker. These similarities include the amplitude of the wave-related meridional wind (4 m s−1) and a cold core of temperature anomaly pattern at low levels. The manner in which the position of the surface confluence line shifts latitudinally with the passage of waves is remarkably similar to that found in GATE Phase III by Chen and Ogura. However, the structure at the upper levels does not show a distinct secondary maxima of the wave-related perturbations, which is a significant feature in the easterly waves observed in other areas.
The positions of both the axis of the strongest deep cloud activity, inferred from the satellite IR data, and the surface wind confluence line exhibit seasonal variation in the eastern Pacific. Nonetheless, they are closely collocated. Further, they are located over the area of maximum sea surface temperature (SST), suggesting a close relationship among SST, the ITCZ, and the confluence of the surface wind.
At 850 and 700 mb, most of the tropical Pacific is dominated by easterly flow. However, both latitudinal and vertical shears of the easterly flow are much weaker than those observed over the eastern Atlantic and western Africa, suggesting that dry barotropic instability alone cannot account for the formation of easterly waves in the eastern Pacific.
Abstract
FGGE revel III-b data provided by the European Centre for Medium-Range Weather Forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large-scale meteorological fields during the Northern Hemisphere summer (May–September) of 1979 over the eastern Pacific. Aside from the summer monsoon area over southern and eastern Asia, the eastern Pacific is the area where strong, deep convection frequently develops in the northern summer. This is also one of the tropical ocean areas that have been least explored meteorologically.
A power spectral analysis using the Maximum Entropy Method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from May through September. The waves with a period of 4–6 days are not only stronger in July and August than those of other months, but active in the regions of 100°–130°W in the eastern Pacific and 130°–160°E in the western Pacific within the zone of 5°–15°N. These waves possess a wavelength of 3000–3500 km and travel westward with a speed of 5–7 m s−1. Deep convection is found to occur at or slightly behind the wave trough axis.
The structure of easterly waves obtained by a composite technique is similar to those of African waves observed in GATE Phase III and easterly waves in the western Pacific determined by Reed and Recker. These similarities include the amplitude of the wave-related meridional wind (4 m s−1) and a cold core of temperature anomaly pattern at low levels. The manner in which the position of the surface confluence line shifts latitudinally with the passage of waves is remarkably similar to that found in GATE Phase III by Chen and Ogura. However, the structure at the upper levels does not show a distinct secondary maxima of the wave-related perturbations, which is a significant feature in the easterly waves observed in other areas.
The positions of both the axis of the strongest deep cloud activity, inferred from the satellite IR data, and the surface wind confluence line exhibit seasonal variation in the eastern Pacific. Nonetheless, they are closely collocated. Further, they are located over the area of maximum sea surface temperature (SST), suggesting a close relationship among SST, the ITCZ, and the confluence of the surface wind.
At 850 and 700 mb, most of the tropical Pacific is dominated by easterly flow. However, both latitudinal and vertical shears of the easterly flow are much weaker than those observed over the eastern Atlantic and western Africa, suggesting that dry barotropic instability alone cannot account for the formation of easterly waves in the eastern Pacific.
Abstract
A two-dimensional, anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line. Initial conditions were adapted from observations of the 22 May 1976 Oklahoma line. Model simulations were made with and without considering the ice phase of water. These model storms have, within the constraints of the model's geometry, replicated the basic multicellular character and general line-normal airflow typical of these lines. In addition, the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable.
Further, it was found that the initial conditions chosen resulted in model storms which were not only long-lasting but also essentially repetitive, indicating that a state of quasi-equilibrium had been attained. The storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations.
The inclusion of ice phase processes resulted in the production of more realistic appearing features in the trailing portion of the storm as well as more widespread precipitation. These were for the most part due to the enhanced rearward transport of precipitation particles from the convective cells which resulted from including ice, particularly low density snow. The underlying structures of these model storms were investigated by averaging model fields across time, smoothing out the transient components. These analyses indicated that the addition of ice had its greatest impact on the scale of the storm's circulation features.
Abstract
A two-dimensional, anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line. Initial conditions were adapted from observations of the 22 May 1976 Oklahoma line. Model simulations were made with and without considering the ice phase of water. These model storms have, within the constraints of the model's geometry, replicated the basic multicellular character and general line-normal airflow typical of these lines. In addition, the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable.
Further, it was found that the initial conditions chosen resulted in model storms which were not only long-lasting but also essentially repetitive, indicating that a state of quasi-equilibrium had been attained. The storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations.
The inclusion of ice phase processes resulted in the production of more realistic appearing features in the trailing portion of the storm as well as more widespread precipitation. These were for the most part due to the enhanced rearward transport of precipitation particles from the convective cells which resulted from including ice, particularly low density snow. The underlying structures of these model storms were investigated by averaging model fields across time, smoothing out the transient components. These analyses indicated that the addition of ice had its greatest impact on the scale of the storm's circulation features.
Abstract
No abstract available.
Abstract
No abstract available.
