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Abstract
Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.
Abstract
Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.
Abstract
A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations.
Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (TB ) field is collocated with the highest point in the convective overshooting region and the TB -height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small.
Another type of storm has a close-in, cold-warm TB couplet with a dimension of approximately 20–40 km and a V-shaped cold TB pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion.
The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators.
The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.
Abstract
A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations.
Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (TB ) field is collocated with the highest point in the convective overshooting region and the TB -height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small.
Another type of storm has a close-in, cold-warm TB couplet with a dimension of approximately 20–40 km and a V-shaped cold TB pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion.
The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators.
The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.
Abstract
A technique was developed for estimating the condensation rates of convective storms using satellite measurements of cirrus anvil expansion rates and radiosonde measurements of environmental water vapor. Three cases of severe conviction in Oklahoma were studied and a diagnostic model was developed for integrating radiosonde data with satellite data.
Two methods were used to measure the anvil expansion rates–the expansion of isotherm contours on infrared image, and the divergent motions of small brightness anomalies tracked on the visible images. The differences between the two methods were large as the storms developed, but these differences became small in the latter stage of all three storms.
A comparison between the three storms indicated that the available moisture in the lowest levels greatly affected the rain rates of the storms. This was evident from both the measured rain rates of the storms and the condensation rates estimated by the model. The possibility of using this diagnostic model for estimating the intensities of convective storms also is discussed.
Abstract
A technique was developed for estimating the condensation rates of convective storms using satellite measurements of cirrus anvil expansion rates and radiosonde measurements of environmental water vapor. Three cases of severe conviction in Oklahoma were studied and a diagnostic model was developed for integrating radiosonde data with satellite data.
Two methods were used to measure the anvil expansion rates–the expansion of isotherm contours on infrared image, and the divergent motions of small brightness anomalies tracked on the visible images. The differences between the two methods were large as the storms developed, but these differences became small in the latter stage of all three storms.
A comparison between the three storms indicated that the available moisture in the lowest levels greatly affected the rain rates of the storms. This was evident from both the measured rain rates of the storms and the condensation rates estimated by the model. The possibility of using this diagnostic model for estimating the intensities of convective storms also is discussed.
Abstract
GOES stereoscopy is applied to the study of severe squall line cells. Short interval (3 min) GOES stereoscopic data from the 2–3 May 1979 SESAME case were used to measure cloud top heights of growing storms as a function of time. A one-dimensional cloud model was used to relate the stereoscopically derived cloud top ascent rates to thunderstorm updraft intensity. Results show ascent rates ranging from 4.4 to 7.7 m s−1 for intense cells in a squall line. These results compare well in magnitude with growth rates determined from simultaneous GOES infrared observations and previous estimates of visual cloud and radar echo top growth rates of other thunderstorms.
Detailed stereoscopic cloud top height contour maps of the mature squall line on 2–3 May 1979 were constructed and are discussed here in terms of the small-scale structure and its variability. Results show that for small-scale features (e.g., 5 km diameter tropopause penetrating towers) the short-interval GOES data are not sufficient for studying the life cycle of such features. The stereoscopic height contours are compared to infrared cloud top temperature patterns observed with intense thunderstorms and used to evaluate various theories on the cause of the infrared V-shaped signatures.
Abstract
GOES stereoscopy is applied to the study of severe squall line cells. Short interval (3 min) GOES stereoscopic data from the 2–3 May 1979 SESAME case were used to measure cloud top heights of growing storms as a function of time. A one-dimensional cloud model was used to relate the stereoscopically derived cloud top ascent rates to thunderstorm updraft intensity. Results show ascent rates ranging from 4.4 to 7.7 m s−1 for intense cells in a squall line. These results compare well in magnitude with growth rates determined from simultaneous GOES infrared observations and previous estimates of visual cloud and radar echo top growth rates of other thunderstorms.
Detailed stereoscopic cloud top height contour maps of the mature squall line on 2–3 May 1979 were constructed and are discussed here in terms of the small-scale structure and its variability. Results show that for small-scale features (e.g., 5 km diameter tropopause penetrating towers) the short-interval GOES data are not sufficient for studying the life cycle of such features. The stereoscopic height contours are compared to infrared cloud top temperature patterns observed with intense thunderstorms and used to evaluate various theories on the cause of the infrared V-shaped signatures.
Abstract
A multichannel statistical approach is used to retrieve rainfall rates from brightness temperatures (TB 's) observed by passive microwave radiometers flown on a high-altitude NASA aircraft. Brightness temperature statistics are based upon data generated by a cloud radiative model. This model simulates variabilities in the underlying geophysical parameters of interest, and computes their associated TB 's in each of the available channels. By further imposing the requirement that the observed TB 's agree with the TB values corresponding to the retrieved parameters through the cloud radiative transfer model, the results can be made to agree quite well with coincident radar-derived rainfall rates. Some information regarding the cloud vertical structure is also obtained by such an added requirement.
The applicability of this technique to satellite retrievals is also investigated. Data which might be observed by satellite-borne radiometers, including the effects of nonuniformly filled footprints, are simulated by the cloud radiative model for this purpose. Results from statistics generated using different hydrometeor vertical profiles in the cloud radiative model are examined. It is found that errors in the retrieved rainfall rates, and retrieval biases, decrease with increasing agreement between simulated TB 's and those corresponding to the retrieved geophysical parameters.
Abstract
A multichannel statistical approach is used to retrieve rainfall rates from brightness temperatures (TB 's) observed by passive microwave radiometers flown on a high-altitude NASA aircraft. Brightness temperature statistics are based upon data generated by a cloud radiative model. This model simulates variabilities in the underlying geophysical parameters of interest, and computes their associated TB 's in each of the available channels. By further imposing the requirement that the observed TB 's agree with the TB values corresponding to the retrieved parameters through the cloud radiative transfer model, the results can be made to agree quite well with coincident radar-derived rainfall rates. Some information regarding the cloud vertical structure is also obtained by such an added requirement.
The applicability of this technique to satellite retrievals is also investigated. Data which might be observed by satellite-borne radiometers, including the effects of nonuniformly filled footprints, are simulated by the cloud radiative model for this purpose. Results from statistics generated using different hydrometeor vertical profiles in the cloud radiative model are examined. It is found that errors in the retrieved rainfall rates, and retrieval biases, decrease with increasing agreement between simulated TB 's and those corresponding to the retrieved geophysical parameters.
Abstract
Aircraft passive microwave observations of deep atmospheric convection at frequencies between 18 and 183 GHz are presented in conjunction with visible and infrared satellite and aircraft observations and ground-based radar observations. Deep convective cores are indicated in the microwave data by negative brightness temperature (TB ) deviations from the land background (270 K) to extreme TB values below 100 K at 37, 92, and 183 GHz and below 200 K at 18 GHz. These TB minima, due to scattering by ice held aloft by the intense updrafts, are well correlated with areas of high radar reflectivity. For this land background case, TB is inversely correlated with rain rate at all frequencies due to TB -ice-rain correlations. Mean ΔT between vertically polarized and horizontally polarized radiance in precipitation areas is approximately 6 K at both 18 GHz and 37 GHz, indicating nonspherical precipitation size ice particles with a preferred horizontal orientation. Convective cores not observed in the visible and infrared data are clearly defined in the microwave observations and borders of convective rain areas are well defined using the high-frequency (90 GHz and greater) microwave observations.
Abstract
Aircraft passive microwave observations of deep atmospheric convection at frequencies between 18 and 183 GHz are presented in conjunction with visible and infrared satellite and aircraft observations and ground-based radar observations. Deep convective cores are indicated in the microwave data by negative brightness temperature (TB ) deviations from the land background (270 K) to extreme TB values below 100 K at 37, 92, and 183 GHz and below 200 K at 18 GHz. These TB minima, due to scattering by ice held aloft by the intense updrafts, are well correlated with areas of high radar reflectivity. For this land background case, TB is inversely correlated with rain rate at all frequencies due to TB -ice-rain correlations. Mean ΔT between vertically polarized and horizontally polarized radiance in precipitation areas is approximately 6 K at both 18 GHz and 37 GHz, indicating nonspherical precipitation size ice particles with a preferred horizontal orientation. Convective cores not observed in the visible and infrared data are clearly defined in the microwave observations and borders of convective rain areas are well defined using the high-frequency (90 GHz and greater) microwave observations.
Abstract
In Part II of the 29 June 1986 case study, a radiative transfer model is used to simulate the aircraft multichannel microwave brightness temperatures presented in Part I and to study the convective storm structure. Ground-based radar data are used to derive hydrometeor profiles of the storm, based on which the microwave upwelling brightness temperatures are calculated. Various vertical hydrometeor phase profiles and the Marshall and Palmer (M-P) and Sekhon and Srivastava (S-S) ice particle size distributions are experimented in the model. The results are compared with the aircraft radiometric data. The comparison reveals that 1) the M-P distribution well represents the ice particle size distribution, especially in the upper tropospheric portion of the cloud; 2) the S-S distribution appears to better simulate the ice particle size at the lower portion of the cloud, which has a greater effect on the low frequency microwave upwelling brightness temperatures; and 3) in deep convective regions, significant supercooled liquid water (∼0.5 g m−3) may be present up to the −30°C layer, while in less convective areas, frozen hydrometeors are predominant above −10°C level.
Abstract
In Part II of the 29 June 1986 case study, a radiative transfer model is used to simulate the aircraft multichannel microwave brightness temperatures presented in Part I and to study the convective storm structure. Ground-based radar data are used to derive hydrometeor profiles of the storm, based on which the microwave upwelling brightness temperatures are calculated. Various vertical hydrometeor phase profiles and the Marshall and Palmer (M-P) and Sekhon and Srivastava (S-S) ice particle size distributions are experimented in the model. The results are compared with the aircraft radiometric data. The comparison reveals that 1) the M-P distribution well represents the ice particle size distribution, especially in the upper tropospheric portion of the cloud; 2) the S-S distribution appears to better simulate the ice particle size at the lower portion of the cloud, which has a greater effect on the low frequency microwave upwelling brightness temperatures; and 3) in deep convective regions, significant supercooled liquid water (∼0.5 g m−3) may be present up to the −30°C layer, while in less convective areas, frozen hydrometeors are predominant above −10°C level.
