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- Author or Editor: F. Ian Harris x
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Abstract
Simple theoretical models of evaporation and sensitive high-resolution Doppler radars are used to study the precipitation and velocity structures resulting from evaporation at the base of layers of ice particles.
The models show that most ice particles will evaporate completely within a 2 km depth of fall, with the depth being dependent primarily upon particle size and relative humidity. Vertical gradients of reflectivity factor of 20–30 dBZ per 750 m are predicted for relative humidities <75% for a non-rimed collection of particles with a distribution of sizes. The evaporative cooling produces a destabilized layer, the depth and intensity of which are most dependent upon the relative humidity and precipitation characteristics. A dynamical model shows that downdrafts of at least 6 m s−1 penetrating to a depth of 2 km can be produced by evaporation. The intensity and penetration depth of the downdrafts depend primarily on the ambient lapse rate of temperature.
The magnitudes of vertical gradients of reflectivity factor predicted by the models were seen in radar observations. On one occasion the base of the precipitation layer lowered with time at 200 m h−1, in excellent agreement with the calculations. Updrafts and downdrafts of 1-3 m s−1 were observed in the region of intense vertical gradients of reflectivity by a vertically pointing Doppler radar. These motions perturbed the precipitation field such that the downdrafts were in downward extending appendages called “stalactites” and updrafts in the holes between.
Doppler radar observations are presented of stalactites and convective motion fields which were associated with a uniformly generated precipitation layer and with trails from cellular generators at cloud top. The motions associated with the trails appeared to be better organized and to have greater vertical extent than those associated with the uniformly generated layer. In the latter case, the largest scales varied from 500 m to 1.5 km with several preferred scales at smaller wavelengths which suggested a tendency toward a breakdown of the stalactite associated motion fields. In the former case the scales were more uniform at 600–900 m.
An evolution of a stalactite layer was observed in the case of the trails. Initially, when the trails entered the top of the dry layer with a small angle of incidence, the stalactites were simply extensions of the trails and the updrafts and downdrafts appeared to follow the stalactites. The vertical extent of these perturbations at this point was 1.5-2 km. The shear at the top of the layer increased with time and the trails became more horizontal. Precipitation tended to be carried out of the trails by downdrafts resulting from locally enhanced chilling by evaporation. This resulted in more vertical stalactites and more vertical updrafts and downdrafts. When the trials became horizontal, they were much more disuse and the whole layer was destabilized. However, the destabilization was not sufficient to result in significant convection. The increase in shear was due to a decrease in the mean horizontal wind at the top of the dry layer which in turn was deduced to be the result of a transfer of energy to the perturbations, probably making them more intense. It appears that this interaction between the perturbations and the mean flow resulted in more intense but shorter lived stalactite associated motion fields than otherwise would have occurred.
Abstract
Simple theoretical models of evaporation and sensitive high-resolution Doppler radars are used to study the precipitation and velocity structures resulting from evaporation at the base of layers of ice particles.
The models show that most ice particles will evaporate completely within a 2 km depth of fall, with the depth being dependent primarily upon particle size and relative humidity. Vertical gradients of reflectivity factor of 20–30 dBZ per 750 m are predicted for relative humidities <75% for a non-rimed collection of particles with a distribution of sizes. The evaporative cooling produces a destabilized layer, the depth and intensity of which are most dependent upon the relative humidity and precipitation characteristics. A dynamical model shows that downdrafts of at least 6 m s−1 penetrating to a depth of 2 km can be produced by evaporation. The intensity and penetration depth of the downdrafts depend primarily on the ambient lapse rate of temperature.
The magnitudes of vertical gradients of reflectivity factor predicted by the models were seen in radar observations. On one occasion the base of the precipitation layer lowered with time at 200 m h−1, in excellent agreement with the calculations. Updrafts and downdrafts of 1-3 m s−1 were observed in the region of intense vertical gradients of reflectivity by a vertically pointing Doppler radar. These motions perturbed the precipitation field such that the downdrafts were in downward extending appendages called “stalactites” and updrafts in the holes between.
Doppler radar observations are presented of stalactites and convective motion fields which were associated with a uniformly generated precipitation layer and with trails from cellular generators at cloud top. The motions associated with the trails appeared to be better organized and to have greater vertical extent than those associated with the uniformly generated layer. In the latter case, the largest scales varied from 500 m to 1.5 km with several preferred scales at smaller wavelengths which suggested a tendency toward a breakdown of the stalactite associated motion fields. In the former case the scales were more uniform at 600–900 m.
An evolution of a stalactite layer was observed in the case of the trails. Initially, when the trails entered the top of the dry layer with a small angle of incidence, the stalactites were simply extensions of the trails and the updrafts and downdrafts appeared to follow the stalactites. The vertical extent of these perturbations at this point was 1.5-2 km. The shear at the top of the layer increased with time and the trails became more horizontal. Precipitation tended to be carried out of the trails by downdrafts resulting from locally enhanced chilling by evaporation. This resulted in more vertical stalactites and more vertical updrafts and downdrafts. When the trials became horizontal, they were much more disuse and the whole layer was destabilized. However, the destabilization was not sufficient to result in significant convection. The increase in shear was due to a decrease in the mean horizontal wind at the top of the dry layer which in turn was deduced to be the result of a transfer of energy to the perturbations, probably making them more intense. It appears that this interaction between the perturbations and the mean flow resulted in more intense but shorter lived stalactite associated motion fields than otherwise would have occurred.
Abstract
The potential for single-Doppler radar determination of wind field characteristics in cyclonic flow is examined. The influence of the four independent first-order derivatives of a wind field, namely curvature, diffluence, downwind shear, and crosswind shear, upon the Doppler radial velocities is studied. Simple models of wind fields containing each of the derivatives defined in natural coordinates are presented. When only one derivative is present at a time, it has been found that there are unique signatures for diffluence and downwind shear and qualitatively similar signatures for curvature and crosswind shear. With a model incorporating all four derivatives, techniques are developed for the recovery of these derivatives. A method is also presented that corrects the mean speed estimate. It is concluded that in most cases the recovery of the downwind shear, diffluence, the sum of curvature and crosswind shear, and mean wind is possible to within 5 percent of the true values.
Application of these techniques to radar data collected from Hurricane Gloria is discussed. A storm strength indicator based on shearing deformation and distance of cyclone center yielded signs of the declining trend of the storm an hour or two before this trend manifested itself significantly in the wind speed as estimated by the Doppler radar, therefore suggesting potential as a forecast tool.
Abstract
The potential for single-Doppler radar determination of wind field characteristics in cyclonic flow is examined. The influence of the four independent first-order derivatives of a wind field, namely curvature, diffluence, downwind shear, and crosswind shear, upon the Doppler radial velocities is studied. Simple models of wind fields containing each of the derivatives defined in natural coordinates are presented. When only one derivative is present at a time, it has been found that there are unique signatures for diffluence and downwind shear and qualitatively similar signatures for curvature and crosswind shear. With a model incorporating all four derivatives, techniques are developed for the recovery of these derivatives. A method is also presented that corrects the mean speed estimate. It is concluded that in most cases the recovery of the downwind shear, diffluence, the sum of curvature and crosswind shear, and mean wind is possible to within 5 percent of the true values.
Application of these techniques to radar data collected from Hurricane Gloria is discussed. A storm strength indicator based on shearing deformation and distance of cyclone center yielded signs of the declining trend of the storm an hour or two before this trend manifested itself significantly in the wind speed as estimated by the Doppler radar, therefore suggesting potential as a forecast tool.
Abstract
A kinematic mesocyclone model is developed to better approximate mesocyclone flows observed by single-Doppler radar. The model is described by two general flow regimes: an inner core region where velocity varies directly with radius from the center of the flow and an outer flow region where velocity varies inversely with radius. The new model differs from the traditional circular mesocyclone model in that the shape of the inner flow is described by an ellipse of specified eccentricity, and the vorticity and divergence structures of the inner flow region are nonuniform and described by simple functions. The effects of flow shape, vorticity and divergence structures, radar viewing angle, and radar resolution on the flow appearance and data interpretation are examined.
One traditional measure of mesocyclone intensity is the shear measured between the relative peaks of incoming and outgoing Doppler velocity. In noncircular flows or flows where the vorticity structure is not uniform, shear is found to be an unreliable measure of mesocyclone intensity. A correction for shear is possible if the flow shape, internal structure, and orientation to the radar are known. Techniques to assess these characteristics from single-Doppler data are presented.
The elliptical mesocyclone model is compared with observations of the 20 May 1977 Del City, Oklahoma, mesocyclone from two Doppler radars. From characteristics of the flow estimated from single-Doppler data, a simulation of the mesocyclone is produced that closely approximates the observed single-Doppler fields. The associated model fields of vorticity and divergence are comparable in structure and magnitude to the fields determined from dual-Doppler analysis.
Abstract
A kinematic mesocyclone model is developed to better approximate mesocyclone flows observed by single-Doppler radar. The model is described by two general flow regimes: an inner core region where velocity varies directly with radius from the center of the flow and an outer flow region where velocity varies inversely with radius. The new model differs from the traditional circular mesocyclone model in that the shape of the inner flow is described by an ellipse of specified eccentricity, and the vorticity and divergence structures of the inner flow region are nonuniform and described by simple functions. The effects of flow shape, vorticity and divergence structures, radar viewing angle, and radar resolution on the flow appearance and data interpretation are examined.
One traditional measure of mesocyclone intensity is the shear measured between the relative peaks of incoming and outgoing Doppler velocity. In noncircular flows or flows where the vorticity structure is not uniform, shear is found to be an unreliable measure of mesocyclone intensity. A correction for shear is possible if the flow shape, internal structure, and orientation to the radar are known. Techniques to assess these characteristics from single-Doppler data are presented.
The elliptical mesocyclone model is compared with observations of the 20 May 1977 Del City, Oklahoma, mesocyclone from two Doppler radars. From characteristics of the flow estimated from single-Doppler data, a simulation of the mesocyclone is produced that closely approximates the observed single-Doppler fields. The associated model fields of vorticity and divergence are comparable in structure and magnitude to the fields determined from dual-Doppler analysis.
