A Z-R Relation for Summertime Convective Clouds over West Texas

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  • a Department of Geosciences, Northeast Louisiana University, Monroe, Louisiana
  • | b Atmospheric Science Group, Texas Tech University, Lubbock, Texas
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Abstract

A set of Z-R relations is derived by linear regression for summertime convective rainfall in West Texas from data gathered with an SWR-75 weather radar and a network of weighing bucket raingages. Data from 21 storms were grouped into storm were dependent on size and degree of organization. Subcloud transport of the rainfall is modeled by using 850-mb winds to advect the rainfall pattern. Subcloud evaporation is modeled by a bulk method. Evaporation was most sensitive to the environmental relative humidity. Below a reflectivity level that is dependent on storm size and degree of organization, no information was obtained from which rainfall rates could be estimated with radar data.

For West Texas, Z=260 R1.02 best estimated overall mean rainfall volume when compared to raingage estimates. No significant reduction in the mean storm rainfall volume errors typical of such techniques occurred when the influence of the physical processes of subcloud transport and evaporation of rainfall was considered and incorporated into the calculations. Because of the small sample size, these results should be interpreted cautiously and the ZR relation should be regarded as a tentative estimate of the one appropriate for convective storms in West Texas.

The limits in attempting to account for the physical processes that contribute to the errors in radar-derived estimates of rainfall are illustrated.

Abstract

A set of Z-R relations is derived by linear regression for summertime convective rainfall in West Texas from data gathered with an SWR-75 weather radar and a network of weighing bucket raingages. Data from 21 storms were grouped into storm were dependent on size and degree of organization. Subcloud transport of the rainfall is modeled by using 850-mb winds to advect the rainfall pattern. Subcloud evaporation is modeled by a bulk method. Evaporation was most sensitive to the environmental relative humidity. Below a reflectivity level that is dependent on storm size and degree of organization, no information was obtained from which rainfall rates could be estimated with radar data.

For West Texas, Z=260 R1.02 best estimated overall mean rainfall volume when compared to raingage estimates. No significant reduction in the mean storm rainfall volume errors typical of such techniques occurred when the influence of the physical processes of subcloud transport and evaporation of rainfall was considered and incorporated into the calculations. Because of the small sample size, these results should be interpreted cautiously and the ZR relation should be regarded as a tentative estimate of the one appropriate for convective storms in West Texas.

The limits in attempting to account for the physical processes that contribute to the errors in radar-derived estimates of rainfall are illustrated.

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