The Relationship Between the Surface Wind Field and Convective Precipitation over the St. Louis Area

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  • 1 State Water Survey Division, Illinois Department of Energy and Natural Resources, Champaign, IL 61820
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Abstract

Rainfall, wind and temperature data at the surface for a mesoscale area surrounding St. Louis, Missouri for seven summer days in 1975 were used to determine qualitative and quantitative relationships between divergence, and the location, timing and intensity of rainfall. The study used 30 prominent raincells that formed over a 1600 km2 wind network under different synoptic and subsynoptic weather conditions.

The results indicate that the physical relationships between convergence and convective rainfall over the Middle West are quite complex. Widespread convective rainfall seldom occurred in the absence of some larger scale forcing. Possible forcing mechanisms occurred at various levels within the troposphere and over several different space scales. On the cell scale, convergence preceded some raincells but not others. Potential mechanisms that could explain the dichotomy were investigated.

The results from several statistical studies include:

1) Pre-rain average network convergence was weakly related to precipitation amount.

2) Significant changes in the subnetwork-scale convergence began as early as 75–90 min before the rain. Correlations maximized at 0.55, 15 min prior to rainfall.

3) Convergence centers that were spatially linked with raincells became established 30 min prior to rainfall on the average, but relatively large correlations (0.55) for these centers were round only at 15 min before the rain began.

4) The Ulanski and Garstang spatial index and a second index of convergence strength developed in this research explained about 10% of the rainfall variance for a subset of raincells for which rainfall totalled more than 1 cm.

Abstract

Rainfall, wind and temperature data at the surface for a mesoscale area surrounding St. Louis, Missouri for seven summer days in 1975 were used to determine qualitative and quantitative relationships between divergence, and the location, timing and intensity of rainfall. The study used 30 prominent raincells that formed over a 1600 km2 wind network under different synoptic and subsynoptic weather conditions.

The results indicate that the physical relationships between convergence and convective rainfall over the Middle West are quite complex. Widespread convective rainfall seldom occurred in the absence of some larger scale forcing. Possible forcing mechanisms occurred at various levels within the troposphere and over several different space scales. On the cell scale, convergence preceded some raincells but not others. Potential mechanisms that could explain the dichotomy were investigated.

The results from several statistical studies include:

1) Pre-rain average network convergence was weakly related to precipitation amount.

2) Significant changes in the subnetwork-scale convergence began as early as 75–90 min before the rain. Correlations maximized at 0.55, 15 min prior to rainfall.

3) Convergence centers that were spatially linked with raincells became established 30 min prior to rainfall on the average, but relatively large correlations (0.55) for these centers were round only at 15 min before the rain began.

4) The Ulanski and Garstang spatial index and a second index of convergence strength developed in this research explained about 10% of the rainfall variance for a subset of raincells for which rainfall totalled more than 1 cm.

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