Information content of differential reflectivity columns for precipitation nowcasting

Abstract

<p>Columns of differential reflectivity, the difference between the horizontal and vertical reflectivity,<br>hereafter Zdr columns, are vertical columns of enhanced Zdr that extend above the environmental 0°C<br>level. These are easily identified when observed by polarimetric radars. Physically, these columns consist<br>of rain dominated by large drops that are being lofted above the freezing level and have been recognized<br>as a proxy for the location of updrafts. Their potential for nowcasting severe weather has been shown in<br>several past studies. We have developed an algorithm that identifies and tracks Zdr columns from<br>volumetric radar data along with 3D wind fields from MultiDoppler analysis to spatially correlate Zdr<br>columns with updrafts. Since Zdr columns are a manifestation of an updraft, different Zdr columns<br>properties for example Zdr column maximum height, volume, and area are expected to be related to<br>updraft intensity levels. In turn intensification of updrafts, as indicated by changes in the Zdr columns<br>properties, should be translated in intensification of observed precipitation at the surface. For the<br>estimation of rain rates, we used a radar-based polarimetric approach, which will allow us to monitor the<br>temporal evolution for a number of identified convective rain cells. These cells will be identified from<br>summer events observed by the C-band polarimetric German network. For each cell, the properties of Zdr<br>columns are correlated with rain rate values. Similarly, correlations are also calculated for updraft<br>volumes, updraft intensity, and other updraft properties. For the nowcasting of observed rain rates, an<br>extrapolation algorithm based on spatial and temporal properties of rain was used. Preliminary results<br>have shown that higher precipitation rates are generally associated with Zdr columns, and cells without a<br>Zdr column produce lower precipitation rates, as expected. Zdr column height and volume show a<br>positive correlation with precipitation intensity at the surface. The time lag between the intensification of<br>the Zdr column and associated increase in precipitation at the surface varies significantly between cells,<br>but it is generally short compared to previous studies, varying mostly between 5 to 15 minutes. An early<br>identification of cells associated with ZDR columns could benefit the skill of the nowcasting of localized<br>rain cells, which often are smoothed during extrapolation.</p>