NWC REU 2007
May 29 - August 2



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Errors in the WSR-88D ZS (Snow) Algorithm

Laura Kruep and Jim LaDue



The WSR-88D’s ZS (snow) Algorithm gives current estimates of snow accumulations. However, underestimations from the radar beam overshooting the dendrite production zones and riming zones can occur. Overestimations can occur from bright banding and sub-beam evaporation/sublimation. This study seeks out a method for forecasting when and where these errors might occur and tests its accuracy.


Ground observations used as ground truths and corresponding radar data was collected for three snow events: Wisconsin for March 1-3, 2007, Pennsylvania for March 16-17, 2007, and Colorado for January 29-31, 2005. Comparing the ground truths to the radar’s snow water equivalent (SWE) rate showed the ZS algorithm is inaccurate in its SWE estimates. A method for forecasting the sources of the previously mentioned errors was devised, but after analysis, it too was considered inaccurate. Regressions were then run on the data to see what factors would lead to better predictors of SWE rates. Out of all the data tested, it is believed that basing SWE rates off the distance between the bottom of the radar beam and the -3°C and -12°C layers. If better predictors could be found, improvements can be made to the ZS snow algorithm.

Full Paper [PDF]