An Examination of Dual-Polarization Radar-Derived Precipitation Estimates over New Mexico Burn Scars during the 2012 Monsoon

Wildfire burn scars pose an especially challenging hydrometeorological dilemma. Much of New Mexico's annual precipitation is received during the summer months and is associated with the North American Monsoon System. The high intensity, short duration rainfall, typical of the summer monsoon thunderstorm events, often produces areas of flash flooding. The risk of damage from flash flooding is greatly increased over and below burn scars as both runoff and debris flows over burned surfaces are many times greater than that of a healthy forest.

Accurate precipitation estimates are vital in these extremely vulnerable areas when quick decision making and timely flash flood warnings can save lives. Even small amounts of precipitation, not normally sufficient for flash flooding, can cause significant, and, in some cases, potentially devastating impacts.

The WSR-88D dual-polarization upgrade was completed for all New Mexico radars in Spring 2012. Improved precipitation estimates due to the addition of new polarimetric variables that describe the size, shape and density of the particles were among the key objectives of the upgrade. Different rainfall algorithms were employed depending upon values of reflectivity, differential reflectivity, and specific differential phase. For enhanced interrogation of dual-polarization rainfall accumulation and rainfall rates, several new precipitation products were developed including One Hour Accumulation, Storm Total Accumulation, and Instantaneous Precipitation Rate, as well as others. These new products were used extensively during the 2012 monsoon with the goals of increasing flash flood warning lead time and enhancing decision support following wildfires that burned nearly 870,000 acres of forested lands in New Mexico during the 2011 and 2012 fire seasons.

This study evaluated several monsoon thunderstorm rain events and compared the performance of the legacy and the dual-polarization quantitative precipitation estimates. Attention was focused over and downstream from recent burn scars due to their high vulnerability and potential impacts. New Mexico suffers from a lack of precipitation observations; however, a dense network of rain gages installed over and near the freshly burned landscapes provided an enhanced network of ground truth verification. Because most burn scars are located at a fairly large distance from the radars, and are also affected by beam blockage associated with New Mexico's diverse terrain, additional non-burn scar verification was completed using various surface observing systems and mesonet arrays.

This study also explored reasons why precipitation estimates from different radars varied widely over the same area, and how this information can be used to further refine local knowledge and dual-polarization radar interpretation. In most cases, the dual-polarization quantitative precipitation estimates were superior to the legacy system; however, a few inferior examples will also be presented.