Evaluation of the Mountain Mapper Product Generated by the Multi-Radar Multi-Sensor System (MRMS) over the Russian River Basin Region in California

The National mosaic & multi-sensor QPE (NMQ), also known as MRMS, is a multi-radar multi-sensor system developed by the National Severe Storms Laboratory (NSSL) [1]. Its goal is to assimilate observations and products from different weather sensors towards producing accurate and high resolution QPE. Various operational products from the MRMS system are used for weather prediction and precipitation estimation over the continental USA. Accurate estimation of rainfall accumulation is a very difficult task, especially in regions, which are dominated by mountains, such as the Russian River Basin in California, USA. Precipitation estimation by operational radars in this complex terrain is limited by several factors, such as radar beam blockage by mountains, discontinuity in vertical reflectivity profile due to coarse vertical resolution etc. The problem is further escalated by radar beam overshooting in the case orographic precipitation during the cool season, which is characterized by very low cloud tops. To address this issue one of the products from MRMS named Mountain Mapper [2] is used by the NWS California Nevada River Forecast Center (CNRFC) for forecast and flash flood warning in the western United States. This procedure is based on real-time observations from Hydrometeorological Automated Data System (HADS) rain gauges along with Parameter-elevation Regressions on Independent Slopes Model (PRISM) [3] climatology.

At places where the rain gauge network is sparsely distributed, for small scale convective events or in regions with variable orographic terrain, the Mountain Mapper product may not give accurate rainfall estimates. Also at times, the PRISM climatology might not capture the real-time variations in the distribution of rainfall. In this study, the Mountain Mapper product is evaluated with the observations from rain gauges in the Russian River watershed operated by the NOAA Physical Science Division (PSD). These rain gauges are independent and are not used in the generation of the Mountain Mapper product. Precipitation data from the years 2015 to 2017 for the months of October to March are used for this study. The MRMS Mountain Mapper is a 1Km by 1Km gridded product. For comparison purposes, the pixels corresponding to the PSD rain gauge locations are selected. Hourly rainfall accumulations as well as 24-hour or daily rainfall accumulations are compared. Statistical scores in terms of Normalized bias, Normalized Standard Error and Correlation Coefficient are presented. Probability distribution of low to high rainfall accumulations is also compared between selected rain gauges and Mountain Mapper product. Preliminary results indicate that the Mountain Mapper product shows better comparison with the rain gauges at low elevation than that of high elevations. This presentation will explore the accuracy of the Mountain Mapper product with respect to terrain, gauge locations, and storm characteristics.

References:

[1] Zhang, J., et al. (2011a). “National mosaic and multi-sensor QPE (NMQ) system: Description, results, and future plans.” Bull. Am. Meteorol. Soc., 92(10), 1321–1338.

[2] Schaake, J., A. Henkel, and S. Cong, 2004: Application of PRISM climatologies for hydrologic modeling and forecasting in the western U.S. Preprints, 18th Conf. on Hydrology, Seattle, WA, Amer. Meteor. Soc., 5.3. [Available online at http://ams.confex.com/ams/ pdfpapers/72159.pdf.]

[3] Daly, C., R. P. Neilson, and D. L. Phillips, 1994: A statistical–topographic model for mapping climatological precipitation over mountainous terrain. J. Appl. Meteor., 33, 140–158.