Use of Radar-Derived Extinction to Diagnose Snow Intensity

Abstract Winter storms create significant impacts on life and property. These impacts depend on, among other things, how quickly snow falls. The current surface observation network does not directly measure snowfall rate with meaningful spatial and temporal resolution. Surface meteorological stations, such as the Automated Surface Observing System, typically report a visibility-derived snow intensity. While this parameter is not a true stand-in for snowfall rate due to its dependence on other factors (humidity, snow morphology, presence of blowing snow, etc.), snow intensity often forms the basis for stakeholder decisions in the absence of other observations to fill the decision support hole. This study evaluates a radar-based visibility algorithm as the basis for a two-dimensional, gridded snow intensity analysis. The algorithm requires only Multi-Radar Multi-Sensor precipitation rate and surface pressure to calculate extinction—the loss of light energy over a distance—which is then used to derive visibility and finally snow intensity. The algorithm’s snow intensities outperform a reflectivity-derived baseline. Changing the characteristics of the particle size distribution in the algorithm impacted the type of snow intensity it verified best against. Verification was performed using both a traditional two-category framework and a three-category framework. An example of the decision support capability is shown using a case study, illustrating the potential of this product to fill in spatial and temporal gaps in coverage within the surface observation network. Significance Statement There are few instruments available to determine how rapidly snow is falling in real time. One of the most commonly used observations is snow intensities reported by surface weather stations. These snow intensities are categories (light, moderate, or heavy) and based on the observed visibility. This study tests a radar-based methodology to derive visibility and determine snow intensity between surface stations. This methodology performed better than using the radar reflectivity alone; radar reflectivity is currently used as a situational awareness tool during snow events, despite known shortcomings.