Dryline Position Errors in Experimental Convection-Allowing NSSL WRF Model Forecasts and the Operational NAM

This study evaluates 24 h forecasts of dryline position from an experimental 4-km grid-spacing version of the Weather Research and Forecasting (WRF) model run daily at the National Severe Storms Laboratory (NSSL), as well as the 12 km grid-spacing North America Mesoscale (NAM) model run operationally by the Environmental Modeling Center of NCEP. For both models, 0000 UTC initializations are examined, and for verification 0000 UTC Rapid Update Cycle (RUC) analyses are used. For the period 1 April to 30 June 2007-2011, 109 cases containing drylines in all three datasets were identified using a manual procedure that considered specific humidity gradient magnitude, temperature, and 10 m wind.

For the 24 h NAM forecasts, no systematic east-west dryline placement errors were found and the majority of east-west errors fell within the range +/- 0.5 degrees longitude. The lack of a systematic bias was generally present across all subgroups of cases categorized according to month, weather pattern, and year. In contrast, a systematic eastward bias was found in 24 h NSSL-WRF forecasts, which was consistent across all subgroups of cases. The eastward biases seemed to be largest for the subgroups that favored “active” (i.e., drylines associated with a progressive synoptic scale weather system) as opposed to “quiescent” drylines that tend to be present with weaker tropospheric flow and have eastward movement dominated by vertical mixing processes in the boundary layer.