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

Abstract

This study evaluates 24-h forecasts of dryline position from an experimental 4-km grid-spacing version of the Weather Research and Forecasting Model (WRF) run daily at the National Severe Storms Laboratory (NSSL), as well as the 12-km grid-spacing North America Mesoscale Model (NAM) 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-30 June 2007-11, 116 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 the east-west errors fell within the range 60.58 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'' drylines (i.e., those 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.

Characteristics and Synoptic Environment of Drylines Occurring over the Higher Terrain of Southeastern Wyoming

“…This particular =q y was chosen in order to be consistent with previous dryline studies (Schaefer 1974b;Hoch and Markowski 2005;Coffer et al 2013). The second and third criteria were chosen because they are commonly observed near drylines over the SGP (e.g., Ziegler and Hane 1993;Miao and Geerts 2007;Geerts 2008), although the second criterion also acts as a means of removing strong eastward-moving cold fronts.…”

The Effect of the Dry Line and Convective Initiation on Drought Evolution over Oklahoma during the 2011 Drought

“…The Control simulations demonstrated that the WRF produced reasonable atmospheric moisture gradients and precipitation patterns compared to observations. While differences were noted, these were typical to the WRF modeling system shown by previous studies [61]. Sensitivity simulations modifying soil moisture were completed to quantify the impacts of soil moisture on the dry line location during the study period.…”

“…by Coffer et al [61] that found an eastward bias of the dry line in the operational NSSL-WRF system. Surface fields were also well represented by the Control simulations, with each simulation incurring small perturbations to the surface temperature and moisture fields.…”

“…This figure indicates two areas of preferred dry line locations, over the central to west central portion of the state and in the northwest portion of the state into the panhandle. Coffer et al [61] mentioned in their analysis of model dry line position errors that 2011 was an active year with frequent passages of shortwave troughs across the central Plains. Hane [62] states that dry lines that form in active synoptic regimes propagate further to the east during the day than those that develop during quiescent periods because the increased westerly momentum associated with the synoptic disturbance aids the vertical turbulent mixing in shifting the boundary towards the east.…”

Assimilating Surface Mesonet Observations with the EnKF to Improve Ensemble Forecasts of Convection Initiation on 29 May 2012

“…A potential reason for the delayed CI within CI3 in CNTL is the westward bias in the placement of the surface dryline (Fig. 17); CNTL does not mix Other studies have found that convection-permitting models mix the dryline too far to the east (Coffer et al 2013). The present case is different in that the dryline is not associated with a synoptic-scale low pressure system, thus its evolution is likely more sensitive to PBL estimates of vertical mixing.…”