Convective Meteorology (Mesoscale Dynamics)

Horizontal grid spacing dependence of idealized WRF simulations and implications for Warn-on-Forecast

Dr. Corey Potvin
Research Scientist
CIMMS / NSSL

26 September 2014, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Continual improvement in computational technology will soon enable real-time ensemble data assimilation and prediction of storms on convection-allowing (e.g., <= 4 km) grids. The overarching goal of the Warn-on-Forecast (WoF) program is to develop and deploy such a system to improve short-term forecasts of tornadoes, flooding, damaging wind and large hail. Until convection-resolving (< 100 m) systems become available, however, resolution errors will degrade the accuracy of ensemble model output. Improved understanding of grid spacing dependence of simulated convection will therefore be needed to properly interpret and calibrate ensemble output, and to optimize tradeoffs between model resolution and other computationally constrained parameters like ensemble size and forecast lead time.

Toward this end, we examine grid spacing sensitivities of idealized WRF-simulated supercells over grid spacings of 333 m - 4 km. Our approach differs from those of previous studies in several important respects. First, we use a range of initialization soundings to gain a more general understanding of the grid spacing sensitivities. Second, we initialize our coarser-resolution (1-4 km) simulations not from a thermal bubble as in the control (333 m) simulations, but from appropriately upscaled versions of the latter after 30 min of integration. This approach is motivated by the need to assimilate several (e.g., 5-10) radar data volumes before skillful forecasts can be obtained from a WoF system. Third, we focus our analysis on model fields most relevant to O[1 hr] convective forecast and warning operations, such as low-level vorticity, rainfall, and surface wind. Finally, we compare our coarser-resolution simulations to both the control simulations and filtered versions thereof in order to isolate impacts of unresolved upscale interactions.

Our results so far suggest that 4 km grid spacing is too coarse to reliably simulate supercell evolution on WoF timescales, often leading to premature storm demise, whereas 3 km grid spacing more often permits forecasts that broadly capture operationally important features, including low-level rotation tracks. The latter result is consistent with real-world 3-km WoF experiments being conducted at CIMMS/NSSL, and is encouraging given that complex evolution of low-level supercell rotation is poorly resolved on grids > 1 km, as our experiments and previous studies demonstrate. Many grid spacing sensitivities vary substantially among our experiments, indicating that calibration of ensemble output may be of limited value until relationships between grid spacing dependence and storm characteristics, environment, and model physics are better understood.

This work is ongoing and suggestions from the audience are strongly encouraged.

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