A Generalized Simple Formulation of Convective Adjustment Timescale for Cumulus Convection Parameterizations
He, J., Kiran Alapaty, AND J. Herwehe. A Generalized Simple Formulation of Convective Adjustment Timescale for Cumulus Convection Parameterizations. 97th Annual American Meteorological Society Meeting, Seattle, Washington, January 22 - 26, 2017.
The results show that GENTAU predicts similar precipitation pattern and intensity as STDTAU and MFXTAU at 108-km and 36-km grid spacing, but predict better precipitation pattern and intensity than STDTAU and MFXTAU at 12-km and 4-km grid spacing, with much better performance at 4-km grid spacing. This demonstrates the new formulation can be applied to both global and regional climate/weather forecasting models, with various grid spacing (e.g., from hundred kilometers to a few kilometers). The benefit of this generalized formulation is also demonstrated at fine grid resolution (e.g., 4-km), which is a need for current and future global and regional climate/weather forecasting models.
Convective adjustment timescale (τ) for cumulus clouds is one of the most influential parameters controlling parameterized convective precipitation in climate and weather simulation models at global and regional scales. Due to the complex nature of deep convection, a prescribed value or ad hoc representation of τ is used in most global and regional climate/weather models making it a tunable parameter and yet still resulting in uncertainties in convective precipitation simulations. In this work, a generalized simple formulation of τ for use in any convection parameterization for shallow and deep clouds is developed to reduce convective precipitation biases at different grid spacing. Unlike existing other methods, our new formulation can be used with field campaign measurements to estimate τ as demonstrated by using data from two different special field campaigns. Then, we implemented our formulation into a regional model (WRF) for testing and evaluation. Results indicate that our simple τ formulation can give realistic temporal and spatial variations of τ across continental U.S. as well as grid-scale and subgrid scale precipitation. We also found that as the grid spacing decreases (e.g., from 36 to 4-km grid spacing), grid-scale precipitation dominants over subgrid-scale precipitation. The generalized τ formulation works for various types of atmospheric conditions (e.g., continental clouds due to heating and large-scale forcing over land/water like in monsoon environments), and various horizontal resolutions (through hundred kilometers to a few kilometers).