Evaluation of a Modified Scheme for Shallow Convection: Implementation of CuP and Case Studies

A new treatment for shallow clouds has been introduced into the Weather Research and Forecasting Model (WRF). The new scheme, called the cumulus potential (CuP) scheme, replaces the ad hoc trigger function used in the Kain-Fritsch cumulus parameterization with a trigger function related to the distribution of temperature and humidity in the convective boundary layer via probability density functions (PDFs). An additional modification to the default version of WRF is the computation of a cumulus cloud fraction based on the time scales relevant for shallow cumuli. Results from three case studies over the U. S. Department of Energy's Atmospheric Radiation Measurement (ARM) site in north-central Oklahoma are presented. These cases were selected because of the presence of shallow cumuli over the ARM site. The modified version of WRF does a much better job predicting the cloud fraction and the downwelling shortwave irradiance than control simulations utilizing the default Kain-Fritsch scheme. The modified scheme includes a number of additional free parameters, including the number and size of bins used to define the PDF, the minimum frequency of a bin within the PDF before that bin is considered for shallow clouds to form, and the critical cumulative frequency of bins required to trigger deep convection. A series of tests were undertaken to evaluate the sensitivity of the simulations to these parameters. Overall, the scheme was found to be relatively insensitive to each of the parameters.