The Met Office Convection Diagnosis Scheme

Abstract

An operational system for diagnosing probability of convective development and structure from NWP model outputs over the British Isles has been developed in the Met Office. The system originated as a research and development project and so for historical reasons is now known as the Convection Diagnosis Project (CDP). The system takes account of expected model errors in temperature and humidity and allows for forcing from sub-grid scale mechanisms on a 1-km scale that are important for mid-latitude convection. A NWP convection scheme is used to determine grid-mean parameters from a range of forcings according to probability of occurrence. The grid-mean values are then used in a simple and variable object-oriented life-cycle model of a convective cloud to diagnose shower structure and expected movement. Using the life-cycle model, a range of probabilistic forecast outputs are produced four times a day from T+6 to T+36 hours ahead for individual grid squares and for larger pre-defined regions. Case studies and rigorous objective verification, making use of an automatic method for identifying convective cloud from radar and satellite data, have shown that the CDP outputs complement the standard NWP outputs and can provide additional useful information in a forecasting environment.