Improvement in the capturing of short-range warm season orographic precipitation in the ECMWF model

Summary

ECMWF short-range deterministic forecasts at T799 resolution (approximately 25 km horizontal resolution) of warm season convective precipitation over the Rockies and the Alps are compared with high-resolution observations. Both a former model cycle (30R1, operational in the first half of 2006, the first model version at resolution T799), and the most recent cycle (32R3) of the ECMWF Integrated Forecasting System (IFS) are investigated in order to assess the impact of substantial modifications to the model’s convection and radiation scheme on the precipitation forecasts.

With 30R1, the strong diurnal cycle of precipitation over the Rocky Mountains is not captured. Significant improvements are obtained with 32R3, including the capturing of a nocturnal rainfall maximum over the Great Plains associated with organized propagating mesoscale convective systems. Part of the improvements are due to non-local feedbacks where increased convective mid-tropospheric heating leads to improved large-scale circulations (moisture advection) maintaining the convection. By contrast, over the Alps both versions of the IFS under/over estimate the precipitation for values smaller/larger than 10 mm/day. This reflects the different meteorology and the east-west orientation of the Alpine mountains, where compared to the Rockies more quasi–stationary convective systems form in a generally moister large-scale environment, and where the large-scale horizontal advection of moisture is less crucial. However, it is suggested that here the overestimation of precipitation is linked to excessive low-level blocking.