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Combining Top-Down and Bottom-Up Approaches to Improve Boundary Layer Parametrization for NWP and Climate

Combining Top-Down and Bottom-Up Approaches to Improve Boundary Layer Parametrization for NWP and Climate

Andrew Brown (1), Bob Beare (1), Anton Beljaars (2), Hans Hersbach (2)
and Adrian Lock (1)

Affiliations: (1) Met Office (2) ECMWF

Two different but complementary methods have been used to identify and reduce biases in the Met Office Unified Model due to the turbulence parametrization. Both the 'top-down' approach of a detailed analysis of short-range NWP errors and the 'bottom-up' approach provided by detailed process studies led to the conclusion that excessive mixing in stable conditions and insufficient mixing in unstable conditions significantly contributed to model bias. Accordingly a modified scheme (featuring reduced mixing in stable conditions over the sea and the inclusion of non-local momentum mixing in convective conditions) has been introduced to both the NWP and climate versions of the Met Office Unified Model.

The top-down approach required short-range NWP errors to be conditionally sampled in a physically relevant way. Over land, it was found that both the Met Office and ECMWF models show systematic biases, with near surface winds which are too weak by day (especially in summer), and too strong by night. Over sea, wind direction was used as a proxy for stability, and strikingly different biases in wind speed and direction (relative to QuikSCAT and conventional observations) were found in northerly and southerly wind conditions.

In the bottom-up approach, single column model simulations were compared with idealized large-eddy simulations and observations. These tests highlighted failings in the traditional parametrizations, and the modified versions were developed.