Why warm conveyor belts matter in NWP
Rodwell, Mark ; Forbes, Richard ; Wernli, Heini
A warm conveyor belt (WCB) is a coherent warm and moist airstream, which originates in the boundary layer of an extratropical cyclone's warm sector. Air within the WCB ascends in a day or two to the upper troposphere while moving poleward. WCBs are the primary cloud- and precipitation-generating flow in extratropical cyclones, and they can be associated with extreme precipitation. In this article, based on work carried out jointly with ECMWF Fellow Heini Wernli and his team at ETH Zurich and discussed at a recent bilateral meeting, we illustrate two more ways in which WCBs are important in numerical weather prediction (NWP): The microphysical processes they involve can have a strong impact on the larger-scale dynamics and they are a major source and magnifier of forecast uncertainty.
Correctly modelling WCBs is of great relevance to ECMWF's Strategy to 2025, which calls for improved forecasts of extreme weather and regime transitions and increased reliability and sharpness of ECMWF ensemble forecasts. Here we suggest two ways to improve the representation of WCBs in weather models: better cloud microphysics and improvements in the initialisation of humidity.
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