Improving the ECMWF model’s representation of supercooled layers in arctic mixed-phase clouds

Description

In November 2010, the ECMWF model’s cloud scheme was extended to include additional prognostic variables for separate cloud liquid and ice species, as well as prognostic rain and snow variables. Instead of a diagnostic split for cloud water species dependent on temperature, this new cloud scheme provides the framework for co-existing cloud water species and an explicit treatment of conversion processes between phases.

Clouds at the North Slope of Alaska (NSA) site are challenging to represent well in a global model due to complicated layer structures, often including supercooled liquid layers. The new cloud scheme is assessed for the Mixed-Phase Arctic Cloud Experiment (M-PACE) during the autumn of 2004.

With the new cloud scheme, the model is now able to produce mixed-phase clouds topped by a liquid layer. In order to improve forecasts during weakly forced conditions, the generation terms for supercooled liquid water and the ice deposition rate near cloud top are modified to allow more supercooled liquid to form. ARM observations from NSA are used to validate these changes.

The impact on liquid and ice water paths and surface radiation for sample cases from the M-PACE experiment are investigated. For the single-layer case (October 9–10, 2004, first intercomparison case), liquid and ice water paths are in good agreement with aircraft observations.