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Using Large Eddy Simulations to parameterise the Convective Cloud Field (LES4CCFM)

Using Large Eddy Simulations to parameterise the Convective Cloud Field (LES4CCFM)

Figure 1 (above): Photo of cumulonimbus cloud over Africa, taken from the International Space Station. Credit: NASA

Project Overview

Despite much effort in improving the representation of physical processes in atmospheric models for climate and numerical weather prediction the representation of clouds including convection, their interaction with the planetary boundary layer and large scale dynamics remains a major source of uncertainty. This limits not only our ability to predict weather and climate but also their value for decision-making on timescales from days to decades. The key issue in representing convection in global models is that the resolutions of these models are too coarse to represent individual convective clouds. Instead, models rely on physically based parameterization of convection. Improvements in the performance of climate and numerical weather prediction models will crucially depend on the development of better parametrizations for convection.

LES4CCFM is part of the joint NERC and Met Office programme ParaCon (Parametrization of Convection). The aim of ParaCon is to significantly improve the representation of convection across model scales from 1-100km. Within LES4CCFM we will use Large Eddy Simulations (LES) to characterize and quantify key elements of convection. The work will be performed within the framework of a new parameterization, the Convective Cloud Field Model (CCFM) coupled to the UK Met Office Unified Model. The novelty of our approach is the explicit distinction between individual clouds and the ensemble or spectrum of clouds in LES and parameterization. Outcomes from this work will not only lead to a better CCFM but more crucially, will inform any new or existing convection parameterization for the UM.

Figure 2

Figure 2: Synthesized C-Band Radar Reflectivity of Mature State from a high-resolution simulation of a convective thunderstorm cloud.

The project started in August 2016. The main focus during the first three years, the exploratory phase of ParaCon, is the improved physical understanding of convection and its representation in multi-plume convection parameterizations like CCFM. To achieve this goal, CCFM will be added to the UK Met Office Unified Model (UM). UM-CCFM will used to translate findings from the LES studies in other work packages and to evaluate the impact of changes in the convection parameterization on large scale dynamics. LES studies will be performed in three consecutive work packages. First, we improve the representation of individual clouds in convection parameterizations by comparing LES output with prediction from the entraining parcel model in CCFM. In a second work package will focus on the convective cloud trigger. As part of a third work package we will characterize the convective cloud spectrum as it evolves over time.

Team

The following people in the Department of Geography and the Department of Chemistry are involved in this project: