1) Implementation of the ECMWF cumulus convection scheme into the GFS,
2) A stochastic approach to cumulus convection parameterization using cellular automata
Lisa Bengtsson
ESRL
13 May, Noon, in 2155
Abstract:
I will use this opportunity to cover two topics related to cumulus
convection parameterization. There is a longstanding challenge in
numerical weather and climate prediction to accurately model the
observed spectrum of tropical wave variability, including convectively
coupled equatorial waves (CCEWs) and the Madden Julian Oscillation. The
difficulty stems from the complex nature of the interaction between
cumulus convection and the large-scale tropical circulation, many
aspects of which must be parameterized using schemes with uncertain
assumptions. In recent years, progress in modeling tropical wave
variability has been made, owing to the adoption of higher resolution
models, improvements in coupling between the atmosphere and ocean, and
the introduction of more sophisticated parameterization schemes of
sub-grid scale processes. For sub-seasonal prediction, the European
Centre for Medium range Weather Forecasts (ECMWF) stands out in
modeling tropical variability and has been shown in a recent study to
be superior to the NOAA Global Forecast System (GFS). In the first part
of the talk, I will cover the topic where we investigate the impacts of
introducing the cumulus convection parameterization from the ECMWF
model into a research version of the GFS. We highlight the importance
of consistent treatment of processes between Planetary Boundary Layer
(PBL), cloud, radiation and convection schemes. We also investigate the
relative role the ECMWF initial conditions and ECMWF cumulus convection
scheme play in forecasting CCEWs within the GFS. In the second part of
the talk I will cover the topic where we introduce a prognostic,
stochastic, equation for the convective updraft area fraction in the
cumulus convection scheme in the GFS that uses a cellular automaton
(CA). The self‐organizational characteristics of the CA allow for
lateral communication between adjacent numerical weather prediction
(NWP) model grid boxes and add additional memory to the deep convection
scheme. The CA acts in two horizontal dimensions, with finer grid
spacing then the NWP model. The distribution of CA cells on the
sub-grid represents a stochastic distribution of convective sub-grid
plumes and follows a skewed distribution. We investigate the impact on
convective organization and CCEWs and precipitation frequency
distribution.