Advancing Organized Convection Representation in the Unified Model:
Implementing and Enhancing Multiscale Coherent Structure
Parameterization
Abstract
To address the effect of stratiform latent heating on meso- to large
scale circulations, an enhanced implementation of the Multiscale
Coherent Structure Parameterization (MCSP) is developed for the Met
Office Unified Model. MCSP represents the top-heavy stratiform latent
heating from under-resolved organized convection in general circulation
models. We couple the MCSP with a mass-flux convection scheme
(CoMorph-A) to improve storm lifecycle continuity. The improved MCSP
trigger is specifically designed for mixed-phase deep convective cloud,
combined with a background vertical wind shear, both known to be crucial
for stratiform development. We also test a cloud top temperature
dependent convective-stratiform heating partitioning, in contrast to the
earlier fixed partitioning. Assessments from ensemble weather forecasts
and decadal simulations demonstrate that MCSP directly reduces cloud
deepening and precipitation areas by moderating mesoscale circulations.
Indirectly, it amends tropical precipitation biases, notably correcting
dry and wet biases over India and the Indian Ocean, respectively.
Remarkably, the scheme outperforms a climate model ensemble by improving
seasonal precipitation cycle predictions in these regions. This
enhancement is partly due to the scheme’s refinement of Madden-Julian
Oscillation (MJO) spectra, achieving better alignment with reanalysis
data by intensifying MJO events and maintaining their eastward
propagation after passing the Maritime Continent. However, the scheme
also increases precipitation overestimation over the Western Pacific.
Shifting from fixed to temperature-dependent convective-stratiform
partitioning reduces the Pacific precipitation overestimation but also
lessens the improvements of seasonal cycle in India. Spatially
correlated biases highlight the necessity for advancements beyond
deterministic approaches to align MCSP with environmental conditions.