Articles | Volume 10, issue 1
Model evaluation paper
06 Jan 2017
Model evaluation paper |  | 06 Jan 2017

Connecting spatial and temporal scales of tropical precipitation in observations and the MetUM-GA6

Gill M. Martin, Nicholas P. Klingaman, and Aurel F. Moise

Abstract. This study analyses tropical rainfall variability (on a range of temporal and spatial scales) in a set of parallel Met Office Unified Model (MetUM) simulations at a range of horizontal resolutions, which are compared with two satellite-derived rainfall datasets. We focus on the shorter scales, i.e. from the native grid and time step of the model through sub-daily to seasonal, since previous studies have paid relatively little attention to sub-daily rainfall variability and how this feeds through to longer scales. We find that the behaviour of the deep convection parametrization in this model on the native grid and time step is largely independent of the grid-box size and time step length over which it operates. There is also little difference in the rainfall variability on larger/longer spatial/temporal scales. Tropical convection in the model on the native grid/time step is spatially and temporally intermittent, producing very large rainfall amounts interspersed with grid boxes/time steps of little or no rain. In contrast, switching off the deep convection parametrization, albeit at an unrealistic resolution for resolving tropical convection, results in very persistent (for limited periods), but very sporadic, rainfall. In both cases, spatial and temporal averaging smoothes out this intermittency. On the  ∼  100 km scale, for oceanic regions, the spectra of 3-hourly and daily mean rainfall in the configurations with parametrized convection agree fairly well with those from satellite-derived rainfall estimates, while at  ∼  10-day timescales the averages are overestimated, indicating a lack of intra-seasonal variability. Over tropical land the results are more varied, but the model often underestimates the daily mean rainfall (partly as a result of a poor diurnal cycle) but still lacks variability on intra-seasonal timescales. Ultimately, such work will shed light on how uncertainties in modelling small-/short-scale processes relate to uncertainty in climate change projections of rainfall distribution and variability, with a view to reducing such uncertainty through improved modelling of small-/short-scale processes.

Short summary
We analyse and evaluate tropical rainfall variability in the MetUM-GA6 configuration at four different horizontal resolutions, plus one in which the convection parameterization has been switched off. Tropical deep convective rainfall in this model tends to be intermittent in space and time. This behaviour is largely independent of model resolution. Switching off the deep convection parameterization (at ~10 km resolution) results in isolated, but persistent, rainfall on the gridscale.