Characteristics of High-Resolution Versions of the Met Office Unified Model for Forecasting Convection over the United Kingdom

Abstract

With many operational centers moving toward order 1-km-gridlength models for routine weather forecasting, this paper presents a systematic investigation of the properties of high-resolution versions of the Met Office Unified Model for short-range forecasting of convective rainfall events. The authors describe a suite of configurations of the Met Office Unified Model running with grid lengths of 12, 4, and 1 km and analyze results from these models for a number of convective cases from the summers of 2003, 2004, and 2005. The analysis includes subjective evaluation of the rainfall fields and comparisons of rainfall amounts, initiation, cell statistics, and a scale-selective verification technique. It is shown that the 4-and 1-kmgridlength models often give more realistic-looking precipitation fields because convection is represented explicitly rather than parameterized. However, the 4-km model representation suffers from large convective cells and delayed initiation because the grid length is too long to correctly reproduce the convection explicitly. These problems are not as evident in the 1-km model, although it does suffer from too numerous small cells in some situations. Both the 4-and 1-km models suffer from poor representation at the start of the forecast in the period when the high-resolution detail is spinning up from the lower-resolution (12 km) starting data used. A scale-selective precipitation verification technique implies that for later times in the forecasts (after the spinup period) the 1-km model performs better than the 12-and 4-km models for lower rainfall thresholds. For higher thresholds the 4-km model scores almost as well as the 1-km model, and both do better than the 12-km model.

The diurnal cycle of precipitation over the Maritime Continent in a high‐resolution atmospheric model

“…The UK Met Office UM, version 7.1, was used in the configuration described by Lean et al (2008). This configuration applies a semi-implicit, semi-Lagrangian scheme to a rotated latitude-longitude grid (Davies et al, 2005) and applies parametrisations to represent boundarylayer turbulence (Lock et al, 2000), and mixed-phase microphysics (Wilson and Ballard, 1999 .…”

Precipitation distributions for explicit versus parametrized convection in a large‐domain high‐resolution tropical case study

“…The 40 km model has similar physics to the 12 km param model, although the convective parametrization closure lacks the vertical velocity adjustment. The 4 km 2Dsmag model uses a CAPE-limited version of the convective parametrization that asymptotes to the same 30 min CAPE time scale at zero CAPE but has a CAPE time scale that rapidly increases with increasing CAPE, such that for typical tropical values virtually all rainfall is generated explicitly (Roberts, 2003;Lean et al, 2008). This model version uses the standard boundary-layer scheme for vertical subgrid mixing but includes Smagorinsky-type subgrid mixing in the horizontal dimensions.…”

A critical assessment of surface cloud observations and their use for verifying cloud forecasts

“…Davies et al (2005), Lean et al (2008) and Hewitt et al (2011) provide further details. In this article, cloud forecasts from the Global (GM, 25 km), North Atlantic European (NAE, 12 km), UK4 (4 km) and UKV (1.5 km) configurations are compared over the UK.…”