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Hill P.G.,MetOffice at Reading | Morcrette C.J.,UK Met Office | Boutle I.A.,UK Met Office
Quarterly Journal of the Royal Meteorological Society

The subgrid-scale spatial variability in cloud water content can be described by a parameter f called the fractional standard deviation. This is equal to the standard deviation of the cloud water content divided by the mean. This parameter is an input to schemes that calculate the impact of subgrid-scale cloud inhomogeneity on gridbox-mean radiative fluxes and microphysical process rates. A new regime-dependent parametrization of the spatial variability of cloud water content is derived from CloudSat observations of ice clouds. In addition to the dependencies on horizontal and vertical resolution and cloud fraction included in previous parametrizations, the new parametrization includes an explicit dependence on cloud type. The new parametrization is then implemented in the Global Atmosphere 6 (GA6) configuration of the Met Office Unified Model and used to model the effects of subgrid variability of both ice and liquid water content on radiative fluxes and autoconversion and accretion rates in three 20-year atmosphere-only climate simulations. These simulations show the impact of the new regime-dependent parametrization on diagnostic radiation calculations, interactive radiation calculations and both interactive radiation calculations and in a new warm microphysics scheme. The control simulation uses a globally constant f value of 0.75 to model the effect of cloud water content variability on radiative fluxes. The use of the new regime-dependent parametrization in the model results in a global mean which is higher than the control's fixed value and a global distribution of f which is closer to CloudSat observations. When the new regime-dependent parametrization is used in radiative transfer calculations only, the magnitudes of short-wave and long-wave top of atmosphere cloud radiative forcing are reduced, increasing the existing global mean biases in the control. When also applied in a new warm microphysics scheme, the short-wave global mean bias is reduced. This article describes the development and impacts of a new cloud-type dependent parametrization of the variability of cloud water content. This image shows a snapshot from the CloudSat observations (courtesy of the CloudSat Data Processing Center) used to derive the parametrization together with the corresponding MODIS view and highlights the clear difference in variability between convective clouds to the centre and right of the image and non-convective clouds to the left. © 2015 Royal Meteorological Society. Source

Davies T.,MetOffice at Reading
Quarterly Journal of the Royal Meteorological Society

Limited area models (LAMs) are widely used in numerical weather prediction and regional climate modelling to obtain high-resolution results that would be too expensive computationally to be obtained with a global model. To run LAMs requires lateral boundary conditions (LBCs) which are normally obtained from a global model or another (lower-resolution) LAM containing the domain of the smaller LAM. It is widely thought that the LBCs are a source of significant errors in LAMs but in the Meteorological Office Unified Model (UM) LBC errors are shown to be a tiny part of the overall error. The source of LBC errors is explained and the LBC procedure employed in the UM is described. The UM LBC procedure is effectively transparent for a nested LAM with the same resolution and grid points as a LAM providing its LBCs. © 2013 John Wiley & Sons, Ltd. Source

Nicol J.C.,University of Reading | Hogan R.J.,University of Reading | Stein T.H.M.,University of Reading | Hanley K.E.,MetOffice at Reading | And 4 more authors.
Quarterly Journal of the Royal Meteorological Society

This study presents an evaluation of the size and strength of convective updraughts in high-resolution simulations by the UK Met Office Unified Model (UM). Updraught velocities have been estimated from range-height indicator (RHI) Doppler velocity measurements using the Chilbolton advanced meteorological radar, as part of the Dynamical and Microphysical Evolution of Convective Storms (DYMECS) project. Based on mass continuity and the vertical integration of the observed radial convergence, vertical velocities tend to be underestimated for convective clouds due to the undetected cross-radial convergence. Velocity fields from the UM at a resolution corresponding to the radar observations are used to scale such estimates to mitigate the inherent biases. The analysis of more than 100 observed and simulated storms indicates that the horizontal scale of updraughts in simulations tend to decrease with grid length; the 200 m grid length agreed most closely with the observations. Typical updraught mass fluxes in the 500 m grid length simulations were up to an order of magnitude greater than observed, and greater still in the 1.5 km grid length simulations. The effect of increasing the mixing length in the sub-grid turbulence scheme depends on the grid length. For the 1.5 km simulations, updraughts were weakened though their horizontal scale remained largely unchanged. Progressively more so for the sub-kilometre grid lengths, updraughts were broadened and intensified; horizontal scale was now determined by the mixing length rather than the grid length. In general, simulated updraughts were found to weaken too quickly with height. The findings were supported by the analysis of the widths of reflectivity patterns in both the simulations and observations. © 2015 Royal Meteorological Society. Source

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