Perturbation growth at the convective scale for CSIP IOP18

Abstract

The direct effects (within one time step) of the perturbations are to generate propagating Lamb and acoustic waves and produce generally small changes in cloud parameters and convective instability. In exceptional cases a perturbation at a specific grid point leads to switching of the diagnosed boundary-layer type or discontinuous changes in convective instability, through the generation or removal of a lid. The indirect effects (during the entire simulation) are changes in the intensity and location of precipitation and in the cloud-size distribution. Qualitatively different behaviour is found for strong (1 K amplitude) and weak (0.01 K amplitude) perturbations, with faster growth after sunrise found only for the weaker perturbations. However, the overall perturbation growth (as measured by the root-mean-square error of accumulated precipitation) reaches similar values at saturation, regardless of the perturbation characterization.

Ensemble forecasts of a flood‐producing storm: comparison of the influence of model‐state perturbations and parameter modifications

“…Indeed, despite the computational expense, experimentation with convective-scale ensembles is already well under way (e.g. Marsigli et al, 2004;Hohenegger et al, 2008;Leoncini et al, 2010) even while the operational use of a single forecast at these resolutions remains in its infancy. At the Met Office, for example, a convective-scale model has been run routinely since 2009 and a test system for ensembles at this scale is currently being built with the intention of trialling it during the spring of 2012.…”

“…Marsigli et al, 2004;Hohenegger et al, 2008;Leoncini et al, 2010;van Weverberg et al, 2010;Yussouf and Stensrud, 2011). The scope of the present study is to compare the uncertainties arising from two classes of forecast model uncertainty.…”

“…We will also consider representativity error in the parametrization of the boundary layer. The methodology has been documented in a previous study of scattered convection in the UK (Leoncini et al, 2010) and an overview is provided in section 4.2. Small perturbations are applied to boundary-layer potential temperature.…”

“…As argued by Leoncini et al (2010), the introduction of boundary-layer state perturbations in a convective-scale forecast addresses the issue that an Reynolds-averagebased parametrization of the boundary layer becomes inappropriate if we intend to represent space and/or time averages at fine scales. The Reynolds average is based upon an ensemble of realizations of the turbulent boundary layer and only tends to the space/time average for large space/time-scales in steady state.…”

Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

“…Marsigli et al, 2004Marsigli et al, , 2005 and has therefore resulted in a movement towards convective-scale ensembles at O(1 km) grid spacing (e.g. Kong et al, 2006Kong et al, , 2007Hohenegger et al, 2008;Clark et al, 2010;Leoncini et al, 2010).…”