Large-eddy Model Simulations Research Articles

A Large Eddy Model Study on the Effect of Overshooting Convection on Lower Stratospheric Water Vapor

AbstractUsing a cloud‐resolving large eddy model (LEM), we investigate how overshooting convection affects the water vapor content in the lower stratosphere. We design and conduct a series of sensitivity experiments to diagnose the effects of dynamical and thermodynamical background conditions on the transport of water vapor into the lower stratosphere associated with overshooting convection. The three‐dimensional LEM simulations capture the bulk properties of the target case and track microphysical processes using a three‐phase microphysical parameterization. The model results indicate that the net effect of overshooting convection on lower stratospheric water content is moistening, primarily due to gravity wave breaking and ice sublimation. The contributions of small‐scale turbulent mixing to water vapor transport from the overshooting turret into the stratosphere are relatively weak. Sensitivity experiments show that convective intensity (as measured by updraft velocity) is directly related to the effect of overshooting convection on lower stratospheric humidity. This impact is quantified for the idealized target case. Changes in vertical wind shear near the tropopause have no significant impact on the extent of overshooting but have important impacts on cross‐tropopause water vapor exchange via their modulation of gravity wave breaking. Larger vertical wind shear in the tropopause layer inhibits the transport of water vapor and ice into the lower stratosphere by overshooting convection.

Simulations of the effects of surface heat flux anomalies on stratification, convective growth, and vertical transport within the Saharan boundary layer

Using two cases based on observations from the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation field campaign, large eddy model (LEM) simulations have been used to investigate the effects of surface flux anomalies on the growth of the summertime Saharan convective boundary layer (CBL) into the Saharan Residual Layer (SRL) above and transport from the CBL into the SRL. Hot surface anomalies generated updrafts and convergence in the CBL that increased transport from the CBL into the SRL. The induced subsidence in regions away from the anomalies inhibited growth of the CBL there. If the domain‐averaged surface fluxes were kept constant, this led to a shallower, cooler CBL. If fluxes outside the anomalies were kept constant so that stronger anomalies led to increased domain‐averaged fluxes, this gave a warmer, shallower CBL. These effects were larger for wider, stronger anomalies with low winds. The low‐level wind speed variations induced by the anomalies were also shown to affect dust uplift rates. Previous observations have shown that the summertime SRL, which often contains sublayers of varying humidity and dust content, can persist until late in the afternoon. The LEM simulations presented show that mesoscale variations in surface fluxes can contribute both to inhibiting the growth of the Saharan CBL into the SRL and to generating layerings within the SRL.

Variable cirrus shading during CSIP IOP 5. I: Effects on the initiation of convection

AbstractObservations from the Convective Storm Initiation Project (CSIP) show that on 29 June 2005 (Intensive Observation Period 5) cirrus patches left over from previous thunderstorms reduced surface sensible and latent heat fluxes in the CSIP area. Large‐eddy model (LEM) simulations, using moving positive surface‐flux anomalies, show that we expect the observed moving gaps in the cirrus cover to significantly aid convective initiation. In these simulations, the timing of the CI is largely determined by the amount of heat added to the boundary layer, but weak convergence at the rear edge of the moving anomalies is also significant.Meteosat and rain‐radar data are combined to determine the position of convective initiation for all 25 trackable showers in the CSIP area. The results are consistent with the LEM simulations, with showers initiating at the rear edge of gaps, at the leading edge of the anvil, or in clear skies, in all but one of the cases. The initiation occurs in relatively clear skies in all but two of the cases, with the exceptions probably linked to orographic effects.For numerical weather prediction, the case highlights the importance of predicting and assimilating cloud cover. The results show that in the absence of stronger forcings, weak forcings, such as from the observed cirrus shading, can determine the precise location and timing of convective initiation. In such cases, since the effects of shading by cirrus anvils from previous convective storms are relatively unpredictable, this is expected to limit the predictability of the convective initiation. Copyright © 2007 Royal Meteorological Society

The effects of wind shear on cirrus: A large‐eddy model and radar case‐study

AbstractWind shear is an almost ubiquitous feature of the troposphere at cirrus altitudes, but there have been few studies focused on investigating its effects on cirrus clouds. In this study we consider a case of strongly sheared frontal ice cloud, which was observed by the Chilbolton radar. This was simulated using the Met Office large‐eddy model (LEM), which included a fully integrated radiation code. It is well known that there can be significant variations between results from different cirrus models, which makes a comparison of the simulations with observations important. So, radar observations of ice water content (IWC) were simulated directly within the framework of the LEM, and true and simulated IWC observations were compared, as well as their Fourier transforms and probability density functions (p.d.f.s). This showed that the LEM was capturing reasonably well the horizontally averaged IWC profile, and also at upper levels the variability in the IWC field, at scales of less than approximately 14 km. At lower levels the IWC field was too homogeneous.Varying the shear within the LEM simulations showed that shear had little effect on the mean IWC profile, but it increased the mixing and so the homogeneity of the IWC field. P.d.f.s of IWC derived from the simulated radar data compared well with observed p.d.f.s, except that the largest IWCs were not captured and there was too little variance at lower levels. Comparing p.d.f.s of IWC and total water for runs with and without shear clearly showed the effects of shear‐induced mixing. LEM results also showed that the variation of the correlation of IWC with vertical separation is shear dependent and initially linear, with or without shear. In this case, shear had little effect on the top‐of‐atmosphere and surface fluxes, or the within‐cloud heating rate profile, but this would be significantly different for a more patchy cloud.A modified case‐study, which allowed Kelvin–Helmholtz wave breaking, showed that wave breaking can significantly affect the microphysical processes, by increasing nucleation, deposition and sublimation rates. Wave breaking can occur in thin layers of limited horizontal extent, which would not be resolved by a global model. These effects were most significant when there was no large‐scale uplift and the vertical velocities from the wave breaking formed a cirrus cloud which did not otherwise occur. Copyright © 2005 Royal Meteorological Society

Estimation of entrainment rate in simple models of convective clouds

A method for estimating lateral entrainment rate in clouds is suggested, linking its magnitude to buoyant production of vertical kinetic energy within a cloud updraught. In single-column model studies the formulation captures the high values of entrainment seen in shallow convection and the values an order of magnitude lower in cases of deep convection, as suggested by observational studies and large-eddy model simulations, together with a realistic vertical variation. Comparison is made with other recent attempts to parametrize entrainment.

First radar detection of a misocyclone in a UK tornadic storm

fully developed to an underlying urban area. Laboratory experiments would enable more controlled measurements that would complement such an observational programme. Many urban areas are too small for the boundary layer to adjust fully, so that it is important to investigate the transition from rural terrain to the urban area. Here large-eddy modelling is the crucial tool, perhaps starting by investigating how the boundary layer responds to simple changes in surface properties. New theoretical concepts are required if the urban areas are to be parametrized effectively both in the largeeddy modelling studies and NWP. Here the expertise amongst the UK community developed in studying boundary-layer flow over hills and through vegetation canopies might well provide clues. Methods for representing the average effects of vegetation on the boundary layer as 'canopies' are now reasonably well developed, and a similar approach to representing buildings within urban areas might prove fruitful. Results from these theoretical studies could suggest further large-eddy model simulations and thence further observational studies. These studies of mixing and transport in urban areas will provide the basis for studying the complex chemical transformations that control pollutants from urban sources. Clearly such an integrated programme of work requires careful co-ordination and planning. UWERN is the ideal vehicle to perform this task.