Description: The governing dynamics of numerically simulated cold fronts, as they collapse towards a minimum cross-frontal scale, are identified by determining force balances and the Rossby and Ekman numbers in frontal regions. A hierarchy of numerical simulations of cold fronts is performed with the Weather Research and Forecasting model and individual terms in the horizontal momentum equations are calculated from model output to construct force balances. An inviscid, three-dimensional, idealised front is simulated at three different horizontal grid spacings (100 km, 20 km and 4 km) and then the experiments are repeated with a planetary boundary layer (PBL) parameterization scheme included. Additionally, a full-physics simulation of the cold front originally analysed by Sanders (1955) is conducted. The leading edge of the idealised, inviscid cold front is characterised by small Rossby numbers and hence balanced dynamics at all three model resolutions. When the PBL scheme is included, large Rossby numbers and unbalanced flow develop at the leading edge of the cold front, but only when the front is simulated at high resolution; at coarse resolution, balanced dynamics remain. The acceleration is in the cross-front direction and arises due to a localised pressure gradient force. Unlike the inviscid experiments, or the coarse-resolution PBL experiments, the front in the high-resolution PBL experiment has a large cross-front thermal gradient, strong forced ascent, and a sharp surface pressure trough which enables the large cross-front pressure gradient force to develop. The dynamics of the Sanders (1955) front qualitatively agree with the idealised PBL simulation, but much larger Rossby numbers occur at the leading edge of the Sanders front. This finding indicates that the dynamics of the Sanders front are more unbalanced than the dynamics of the idealised simulations.

Description: The collapse of turbulence in a pressure driven, cooled channel flow is studied by using 3-D direct numerical simulations (DNS) in combination with theoretical analysis using a local similarity model. Previous studies with DNS reported a definite collapse of turbulence in case when the normalized surface cooling h/L (with h the channel depth and L the Obukhov length) exceeded a value of 0.5. A recent study by the present authors succeeded to explain this collapse from the so-called Maximum Sustainable Heat Flux (MSHF) theory. This states that collapse may occur when the ambient momentum of the flow is too weak to transport enough heat downward to compensate for the surface cooling. The MSHF theory predicts that in pressure driven flows, acceleration of the fluid after collapse eventually will cause a regeneration of turbulence, thus in contrast with the aforementioned DNS results. Also it predicts that the flow should be able to survive ’supercritical’ cooling rates, in case when sufficient momentum is applied on the initial state. Here, both predictions are confirmed using DNS simulations. It is shown that also in DNS a recovery of turbulence will occur naturally, provided that perturbations of finite amplitude are imposed to the laminarized state and provided that sufficient time for flow acceleration is allowed. As such, it is concluded that the collapse of turbulence in this configuration is a temporary, transient phenomenon for which a universal cooling rate does not exist. Finally, in the present work a one-to-one comparison between a parameterized, local similarity model and the turbulence resolving model (DNS), is made. Although, local similarity originates from observations that represent much larger Reynolds numbers than those covered by our DNS simulations, both methods appear to predict very similar mean velocity (and temperature) profiles. This suggests that in-depth analysis with DNS can be an attractive complementary tool to study atmospheric physics in addition to tools which are able to represent high Reynolds number flows like Large Eddy Simulation.

Description: In this paper we evaluated the Weather Research and Forecasting (WRF) mesoscale meteorological model for stable conditions at clear skies with low wind speeds. Three contrasting terrains with snow covered surfaces are considered, namely Cabauw (Netherlands, snow over grass), Sodankylä (Finland, snow over a needle-leaf forest) and Halley (Antarctica, snow over an ice shelf). We used the full 3D model and the single-column versions of the WRF model. The SCM was driven by realistic forcings of the WRF-3D field. Several sets of SCM forcings were tested: A. no advection, B. varying geostrophic wind in time, C. momentum advection in addition to B, D. temperature and moisture advection in addition to C, and E. forcing the SCM field to the 3D field above a threshold height. The WRF-3D model produced overall good results for wind speed, but the near-surface temperatures and specific humidity were overestimated for Cabauw and Sodankylä, and underestimated for Halley. Prescribing advection for momentum, temperature and moisture gave the best results for the WRF-SCM, and simulations deviated strongly from reality without advection. Nudging the SCM field to the 3D field above a threshold height lead to an unrealistic behaviour of the variables below this height and is not recommended. Detailed prescription of the surface characteristics, e.g. adjusting the snow cover and vegetation fraction, improved the 2 m temperature simulation. For all three sites, the simulated temperature and moisture inversion was underestimated, though this improved when prescribing advection. Overall, in clear-sky conditions, the stable boundary layer over snow and ice can be modelled to a good approximation if all processes are taken into account at high resolution, and if land surface properties are carefully prescribed.

Description: The intraseasonal moisture budget is analyzed in simulations with the UK Met Office climate model , which produces a reasonable representation of the Madden-Julian Oscillation when the entrainment rate in the convective parameterization is increased by 50%. Analysis of the moisture budget shows that parameterized convection tends to dry the troposphere and that large scale vertical advection moistens the troposphere. These two tendencies mostly balance each other. However, the total moisture tendency is asymmetric relative to the maximum precipitation, corresponding to the recharge and discharge process of organized convection in the Tropics. This moistening before and drying after the maximum precipitation is largely due to large-scale horizontal advection of moisture. By comparing to the control run that does not simulate a realistic MJO, we find that increasing the entrainment acts to reduce deep convection in the relatively dry environment by increasing the mixing of the dry air. As a result, large scale convection and large-scale advective processes play a stronger role during the development stage, which implies that convection is necessarily better organized.

Description: Convection-permitting limited-area models based on the same spectral semi-implicit semi-Lagrangian (SL) techniques which are used in the ECMWF global model, are run operationally in several countries of the ALADIN/HIRLAM consortium. Forecasters reported a general tendency for these models to produce overestimated precipitation and unrealistic divergent winds at the edges of the cold outflows generated by the precipitation evaporation in the vicinity of the convective clouds. These grid-point storms have been associated with a spurious behaviour of the pointwise interpolation used in the semi-Lagrangian scheme, where grid-scale buoyant updrafts create strong small scale convergence near the surface. A modification of the interpolation weights in the SL transport scheme introduces the concept of cell-averaging into the traditional pointwise SL scheme which improves the conservation property of the scheme and eliminates the spurious mode. The COMAD (for COntinuous Mapping about Departure points) correction applied to the standard interpolation weights takes into account the deformation of the air parcels along each direction of interpolation in order to improve the continuity and the conservative property of the re-mapping between the model grid-points and the origin points of the backward trajectories. The method has been validated with the small planet configuration of the Integrated Forecast System (IFS) at ECMWF and with the limited area version of the same dynamics used for the AROME (Meteo-France) and HARMONIE (HIRLAM) models. The pathological behaviour of grid-scale buoyant flows permitted by these dynamical cores is corrected by the COMAD interpolations. The precipitation forecast in the convection-permitting models AROME/HARMONIE which shows an overestimation of intense convective precipitation is systematically improved by the new weights.

Description: 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 that is closer to CloudSat observations. When the new regime-dependent parametrization is used in radiative transfer calculations only, the magnitudes of shortwave and longwave top of atmosphere (TOA) cloud radiative forcing (CRF) are reduced, increasing the existing global mean biases in the control. When also applied in a new warm microphysics scheme, the shortwave global mean bias is reduced.

Description: The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day −1 , they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation. Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone.

Description: This study examines the influence of moisture, baroclinicity, and static stability on developing disturbances at high latitudes by utilising an idealised baroclinic channel model. The setup is composed of environmental baroclinicity defined by a zonally uniform jet in thermal wind balance with a meridional temperature gradient and moisture content defined by relative humidity profiles. Initiation of disturbance growth is achieved by superimposing a weak, surface-based warm-cored cyclonic disturbance to the setup. The experiments show that the disturbance is able to amplify within such an environment in absence of an upper-level perturbation, surface fluxes, friction, or radiation. Separation between developing and non-developing disturbances is feasible by considering the baroclinic and diabatic contributions to eddy available potential energy. Developing disturbances show a clear diabatic dominance during the early stage of development, whereas experiments lacking this diabatic boost fail to intensify within a three-day time-window. A comparison with the conceptual framework of the diabatic Rossby vortex (DRV) growth mechanism provides insight into the dynamical pathway potentially underlying the enhanced amplification. The emerging disturbance qualitatively resembles the postulated DRV structure, the major difference being a decreased depth in comparison with simulated and observed DRVs in midlatitudes. A suit of sensitivity experiments examines the range of atmospheric conditions in which enhanced amplification by diabatic processes is possible. Threshold values for moisture content and isentropic slopes are identified.

Description: The increasing number of physics parameterization schemes adopted in numerical weather forecasting models has resulted in a proliferation of inter-comparison studies in recent years. Many of these studies concentrated on determining which parameterization yields results closest to observations rather than analyzing the reasons underlying the differences. In this work, we study the performance of two 1.5-order boundary layer parameterizations, the Quasi-Normal Scale Elimination (QNSE) and Mellor-Yamada-Janjić (MYJ) schemes, in the Weather Research and Forecasting model. Our objectives are to isolate the effect of stability functions on the near-surface values and vertical profiles of virtual temperature, mixing ratio and wind speed. The results demonstrate that the QNSE stability functions yield better error statistics for 2-m virtual temperature but higher up the errors related to QNSE are slightly larger for virtual temperature and mixing ratio. A surprising finding is the sensitivity of the model results to the choice of the turbulent Prandtl number for neutral stratification (Pr t0 ): in the Monin-Obukhov similarity function for heat, the choice of Pr t0 is sometimes more important than the functional form of the similarity function itself. There is a stability-related dependence to this sensitivity: with increasing near-surface stability, the relative importance of the functional form increases. In near-neutral conditions, QNSE exhibits too strong vertical mixing attributed to the applied turbulent kinetic energy subroutine and the stability functions including the effect of Pr t0 .

Description: Stratospheric Sounding Units (SSU) on the NOAA polar orbiting satellites measured infrared radiances in the 15 micron CO 2 band between late 1978 and mid-2006. From these radiances a time series of layer mean stratospheric temperatures has been derived by several groups. Discrepancies in these temperature analyses have been highlighted recently and efforts are now underway to resolve the differences between them. This paper is the Met Office response summarising the issues to be resolved in creating a climate data record from the different SSUs, including corrections for radiometric, spectroscopic and tidal differences. Calibration issues identified include the SSU space view anomaly and radiometric anomalies in the NOAA-9 observations. The spectroscopic correction required for changing pressures in the pressure modulator cells is also outlined. The most important correction for the time series is for the solar diurnal and semi-diurnal tides as the satellite overpass local times change. Comparisons with other stratospheric temperature trend analyses are made and the reasons for the differences discussed. The time series presented here show sustained drops in stratospheric temperatures at all levels after the El Chichon and Pinatubo eruptions but only small trends to lower temperatures between eruptions.