The Quarterly Journal of the Royal Meteorological Society

Bollmeyer, C.; Keller, J. D.; Ohlwein, C.; Wahl, S.; Crewell, S.; Friederichs, P.; Hense, A.; Keune, J.; Kneifel, S.; Pscheidt, I.; Redl, S.; Steinke, S.

doi: 10.1002/qj.2486pmid: N/A

Atmospheric reanalyses covering the European region are mainly available as part of relatively coarse global reanalyses. The aim of this article is to present the development and evaluation of a next generation regional reanalysis for the European CORDEX EUR‐11 domain with a horizontal grid spacing of approximately 6 km. In this context, a reanalysis is understood to be an assimilation of heterogeneous observations with a physical model such as a numerical weather prediction (NWP) model. The reanalysis system presented here is based on the NWP model COSMO by the German Meteorological Service (Deutscher Wetterdienst) using a continuous nudging scheme. In order to assess the added value of data assimilation, a dynamical downscaling experiment has been conducted, i.e. an identical model set‐up but without data assimilation. Both systems have been evaluated for a 1 year test period, employing standard measures such as analysis increments, biases, or log‐odds ratios, as well as tests for distributional characteristics. An important aspect is the evaluation from different perspectives and with independent measurements such as satellite infrared brightness temperatures using forward operators, integrated water vapour from GPS stations, and ceilometer cloud cover. It can be shown that the reanalysis better resolves local extreme events; this is basically an effect of the higher spatio‐temporal resolution, as known from dynamical downscaling approaches. However, an important criterion for regional reanalyses is the coherence with independent observations of high temporal and spatial resolution, resulting in significant improvement over dynamical downscaling. The system is intended to become operational within a year, continuously reprocessing and evaluating longer time periods. The reanalysis data are planned to become available to the research community within a year.

Bahaga, T. K.; Mengistu Tsidu, G.; Kucharski, F.; Diro, G. T.

doi: 10.1002/qj.2338pmid: N/A

In this article, the predictability of the 20th century sea‐surface temperature (SST) forced East African short rains variability is analyzed using observational data and ensembles of long atmospheric general circulation model (AGCM) simulations. To our knowledge, such an analysis for the whole 20th century using a series of AGCM ensemble simulations is carried out here for the first time. The physical mechanisms that govern the influence of SST on East African short rains in the model are also investigated. It is found that there is substantial skill in reproducing the East African short rains variability, given that the SSTs are known. Consistent with previous recent studies, it is found that the Indian Ocean and in particular the western pole of the Indian Ocean dipole (IOD) play a dominant role for the prediction skill, whereas SSTs outside the Indian Ocean play a minor role. The physical mechanism for the influence of the western Indian Ocean on East African rainfall in the model is consistent with previous findings and consists of a gill‐type response to a warm (cold) anomaly that induces a westerly (easterly) low‐level flow anomaly over equatorial Africa and leads to moisture flux convergence (divergence) over East Africa. On the other hand, a positive El Niño–Southern Oscillation (ENSO) anomaly leads to a spatially non‐coherent reducing effect over parts of East Africa, but the relationship is not strong enough to provide any predictive skill in our model. The East African short rains prediction skill is also analyzed within a model‐derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the model potential prediction skill. Low‐frequency variations of the prediction skill are mostly related to SSTs outside the Indian Ocean region and are likely due to an increased interference of ENSO with the Indian Ocean influence on East African short rains after the mid‐1970s climate shift.

Lapworth, A.

doi: 10.1002/qj.2340pmid: N/A

Values of 10 m wind speed have been plotted against screen temperature during morning warming and averaged over several years for a number of sites within Great Britain. When the temperature is referenced to that at the time of the coming evening transition the plots give consistent lines with a gradient dependent on geostrophic wind. However the gradient for a given geostrophic wind varies between sites. Comparison with similar plots made during evening cooling shows that the wind plot against temperature increases relatively abruptly at some stage during the early hours of the morning and then increases to its midday maximum with a reduced wind–temperature gradient to that in the evening. However, as with the evening plots, the gradients of the morning plots increase in hillier terrain. The effect of wind direction on the gradients during morning and evening is also described in order to verify that the effect of topography on the plots is consistent. A one‐dimensional numerical model is used to simulate both transitions. It is shown that external drag factors such as form drag in the region of low hills are not sufficient to explain the observations. Only an external drag of the type given by a simple formulation of gravity wave generation and absorption within the stable boundary layer gives good agreement with the observations. This suggests that an important factor causing the wind to drop during the evening cooling is gravity wave drag and its relative absence, due to the near‐surface convective layer, is a factor during the morning wind increase.

Ragone, Francesco; Fraedrich, Klaus; Borth, Hartmut; Lunkeit, Frank

doi: 10.1002/qj.2331pmid: N/A

We propose a strategy to couple a stochastic lattice gas model of a cloud system to a rather general class of convective parametrization schemes. As proposed in similar models recently presented in the literature, a cloud system in a grid box of a general circulation model (GCM) is modelled as a subgrid lattice of N elements that can be in one of S states, each corresponding to a different convective regime. The time evolution of each element of the lattice is represented as a Markov process characterized by transition rates dependent on large‐scale fields and/or local interactions. In order to make application to GCMs computationally feasible, we propose a reduction method leading to a system of S − 1 stochastic differential equations with multiplicative noise. The accuracy of the reduction method is tested in a minimal version of the model. The coupling to a convective scheme is performed in such a way that, in the limit of space‐ and time‐scale separation, the modified stochastic parametrization converges to the original deterministic version of the host scheme. Experiments with a real GCM are then performed, coupling the minimal version of the stochastic model to the Betts–Miller scheme in an aquaplanet version of the Planet Simulator. In this configuration, the stochastic extension of the parametrization keeps the climatology of its deterministic limit but strongly impacts the statistics of the extremes of daily convective precipitation.

O'Reilly, Christopher H.; Czaja, Arnaud

doi: 10.1002/qj.2334pmid: N/A

An index of the Kuroshio Extension front strength is produced using a maximum covariance analysis between sea‐surface temperature (SST) and sea‐surface height (SSH) gradient observations, and composites of the atmospheric state are presented during its positive and negative phases using reanalysis data (1992–2011). It is found that when the Kuroshio Extension is less (more) meandering, with a stronger (weaker) SST front, the atmospheric heat transport by transient eddies is increased in the western (eastern) Pacific region, consistent with an increase (decrease) in low‐level baroclinicity. Analysis of the eddy–mean flow interaction shows that this zonal shift in heat transport forces anomalous barotropic flow in the Eastern Pacific, where blocking frequency is strongly influenced. The above relationships cannot be reconciled with the known response of the North Pacific storm track to remote forcing from the Tropical Pacific, nor can they be explained by the response of the ocean to atmospheric forcing via surface heat fluxes or winds. Rather, the zonal shift in the storm track highlighted here, and the associated changes in the large‐scale circulation, are interpreted as a response to the interannual variability of the Kuroshio Extension front.

Marquet, Pascal

doi: 10.1002/qj.2335pmid: N/A

The specific thermal enthalpy of a moist‐air parcel is defined analytically following a method in which specific moist entropy is derived from the Third Law of Thermodynamics. Specific thermal enthalpy is computed by integrating specific heat content with respect to absolute temperature and including the impacts of various latent heats (i.e. solid condensation, sublimation, melting, and evaporation). It is assumed that thermal enthalpies can be set to zero at 0 K for the solid form of the main chemically inactive components of the atmosphere (solid‐α oxygen and nitrogen, hexagonal ice). The moist thermal enthalpy is compared to already existing formulations of moist static energy (MSE). It is shown that the differences between thermal enthalpy and the thermal part of MSE may be quite large. This prevents the use of MSE for evaluating accurately the enthalpy budget of a moist atmosphere, a situation that is particularly true when dry air and cloud parcels mix because of entrainment/detrainment processes along the edges of cloud. Other differences are observed when MSE or moist‐air thermal enthalpy is plotted on a psychrometric diagram or when vertical profiles of surface deficit are plotted.

Mortarini, L.; Anfossi, D.

doi: 10.1002/qj.2336pmid: N/A

Low‐wind spectra are investigated through the analysis of sonic anemometer observations gathered in three experimental campaigns: the Urban Turbulence Project, the NOrthern hemisphere climate Processed land‐surface EXperiment and the Graz experiment. From the comparison of the horizontal Eulerian experimental spectra computed in this work with those available in the literature it is shown that in low‐wind conditions the spectra present an evident peak in the lower frequency range, that it is not accounted for in the classical formulations. This is a consequence of the oscillatory behaviour of the horizontal wind, and the peak frequency depends on the meandering time‐scale of the horizontal wind components. In contrast with previous studies the meandering behaviour is observed in both stable and unstable conditions. A new formula for the low‐wind velocity spectrum is proposed and evaluated.

Bieli, Melanie; Pfahl, Stephan; Wernli, Heini

doi: 10.1002/qj.2339pmid: N/A

Since temperature extremes have a strong impact on environment and society, it is crucial to understand their underlying mechanisms. While their relationship to the large‐scale atmospheric circulation has been comprehensively investigated, the role of physical processes in the formation of air masses with extreme temperatures is less well understood. This study presents a Lagrangian analysis of the 0.1% most extreme six‐hourly hot and cold events in three European regions (UK, Central Europe, Balkans) for the time period 1989–2009. The results provide insight into typical transport patterns and physical processes (adiabatic compression, radiation, surface heat fluxes) occurring along the trajectories of extreme temperature air masses. Cold events in Europe are most frequently induced by advection of cold air masses from the Arctic and Russia. The transport to the target region is characterized by a temperature increase caused by adiabatic compression and, in the maritime setting of the UK, also by diabatic heating due to surface sensible heat fluxes. Despite the warming along the trajectories, the extremeness of the associated 2 m temperature increases, due to the dislocation of the air mass to regions with a milder climate. Hot events are generally associated with weaker horizontal transport, but strong adiabatic warming and local temperature increase caused by enhanced radiation and surface heat fluxes. This in situ warming is particularly strong in Central Europe. Evaluating the temperature evolution along the trajectories reveals that hot and cold extremes develop on a similar time‐scale of 2–3 days. This time‐scale is mostly set by physical processes for hot extremes and controlled by advective transport for cold extremes. The diagnostics applied in this study lead to an improved process understanding that can provide a basis for more accurate predictions of temperature extremes.

Mateos, D.; di Sarra, A.; Bilbao, J.; Meloni, D.; Pace, G.; de Miguel, A.; Casasanta, G.

doi: 10.1002/qj.2341pmid: N/A

Measurements of spectral global and diffuse ultraviolet (UV) irradiance, and of spectral actinic flux were made jointly with determinations of cloud properties during a two‐ month experimental campaign that took place in southern Italy in May–June 2010. The cloud modification factor (CMF) which is the ratio between the incoming radiation under cloudy and in cloud‐free conditions, was obtained for the global (direct plus diffuse) and diffuse components of the irradiance, and for the actinic flux in the UV spectral range for overcast conditions. Measurements of the diffuse irradiance and actinic flux CMFs are very scarce, and are particularly important to characterize the radiation field and verify the behaviour of radiative transfer models in cloudy conditions. In addition to measurements, one‐dimensional radiative transfer simulations are used to investigate the cloud influence on the spectral solar UV radiation. The analysis shows that the wavelength dependence of CMF for the global irradiance is mainly due to the normalization for cloud‐free conditions. The CMF for the diffuse irradiance and for the actinic flux displays a strong dependence on the solar zenith angle. This dependence appears to be mainly driven by the large role played by the zenith radiation in overcast conditions. Radiative transfer model determinations of CMF satisfactorily reproduce observations.

Baba, Yuya

doi: 10.1002/qj.2342pmid: N/A

Sensitivity of the atmospheric energy budget to two‐moment (2M) representation of cloud microphysics in idealized convective radiative quasi‐equilibrium (CRQE) is studied using partial and full 2M schemes. The focus is especially given to the sensitivity to 2M scheme for large hydrometeors. The atmospheric energy budget is found to be sensitive to the first indirect effect, but the energy budget is more sensitive to the 2M treatment for large hydrometeors. The top and bottom of atmosphere (TOA and BOA) energy budgets depend on 2M treatment especially for large ice hydrometeors (snow and graupel). The predicted number concentration of graupel dominates BOA energy budgets by increasing rain size leading to decrease of sensible heat flux, and that of snow dominates TOA energy budgets by enhancing updraught cloud mass flux in both low and middle clouds. In conclusion, the energy budgets are controlled not only by 2M representation of rain but also by representation of large ice hydrometeors. Sensitivity of the energy budget to 2M treatment for large ice hydrometeors is more significant than that for rain because the treatment impacts properties of surface heat fluxes and both low and mid‐level clouds.