The Quarterly Journal of the Royal Meteorological Society

Berrisford, P.; Kållberg, P.; Kobayashi, S.; Dee, D.; Uppala, S.; Simmons, A. J.; Poli, P.; Sato, H.

doi: 10.1002/qj.864pmid: N/A

We study the global atmospheric budgets of mass, moisture, energy and angular momentum in the latest reanalysis from the European Centre for Medium‐Range Weather Forecasts (ECMWF), ERA‐Interim, for the period 1989–2008 and compare with ERA‐40. Most of the measures we use indicate that the ERA‐Interim reanalysis is superior in quality to ERA‐40. In ERA‐Interim the standard deviation of the monthly mean global dry mass of 0.7 kg m−2 (0.007%) is slightly worse than in ERA‐40, and long time‐scale variations in dry mass originate predominately in the surface pressure field. The divergent winds are improved in ERA‐Interim: the global standard deviation of the time‐averaged dry mass budget residual is 10 kg m−2 day−1 and the quality of the cross‐equatorial mass fluxes is improved. The temporal variations in the global evaporation minus precipitation (E − P) are too large but the global moisture budget residual is 0.003 kg m−2 day−1 with a spatial standard deviation of 0.3 kg m−2 day−1. Both the E − P over ocean and P − E over land are about 15% larger than the 1.1 Tg s−1 transport of water from ocean to land. The top of atmosphere (TOA) net energy losses are improved, with a value of 1 W m−2, but the meridional gradient of the TOA net energy flux is smaller than that from the Clouds and the Earth's Radiant Energy System (CERES) data. At the surface the global energy losses are worse, with a value of 7 W m−2. Over land however, the energy loss is only 0.5 W m−2. The downwelling thermal radiation at the surface in ERA‐Interim of 341 W m−2 is towards the higher end of previous estimates. The global mass‐adjusted energy budget residual is 8 W m−2 with a spatial standard deviation of 11 W m−2, and the mass‐adjusted atmospheric energy transport from low to high latitudes (the sum for the two hemispheres) is 9.5 PW. Copyright © 2011 Royal Meteorological Society

Stickler, A.; Brönnimann, S.

doi: 10.1002/qj.854pmid: N/A

The strong interannual‐to‐decadal variability of the West African Monsoon is subject to an active field of climate research that tries to disentangle its influencing factors and explore its predictability. Reliable observation‐based data over a preferably long period are arguably the most important basis for such efforts. Here, we try to explore the quality of several data products available for the earlier period of upper‐air observations (1940–1957): the Comprehensive Historical Upper‐Air Network (CHUAN), the NCEP/NCAR Reanalysis (NNR), and the Twentieth Century Reanalysis (20CR). To do so, we compare wind soundings from 37 pilot balloon stations contained in CHUAN (10°S–30°N, 20°W–20°E) with the reanalyses. The comparison with the NNR reveals seasonally and diurnally varying significant differences relative to the observations over West and Central Africa. The differences reach absolute values of several metres per second, and their spatially coherent structure strongly points to a deficiency of the reanalysis rather than observational errors. The difference fields indicate an overestimation of the strength and thickness of the low‐level monsoon in all seasons and an underestimation of the Harmattan winds over the Sahel in winter. At higher levels, they point to an overestimation of the mid‐tropospheric monsoon return flow and African easterly jet. For the 20CR, the fields reveal again significant differences up to several metres per second on all levels and in all seasons. However, the direction relative to the observed monsoon flow and Harmattan/trades is opposite at the low levels. Additionally, the differences tend to be smaller and more confined to the coastal region. Further analysis demonstrates that the observed interannual variability is only insufficiently modelled in both reanalyses. Together with the diurnal cycle of the differences, this precludes a simple correction of the reanalyses and demonstrates that, depending on the purpose of a study, one should be extremely cautious when using reanalysis products. Copyright © 2011 Royal Meteorological Society

Sylla, M. B.; Giorgi, F.; Ruti, P. M.; Calmanti, S.; Dell'Aquila, A.

doi: 10.1002/qj.853pmid: N/A

The role of the representation of deep convection on key elements of the West African summer monsoon climate is addressed using the Regional Climate Model RegCM3. Two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared. Results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower‐level baroclinicity favoring intensification of the jet core and leading to a more realistic African easterly jet. In addition, although the isentropic potential vorticity (IPV) is lower when the convection scheme is switched off, African easterly waves (AEWs) are still generated and propagate westwards but they dissipate around the west coast. Substantial differences between the two simulations occur mainly at the 6‐ to 9‐day time‐scale over land, when much weaker activity is simulated in the absence of convection. This indicates that orographic friction and low‐level large‐scale moisture convergence, generating high values of latent heat and IPV, may play the dominant role in the genesis and growth of AEWs and that deep convection acts to strengthen the overall wave activity and to favor their west coast development. Copyright © 2011 Royal Meteorological Society

Schepanski, K.; Knippertz, P.

doi: 10.1002/qj.850pmid: N/A

According to a classical synoptic concept, Soudano‐Saharan depressions (SSDs) are surface lows that track westward over tropical West Africa, curve anticyclonically across the Sahara and may then transform into eastward‐moving Mediterranean cyclones. Occurrence frequency and track location undergo a marked seasonal cycle. Interactions between troughs in the upper‐level westerlies and mid‐level African easterly waves have been suggested as a mechanism for their formation. SSDs have been reported to be associated with dust‐storms and precipitation over northern Africa. This paper presents the first‐ever systematic investigation of SSDs using re‐analysis and satellite data. Depressions are identified and tracked objectively based on closed contours in 0000 UTC fields of 925 hPa geopotential height from the European Centre for Medium‐Range Weather Forecasts ERA‐Interim re‐analysis (1989–2008). To classify as potential SSDs, tracks must: (i) start to the south of 20°N; (ii) intersect 15°–30°N, 10°W–30°E; (iii) cover a meridional distance of at least 10° latitude; and (iv) have a minimum lifetime of 24 hours. Even with a relatively low threshold of 4 gpm, only 50 potential SSDs are found (annual average 2.5, monthly range 0–6). Lagrangian and Eulerian composite analyses reveal that the identified systems: (i) are mostly shallow lee troughs of the central Saharan and Atlas Mountains during the warm season without a well‐defined cyclonic wind field; (ii) do not show the seasonal track variation described in the literature; (iii) mostly occur in association with high‐pressure anomalies over the Mediterranean Sea; and (iv) are not associated with significant increases in dustiness and precipitation. These results strongly suggest that the disturbances described as SSDs do not manifest themselves as traceable low‐level depressions, calling for a fundamental revision of the classical concept in the literature. Copyright © 2011 Royal Meteorological Society

Bertotti, Luciana; Bidlot, Jean‐Raymond; Buizza, Roberto; Cavaleri, Luigi; Janousek, Martin

doi: 10.1002/qj.861pmid: N/A

The accuracy of deterministic and probabilistic forecasts of storms in the Adriatic Sea that lead to the flooding of Venice is discussed. We consider ECMWF state‐of‐the‐art high‐resolution single deterministic and lower‐resolution ensemble‐based forecasts of meteorological and sea states (waves) for five storms that affected Venice between 1966 and 2008. Notwithstanding the complicated local orographic situation, it is shown that ECMWF single, deterministic forecasts provide accurate information up to 3–4 days in advance. This range is further extended to between 4 and 6 days if ensemble‐based, probabilistic forecasts are considered. The assessment of the quality of Ensemble Prediction System (EPS) probabilistic forecasts during the winters of 2008–2009 and 2009–2010 over the Adriatic and the Mediterranean Seas, and the North Atlantic Ocean, is also discussed to provide a proper statistical evaluation of the accuracy of EPS‐based probabilistic forecasts of the wind over the sea. Average results indicate that EPS probabilistic forecasts over these areas are skilful for the whole forecast range considered in this study. Copyright © 2011 Royal Meteorological Society

Garcies, L.; Homar, V.

doi: 10.1002/qj.872pmid: N/A

A variety of sensitivity climatologies of Mediterranean intense cyclones have been recently built owing to the growing international interest in contributing to the basic understanding and the short‐range forecasting of high‐impact weather events. The verification of these climatologies is essential to ensure the reliability of the sensitivity products and ultimately provide robust guidance to policy‐makers on plans to redefine routine observational strategies. This work tackles the arduous task of verifying the available (an adjoint‐based and two different ensemble‐based) sensitivity climatologies of Mediterranean intense cyclones. We perform Observing System Simulation Experiments (OSSE) with the WRF ARW model for 25 of the most intense Mediterranean cyclones detected in the ERA‐40 database to test the ability of each method in identifying areas where perturbations in the initial conditions derived from the sensitivity fields lead to a greater impact on the forecast of intense cyclones. For the sake of a sensible reference, the performance of the available sensitivity climatologies is tested against the judgement of an experienced severe weather meteorologist. In addition, a control measure of the background random response is also carried out. The impact on the prediction of intense Mediterranean cyclones of prescribed perturbations to the initial conditions is evaluated comparing each perturbed experiment with a control simulation. Furthermore, a quantitative study of the linearity of the evolution of the perturbations is performed using twin perturbations. Results confirm a statistically significant superior skill of the human and adjoint sensitivity fields against both ensemble sensitivity climatologies. Climatological ensemble sensitivities only show a noticeable improvement upon non‐sensitivity experiments when an ad hoc classification of cyclones is used. This reveals one fundamental limitation of the ensemble sensitivity technique in climatological mode when it is applied to rare events insufficiently sampled in the available datasets. Copyright © 2011 Royal Meteorological Society

Gray, S. L.; Martínez‐Alvarado, O.; Baker, L. H.; Clark, P. A.

doi: 10.1002/qj.859pmid: N/A

Sting jets are transient mesoscale jets of air that descend from the tip of the cloud head towards the top of the boundary layer in severe extratropical cyclones and can lead to damaging surface wind gusts. This recently identified jet is distinct from the well‐documented jets associated with the cold and warm conveyor belts. One mechanism proposed for their development is the release of conditional symmetric instability (CSI). Here the spatial distribution and temporal evolution of several CSI diagnostics in four severe storms are analysed. A sting jet has been identified in three of these storms; for comparison, we also analysed one storm that did not have a sting jet, even though it had many of the apparent features of sting‐jet storms. The sting‐jet storms are distinct from the non‐sting‐jet storms by having much greater and more extensive conditional instability (CI) and CSI. CSI is released by ascending air parcels in the cloud head in two of the sting‐jet storms and by descending air parcels in the other sting‐jet storm. By contrast, only weak CI to ascending air parcels is present at the cloud‐head tip in the non‐sting‐jet storm. CSI released by descending air parcels, as diagnosed by decaying downdraught slantwise convective available potential energy (DSCAPE), is collocated with the sting jets in all three sting‐jet storms and has a localised maximum in two of them. Consistent evolutions of saturated moist potential vorticity are found. We conclude that CSI release has a role in the generation of the sting jet, that the sting jet may be driven by the release of instability to both ascending and descending parcels, and that DSCAPE could be used as a discriminating diagnostic for the sting jet based on these four case‐studies. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Field, P. R.; Bodas‐Salcedo, A.; Brooks, M. E.

doi: 10.1002/qj.858pmid: N/A

Midlatitude cyclones taken from 18 months of global operational Met Office Unified Model analysis archives were combined to form three‐dimensional composite cyclones. Given an accurate dynamical and thermodynamical representation of the atmosphere from 4D‐Var analysis, this study attributes differences between observations and the model cloud/precipitation to potential shortcomings in the physical parametrizations that control cloud/precipitation. Coincident ( ± 6 h) data from CloudSat radar, AMSR‐E microwave, and ISCCP flux products were collected into composites for comparison. Only cyclones over the ocean were analysed here. Considering all of the observations in a single composite shows that horizontal slices through the composite display qualitative agreement between the mean reflectivity structures seen in the UM and CloudSat data. Splitting the composite cyclone into quadrants reveals that the UM underpredicts fractions of reflectivities greater than –20 dBZ and + 10 dBZ at heights above 2 km in the poleward quadrants. This lack of cloud is manifested in smaller short‐wave top‐of‐atmosphere fluxes from the UM composite cyclone than from the observations. Comparison of UM precipitation rates with CloudSat shows agreement to within the assumed potential bias of the observations. Copyright © 2011 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Wagner, J. S.; Gohm, A.; Dörnbrack, A.; Schäfler, A.

doi: 10.1002/qj.857pmid: N/A

The mesoscale structure of a mature polar low was studied on the basis of high‐resolution airborne measurements and numerical modelling. A polar low was measured by light detection and ranging (lidar) and dropsonde observations over the Norwegian Sea on 3 and 4 March 2008. Lidar observations provided cross‐sections of water‐vapour mixing ratio, backscatter ratio and horizontal wind speed around the polar low and through its centre. Mesoscale structures, such as shallow convection in a cold‐air outbreak, a dry intrusion in the eye‐like centre of the cyclone and deep convection surrounding it could be identified. Numerical simulations were performed with the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) and a high‐resolution, polar version of the Weather Research and Forecasting (WRF) model. WRF simulations reproduced these structures and showed that the polar low had a warm, upper‐level core with descending motions. The eye‐like centre had a diameter of about 100–150 km and was characterized by rather stable stratification, horizontally constant potential temperatures and calm winds. Beyond the centre, wind speeds increased rapidly. The observed radial wind and temperature profiles support previous idealized simulations. Several WRF sensitivity tests showed the influence of the initialization time and sensible and latent heat fluxes from the surface on the simulated polar‐low development. The polar‐low simulations were more accurate in runs starting at the mature stage. Heat fluxes from the surface were important for the polar‐low energetics, especially at the final stages. Copyright © 2011 Royal Meteorological Society

Harnisch, F.; Weissmann, M.; Cardinali, C.; Wirth, M.

doi: 10.1002/qj.851pmid: N/A

A unique airborne differential absorption lidar (DIAL) for water vapour observations was developed at the Deutsches Zentrum für Luft‐ und Raumfahrt (DLR). Installed on board the DLR Falcon 20 aircraft, the system measured a dataset of about 3900 water vapour profiles during the T‐PARC field campaign. These high‐resolution humidity observations were assimilated into the European Centre for Medium‐Range Weather Forecasts (ECMWF) global model using a version of the operational four‐dimensional variational data assimilation system. The assimilation system is able to extract the information for DIAL observations, and verification with independent dropsonde observations shows a reduction in the analysis error when DIAL water vapour observations are assimilated. The forecast influence of the humidity observations is found to be small in most cases, but the observations are able to affect the forecast considerably under certain conditions. Systematic errors are investigated by comparison between humidity model fields, DIAL and dropsonde observations. Overall, DIAL observations are roughly 7–10% drier than model fields throughout the troposphere. Comparison with dropsonde observations suggests that the DIAL observations are too dry in the lower troposphere but not above it. Copyright © 2011 Royal Meteorological Society