ALBERT

Description: Here it is argued that air pollution over West African cities needs greater consideration. The effects of aerosol pollution on clouds and solar and thermal radiation can be expected to alter regional climate and impact human health and food security. Nature Climate Change 5 815 doi: 10.1038/nclimate2727

Description: This study describes different processes leading to heat waves in Europe. Employing backward trajectories, three clusters with coherent thermodynamic characteristics and vertical motions are identified. In two of the three clusters, subsidence is of first‐order importance for high near‐surface temperatures, whereas the third cluster is primarily heated diabatically due to surface sensible heat fluxes. Western Russia, in particular, is largely affected by remote surface fluxes, whereas the British Isles are largely affected by subsidence and adiabatic warming. This study presents a comprehensive analysis of processes determining heat waves across different climates in Europe for the period 1979–2016. Heat waves are defined using a percentile‐based index and the main processes quantified along trajectories are adiabatic compression by subsidence and local and remote diabatic processes in the upper and lower troposphere. This Lagrangian analysis is complemented by an Eulerian calculation of horizontal temperature advection. During typical summers in Europe, one or two heat waves occur, with an average duration of five days. Whereas high near‐surface temperatures over Scandinavia are accompanied by omega‐like blocking structures at 500 hPa, heat waves over the Mediterranean are connected to comparably flat ridges. Tracing air masses backwards from the heat waves, we identify three trajectory clusters with coherent thermodynamic characteristics, vertical motions, and geographic origins. In all regions, horizontal temperature advection is almost negligible. In two of the three clusters, subsidence in the free atmosphere is very important in establishing high temperatures near the surface, while the air masses in the third cluster are warmed primarily due to diabatic heating near the surface. Large interregional differences occur between the British Isles and western Russia. Over the latter region, near‐surface transport and diabatic heating appear to be very important in determining the intensity of the heat waves, whereas subsidence and adiabatic warming are of first‐order importance for the British Isles. Although the large‐scale pattern is quasistationary during heat wave days, new air masses are entrained steadily into the lower troposphere during the life cycle of a heat wave. Overall, the results of the present study provide a guideline as to which processes and diagnostics weather and climate studies should focus on to understand the severity of heat waves.

Description: Annual and monthly mean rainfall for the period 1981–2015 at 13 stations in the CH. Abstract In this study, different criteria to determine the rainy season onset date (RSOD) and its predictability in the Central Highlands (CH) of Vietnam are investigated. Using daily rainfall amounts from 10 meteorological stations for the period 1981–2015, four criteria to determine the RSOD were tested in order to select the one that most reasonably depicts the climatology of the RSOD over the CH. Results show that the RSOD varies strongly from year to year. In the long‐term mean, the onset starts first in southern parts of the CH and then progresses to the northern parts. The earliest onset date is around the beginning of April, and the latest in the second half of May. The average RSOD is on 28 April with a standard deviation of 14 days. Thus, the RSOD is distinct from the summer monsoon onset, namely, leading the mean summer monsoon onset by about 3 weeks in some years. In terms of remote influences, the RSOD in the CH has a high correlation with the El Niño–Southern Oscillation phenomenon, with most RSODs being later during El Niño years, while being earlier during La Niña years. The RSOD in the CH also shows high correlations with sea surface temperatures (SSTs), 850‐hPa zonal winds (U850) and mean sea level pressures (PMSL) over certain regions of the Pacific and Indian Oceans. Building on these relationships, the RSOD can be successfully predicted using large‐scale fields of SST, U850 and PMSL as predictors in two different approaches, namely the field mean and principal component analysis. This suggests that the overall approach can also be applied in predicting the RSOD in the CH on sub‐seasonal to seasonal timescales.

Description: Journal of Climate, Ahead of Print. 〈br/〉

Description: Bulletin of the American Meteorological Society, Ahead of Print. 〈br/〉

Description: Synoptic observations and various satellite products have been utilized for computing climatologies of low-level stratus over southern West Africa for the wet monsoon seasons July-September 2006-2011. Previous studies found inconsistencies between satellite cloud products; climate models often fail to reproduce the extensive stratus decks. Therefore a better observational reference and an understanding of its limitations are urgently needed to better validate models. Most detailed information of the spatio-temporal characteristics of low-level clouds was obtained from two Meteosat Second Generation (MSG) satellite based data sets. However, CALIPSO and CloudSat cross sections of cloud occurrence frequency suggest that both MSG products underestimate the low-level cloudiness over Nigeria due to shielding by abundant upper- and mid-level clouds and reveal that the stratus is lower over the continent than over the ocean. The Terra MISR product appears to overestimate the morning extent of low-level clouds. The climatology presented here shows that the zone of abundant low-level stratiform clouds is at its diurnal minimum south of 6-7°N around sunset (~1800 UTC). Thereafter, it starts to spread inland and reaches its maximum northward extent of 10-11°N between 0900 and 1000 UTC. The maximum affected area is approximately 800,000 km 2 . After about 1000 UTC, the northern boundary gets fragmented due to the breakup of stratus decks into fair-weather cumuli. The stratus is most frequent around Cape Palmas, over Ivory Coast, and at the windward sides of the Mampong Range (Ghana) and Oshogbo Hills (Nigeria).

Description: New ground- and space-based observations show that summertime southern West Africa is frequently affected by an extended cover of shallow, non-precipitating clouds only few hundred meters above the ground. These clouds are associated with nocturnal low-level wind speed maxima and frequently persist into the day, considerably reducing surface solar radiation. While the involved phenomena are well represented in re-analysis data, climate models show large errors in low-level wind, cloudiness, and solar radiation of up to 90 W m−2. Errors of such a magnitude could strongly affect the regional energy and moisture budgets, which might help to explain the notorious difficulties of many models to simulate the West African climate. More effort is needed in the future to improve the monitoring, modeling, and physical understanding of these ultra-low clouds and their importance for the West African monsoon system.

Description: A novel version of the classical surface pressure tendency equation (PTE) is applied to ERA-Interim reanalysis data to quantitatively assess the contribution of diabatic processes to the deepening of extratropical cyclones relative to effects of temperature advection and vertical motions. The five cyclone cases selected, Lothar and Martin in December 1999, Kyrill in January 2007, Klaus in January 2009, and Xynthia in February 2010, all showed explosive deepening and brought considerable damage to parts of Europe. For Xynthia, Klaus and Lothar diabatic processes contribute more to the observed surface pressure fall than horizontal temperature advection during their respective explosive deepening phases, while Kyrill and Martin appear to be more baroclinically driven storms. The powerful new diagnostic tool presented here can easily be applied to large numbers of cyclones and will help to better understand the role of diabatic processes in future changes in extratropical storminess.

Description: The detailed structure of an African easterly wave (AEW) observed during the AMMA field campaign is analysed. The wave was present from 25 to 29 July 2006. A complex circulation pattern was observed: the overall structure of convection and the positive vorticity of the trough region had an elongated inverted-V appearance, wrapped around an area of low winds and clear skies. Satellite imagery showed that the AEW was a significant influence on the modulation of convection on the large scale. The wave was identified initially through its strong signature on soil moisture and convection. The AEW structure observed was not anticipated and has not been discussed in previous literature. In addition, wave tracking using a Hovmöller diagram of meridional winds did not detect the wave, and a Hovmöller of vorticity showed the wave moved at a slower speed than other AEWs in July. New schematics explaining the structure are presented, describing the case as observed by satellites and analysed by a limited-area version of the Met Office Unified Model. It is proposed that the positive vorticity branches of the inverted-V can be regarded as analogous to atmospheric fronts, with characteristic gradients in winds and thermodynamic properties, acting as locations for enhanced convection. The implications of the new case are discussed in relation to previous theory and it is suggested that the accepted model of an idealised AEW is incomplete and should be extended to include more complex structures. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: Recent debate on records of southern midlatitude glaciation has focused on reconstructing glacier dynamics during the last glacial termination, with different results supporting both in-phase and out-of-phase correlations with Northern Hemisphere glacial signals. A continuing major weakness in this debate is the lack of robust data, particularly from the early...