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Declan L. Finney
,
John H. Marsham
,
Lawrence S. Jackson
,
Elizabeth J. Kendon
,
David P. Rowell
,
Penelope M. Boorman
,
Richard J. Keane
,
Rachel A. Stratton
, and
Catherine A. Senior

Abstract

The precipitation and diabatic heating resulting from moist convection make it a key component of the atmospheric water budget in the tropics. With convective parameterization being a known source of uncertainty in global models, convection-permitting (CP) models are increasingly being used to improve understanding of regional climate. Here, a new 10-yr CP simulation is used to study the characteristics of rainfall and atmospheric water budget for East Africa and the Lake Victoria basin. The explicit representation of convection leads to a widespread improvement in the intensities and diurnal cycle of rainfall when compared with a parameterized simulation. Differences in large-scale moisture fluxes lead to a shift in the mean rainfall pattern from the Congo to Lake Victoria basin in the CP simulation—highlighting the important connection between local changes in the representation of convection and larger-scale dynamics and rainfall. Stronger lake–land contrasts in buoyancy in the CP model lead to a stronger nocturnal land breeze over Lake Victoria, increasing evaporation and moisture flux convergence (MFC), and likely unrealistically high rainfall. However, for the mountains east of the lake, the CP model produces a diurnal rainfall cycle much more similar to satellite estimates, which is related to differences in the timing of MFC. Results here demonstrate that, while care is needed regarding lake forcings, a CP approach offers a more realistic representation of several rainfall characteristics through a more physically based realization of the atmospheric dynamics around the complex topography of East Africa.

Open access
Rory G. J. Fitzpatrick
,
Douglas J. Parker
,
John H. Marsham
,
David P. Rowell
,
Francoise M. Guichard
,
Chris M. Taylor
,
Kerry H. Cook
,
Edward K. Vizy
,
Lawrence S. Jackson
,
Declan Finney
,
Julia Crook
,
Rachel Stratton
, and
Simon Tucker

Abstract

Extreme rainfall is expected to increase under climate change, carrying potential socioeconomic risks. However, the magnitude of increase is uncertain. Over recent decades, extreme storms over the West African Sahel have increased in frequency, with increased vertical wind shear shown to be a cause. Drier midlevels, stronger cold pools, and increased storm organization have also been observed. Global models do not capture the potential effects of lower- to midtropospheric wind shear or cold pools on storm organization since they parameterize convection. Here we use the first convection-permitting simulations of African climate change to understand how changes in thermodynamics and storm dynamics affect future extreme Sahelian rainfall. The model, which simulates warming associated with representative concentration pathway 8.5 (RCP8.5) until the end of the twenty-first century, projects a 28% increase of the extreme rain rate of MCSs. The Sahel moisture change on average follows Clausius–Clapeyron scaling, but has regional heterogeneity. Rain rates scale with the product of time-of-storm total column water (TCW) and in-storm vertical velocity. Additionally, prestorm wind shear and convective available potential energy both modulate in-storm vertical velocity. Although wind shear affects cloud-top temperatures within our model, it has no direct correlation with precipitation rates. In our model, projected future increase in TCW is the primary explanation for increased rain rates. Finally, although colder cold pools are modeled in the future climate, we see no significant change in near-surface winds, highlighting avenues for future research on convection-permitting modeling of storm dynamics.

Open access
Belen Rodríguez-Fonseca
,
Elsa Mohino
,
Carlos R. Mechoso
,
Cyril Caminade
,
Michela Biasutti
,
Marco Gaetani
,
J. Garcia-Serrano
,
Edward K. Vizy
,
Kerry Cook
,
Yongkang Xue
,
Irene Polo
,
Teresa Losada
,
Leonard Druyan
,
Bernard Fontaine
,
Juergen Bader
,
Francisco J. Doblas-Reyes
,
Lisa Goddard
,
Serge Janicot
,
Alberto Arribas
,
William Lau
,
Andrew Colman
,
M. Vellinga
,
David P. Rowell
,
Fred Kucharski
, and
Aurore Voldoire

Abstract

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decadal forecasting. The agreement among future projections has improved from CMIP3 to CMIP5, with a general tendency for slightly wetter conditions over the central part of the Sahel, drier conditions over the western part, and a delay in the monsoon onset. The role of the Indian Ocean, the stationarity of teleconnections, the determination of the leader ocean basin in driving decadal variability, the anthropogenic role, the reduction of the model rainfall spread, and the improvement of some model components are among the most important remaining questions that continue to be the focus of current international projects.

Full access
Catherine A. Senior
,
John H. Marsham
,
Ségolène Berthou
,
Laura E. Burgin
,
Sonja S. Folwell
,
Elizabeth J. Kendon
,
Cornelia M. Klein
,
Richard G. Jones
,
Neha Mittal
,
David P. Rowell
,
Lorenzo Tomassini
,
Théo Vischel
,
Bernd Becker
,
Cathryn E. Birch
,
Julia Crook
,
Andrew J. Dougill
,
Declan L. Finney
,
Richard J. Graham
,
Neil C. G. Hart
,
Christopher D. Jack
,
Lawrence S. Jackson
,
Rachel James
,
Bettina Koelle
,
Herbert Misiani
,
Brenda Mwalukanga
,
Douglas J. Parker
,
Rachel A. Stratton
,
Christopher M. Taylor
,
Simon O. Tucker
,
Caroline M. Wainwright
,
Richard Washington
, and
Martin R. Willet

Abstract

Pan-Africa convection-permitting regional climate model simulations have been performed to study the impact of high resolution and the explicit representation of atmospheric moist convection on the present and future climate of Africa. These unique simulations have allowed European and African climate scientists to understand the critical role that the representation of convection plays in the ability of a contemporary climate model to capture climate and climate change, including many impact-relevant aspects such as rainfall variability and extremes. There are significant improvements in not only the small-scale characteristics of rainfall such as its intensity and diurnal cycle, but also in the large-scale circulation. Similarly, effects of explicit convection affect not only projected changes in rainfall extremes, dry spells, and high winds, but also continental-scale circulation and regional rainfall accumulations. The physics underlying such differences are in many cases expected to be relevant to all models that use parameterized convection. In some cases physical understanding of small-scale change means that we can provide regional decision-makers with new scales of information across a range of sectors. We demonstrate the potential value of these simulations both as scientific tools to increase climate process understanding and, when used with other models, for direct user applications. We describe how these ground-breaking simulations have been achieved under the U.K. Government’s Future Climate for Africa Programme. We anticipate a growing number of such simulations, which we advocate should become a routine component of climate projection, and encourage international coordination of such computationally and human-resource expensive simulations as effectively as possible.

Full access