Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model [presentation]

[1] Predicting the West African monsoon (WAM) remains a major challenge for weather and climate models. We compare multiday continental-scale simulations of the WAM that explicitly resolve moist convection with simulations which parameterize convection. Simulations with the same grid spacing but differing representations of convection isolate the impact of the representation of convection. The more realistic explicit convection gives greater latent and radiative heating farther north, with latent heating later in the day. This weakens the Sahel-Sahara pressure gradient and the monsoon flow, delaying its diurnal cycle and changing interactions between the monsoon and boundary layer convection. In explicit runs, cold storm outflows provide a significant component of the monsoon flux. In an operational global model, biases resemble those in our parameterized case. Improved parameterizations of convection that better capture storm structures, their diurnal cycle, and rainfall intensities will therefore substantially improve predictions of the WAM and coupled aspects of the Earth system.

The role of moist convection in the West African monsoon

This study evaluates the use of European Centre for Medium-Range Weather Forecasts (ECMWF) products in monitoring and forecasting drought conditions during the recent 2010–2011 drought in the Horn of Africa (HoA). The region was affected by a precipitation deficit in both the October–December 2010 and March–May 2011 rainy seasons. These anomalies were captured by the ERA-Interim reanalysis (ERAI), despite its limitations in representing the March–May interannual variability. Soil moisture anomalies of ERAI also identified the onset of the drought condition early in October 2010 with a persistent drought still present in September 2011. This signal was also evident in normalized difference vegetation index (NDVI) remote sensing data. The precipitation deficit in October–December 2010 was associated with a strong La Nina event. The ECMWF seasonal forecasts for the October–December 2010 season predicted the La Nina event from June 2010 onwards. The forecasts also predicted a below-average October–December rainfall, from July 2010 onwards. The subsequent March–May rainfall anomaly was only captured by the new ECWMF seasonal forecast system in the forecasts starting in March 2011. Our analysis shows that a recent (since 1999) drying in the region during the March–May season is captured by the new ECMWF seasonal forecast system and is consistent with recently published results. The HoA region and its population are highly vulnerable to future droughts, thus global monitoring and forecasting of drought, such as that presented here, will become increasingly important in the future. Copyright © 2012 Royal Meteorological Society

The 2010-2011 drought in the Horn of Africa

Mesoscale convective systems (MCSs) describe organized groupings of thunderstorms in the tropics and mid-latitudes that span thousands of square kilometres. While recognized for over a century, the advent of satellite and radar observations, as well as atmospheric-model simulations, has brought about their increased understanding. In this Review, we synthesize current knowledge on MCS formation, climatological characteristics, hazardous weather, predictive capacity and projected changes with anthropogenic warming. Driven by typical deep moist convective processes (moisture, lift and instability) and vertical wind shear, MCS formation occurs preferentially in locations where these ingredients are present and can be maintained by large-scale ascent and the cold pools that they produce. MCSs also generate hazardous weather, including extreme rainfall, flooding, derechos and, sometimes, tornadoes and hail, all of which have substantial economic and societal impacts. Given that MCSs also produce a large fraction of warm-season rainfall, there is critical need for both short-term forecasts and long-term projections, presently challenged by inadequate model resolution. Yet, with continually improving modelling capabilities, as well as greater theoretical basis, it is suggested that MCSs might increase in frequency and intensity under a warming climate. Further modelling progress, in turn, offers improved understanding of MCS characteristics, from their life cycle through to impacts. Mesoscale convective systems are an important source of precipitation in many tropical and mid-latitude regions, but can also produce hazardous weather, such as extreme rain, derechos and tornadoes. This Review discusses the formation of mesoscale convective systems, their hazardous weather, predictive capabilities and projected changes with anthropogenic warming.

The formation, character and changing nature of mesoscale convective systems

Convection-permitting modeling with regional climate models: Latest developments and next steps

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.

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What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change

Ensemble simulations with a regional climate model on a large domain and 30-km resolution are used to understand why projected precipitation changes under greenhouse gas-forcing are asymmetric across seasons in equatorial East Africa, with rainfall increasing during the short rains (October through December) but not during the long rains (March through May) The model captures an accurate simulation of observed East African precipitation improving over coupled GCM simulations Future simulations are generated by increasing atmospheric CO2 according to the RCP85 scenario and adding anomalies to observed SSTs as well as initial and lateral boundary conditions derived from coupled GCM simulations In November, simulated rainfall rates increase by approximately one-third over much of equatorial East Africa by the mid-twenty-first century, and double by the end of the century The long rains are not significantly increased The difference in the seasonal response is attributed to differences in the background state The East African short rains are greatest in November, more than 1 month after the autumnal equinox, when the climatological basic state is in a solstitial pattern The well-defined heat low over southern Africa and the South Indian Ocean subtropical high to its east are intensified, leading to enhanced moisture convergence over equatorial East Africa In contrast, the long rains are near their maximum on the vernal equinox, with a continental thermal low centered near the equator that is insensitive to twenty-first century greenhouse gas-induced changes in the subtropical atmospheric hydrodynamics

Seasonal asymmetry of equatorial East African rainfall projections: understanding differences between the response of the long rains and the short rains to increased greenhouse gases

Rory G. J. Fitzpatrick

Papers

What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change

Seasonal asymmetry of equatorial East African rainfall projections: understanding differences between the response of the long rains and the short rains to increased greenhouse gases