New Paper “Frequency Of Sahelian Storm Initiation Enhanced Over Mesoscale Soil-Moisture Patterns” By Taylor Et Al 2011

There is an important new paper that documents the major role of landscape heterogeneity on rainfall. It is

Christopher M. Taylor, Amanda Gounou, Françoise Guichard, Phil P. Harris, Richard J. Ellis, Fleur Couvreux, Martin De Kauwe, ,Nature Geoscience, 2011: Frequency of Sahelian storm initiation enhanced over mesoscale soil-moisture patterns. Nature Geosciences doi:10.1038/ngeo1173

The abstract reads [highlight added]

“Evapotranspiration of soil moisture can affect temperature and humidity in the lower atmosphere, and thereby the development of convective rain storms. Climate models have illustrated the importance of soil-moisture–precipitation feedbacks for weekly rainfall totals in semi-arid regions, such as the Sahel. However, large variations exist between model feedbacks, and the mechanisms governing the strength and sign of the feedback are uncertain. Here, we use satellite observations of land surface temperatures and convective cloud cover overWest Africa—collected during the wet seasons between 2006 and 2010—to determine the impact of soil moisture on rainfall in the Sahel. We show that variations in soil moisture on length scales of approximately 10–40 km exert a strong control on storm initiation—as evidenced by the appearance of convective cloud. The probability of convective initiation is doubled over strong soil-moisture gradients compared with that over uniform soil-moisture conditions. We find that 37% of all storm initiations analysed occurred over the steepest 25% of soil-moisture gradients. We conclude that heterogeneities in soil moisture on scales of tens of kilometres have a pronounced impact on rainfall in the Sahel, and suggest that similar processes may be important throughout the semi-arid tropics.”

In their conclusions, they write

“The observed relationships between convective initiation and soil-moisture patterns presented here shed new light on land-atmosphere coupling mechanisms in the real world. Though focused on the Sahel, the conclusions are likely to be relevant for many semi-arid regions, particularly in the tropics, where a short growing season is driven by the seasonal migration of the ITCZ. It is important to note that the feedbacks highlighted here occur on length scales which are not represented in current climate models, al- though their effects have large scale consequences. This raises questions about the sensitivity of climate models to soil moisture, and their predictions for future rainfall changes in the semi-arid tropics.”

Their paper provides additional support for the role of landscape heterogeneity as a major climate forcing in all land regions that have deep cumulus convection as we report in the papers

Pielke, R.A. and R. Avissar, 1990: Influence of landscape structure on local and regional climate. Landscape Ecology, 4, 133-155.

Pielke Sr., R.A., 2001: Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall. Rev. Geophys., 39, 151-177.

Pielke, R.A. Sr., J. Adegoke, A. Beltran-Przekurat, C.A. Hiemstra, J. Lin, U.S. Nair, D. Niyogi, and T.E. Nobis, 2007: An overview of regional land use and land cover impacts on rainfall. Tellus B, 59, 587-601.

In the abstract of my 2001 paper I wrote

“This paper uses published work to demonstrate the link between surface moisture and heat fluxes and cumulus convective rainfall. The Earth’s surface role with respect to the surface energy and moisture budgets is examined. Changes in land-surface properties are shown to influence the heat and moisture fluxes within the planetary boundary layer, convective available potential energy, and other measures of the deep cumulus cloud activity. The spatial structure of the surface heating, as influenced by landscape patterning, produces focused regions for deep cumulonimbus convection. In the tropics, and during midlatitude summers, deep cumulus convection has apparently been significantly altered as a result of landscape changes. These alterations in cumulus convection teleconnect to higher latitudes, which significantly alters the weather in those regions. The effect of tropical deforestation is most clearly defined in the winter hemisphere. In the context of climate, landscape processes are shown to be as much a part of the climate system as are atmospheric processes.”

The Taylor et al 2011 paper, while focusing of soil moisture content, necessarily applies wherever deep cumulus convection occurs over regions with significant spatial variations in surface sensible and latent heat flux. This is yet another climate forcing that was not adequately considered in the 2007 IPCC climate assessments.

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