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Impact of Surface Properties on Convective Initiation in the Sahel

Impact of Surface Properties on Convective Initiation in the Sahel. Amanda Gounou (1) , Christopher Taylor (2) , Francoise Guichard (1) , Phil Harris (2) , Richard Ellis (2) , Fleur Couvreux (1) and Martin De Kauwe (3) CNRM (CNRS, Météo France), Toulouse, France

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Impact of Surface Properties on Convective Initiation in the Sahel

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  1. Impact of Surface Properties on Convective Initiation in the Sahel Amanda Gounou(1), Christopher Taylor(2) , Francoise Guichard(1), Phil Harris(2) , Richard Ellis(2), Fleur Couvreux(1) and Martin De Kauwe(3) CNRM (CNRS, Météo France), Toulouse, France CEH, Wallingford, United Kingdom Macquarie University, Sydney, Australia

  2. Introduction and Motivations • 80% rainfallgenerated by MCSs AMSR-E soilmoisture • MCS createstrong surface moistureheterogeneities in the Sahel • Boundary layer properties (& instability) • largely influenced by soil moisture • (Taylor et al.,2007, Couvreux et al., 2009) • Cases studies (obs, modelling) suggest that convection is more favoured over dry surface and moisture gradients could be important for initiations • (Taylor and Ellis, 2006, Taylor, 2009, Gantner and Kalthoff, 2009) Fig. Taylor • Whatis the climatologicalsignificance of thesemechanisms?

  3. Methodology of the study => StatisticalApproachbased on large datasets of satellite observations • Combine datasets of: • satellite observations of MCS initiations • satellite estimates of surface properties • Atdifferentspacescales: • mesoscale (1°x1°) • atfinerscale (few tens of kms)

  4. Satellite Datasets Location of daytime MCS initiations Backtrackingmethod: IRT MSG contours at -40°C (CEH) Domain: Sahel (10N-20N and 10E-10W) 5 years of data: Jun-Sept 2006-2010 Underlying Surface PropertyEstimates SoilMoistureEstimates ~1cm MicrowaveAMSR-E (AQUA) ……………… Resolution: ~40km ………………………… Data available in all atmospheric conditions…. Land Surface Temperature Anomalies (LSTA) (Proxi for soilmoisture anomalies) Infrared MSG Resolution: ~3km Data available in clearsky conditions Precise location of more than 3000 observed initiations of MCS Underlying surfaces characterised by twodifferentproducts

  5. Systems preferentiallydevelop over particular surface properties? AMSR-E soilmoisture More variability Dry 1°x1° Initiation? Land Surface Temperature Anomalies yes no More variability Initiation distribution Reference Distribution Warm (dry) MCS initiations more likely over drier surface presenting more variabilityat 1°x1° scale

  6. Atfinerscale, influence of LSTA gradients? Computation of LSTA gradient alongwind direction: -centeredat initiation (I) - in the environment I 2° Synoptic wind + - 4° 44 gradients sampled in the environment on a regulargridcentered on initiation Sampling of the gradients at initiation location compared to itsenvironment negative gradients

  7. Atfinerscale, influence of LSTA gradients? 38% of initiations occur over gradients < 1st decile (25%) =>13% additional initiations over large negative gradients => 1 in 8 of all MCS Synoptic wind + - MCS initiations preferentially over warm to cool transitions along the wind direction (1 in 8 of all MCS) negative gradients

  8. Atfinerscale, influence of LSTA gradients? Composite of LSTA centered on initiations Variability of LSTA sorted as a function of LFC Low LFC Cross-wind distance (km) High LFC= Lessfavourable for MCS initiation Along-wind distance (km) High LFC: initiations increasewithsoilmoisturevariability. => Heterogeneities help the triggering of convection thatwould not happenotherwise Initiations preferentially over the warm part of a cold-warm-cold transition Circulation opposite to synopticwind direction induced by moisture gradient allows to overcome inhibition

  9. Conclusions At 1°x1° scale: development of convection over dry surfaces presenting more variability (sameresultswith 2 differentproducts) At few tens of km scale: Development of MCS over negative gradients mesoscale circulation in opposite direction withsynopticwindthathelpstriggering of deep convection AdaptedfromGarcia-Carreras et al., 2010 1 MCS over 8 triggered over large negative gradients => Important triggeringmechanism

  10. Perspectives • Need to parameterisethismechanism in numericalmodels? • Is thismechanismobserved in other parts of the world?

  11. Thankyou for your attention Taylor, Gounou, Guichard, Harris, Ellis, Couvreux and De Kauwe: Frequency of Sahelian storm initiation doubled over mesoscale soil moisture patterns, Nature Geosciences, in revision See also: Poster XY30: Simulation of daytime convective initiation in the Sahel: an evaluation of numericalweatherforecastmodelswith satellite dataFrançoise Guichard, Christopher M. Taylor, Amanda Gounou, Fleur Couvreux, Richard J. Ellis, and Philip P. Harris

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