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) (1)CNRM (CNRS, Météo France), Toulouse, France (2)CEH, Wallingford, United Kingdom (3)Macquarie University, Sydney, Australia

AMSR-E soil moisture Fig. Taylor 80% rainfall generated by MCSs Introduction and Motivations MCS create strong surface moisture heterogeneities in the Sahel Boundary layer properties (& instability) largely influenced by soil moisture (Taylor et al.,2007, Couvreux et al., 2009) What is the climatological significance of these mechanisms? 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)

Methodology of the study Combine datasets of:  satellite observations of MCS initiations  satellite estimates of surface properties At different space scales:  mesoscale (1°x1°)  at finer scale (few tens of kms) => Statistical Approach based on large datasets of satellite observations

Satellite Datasets Backtracking method: IRT MSG contours at -40°C (CEH) Domain: Sahel (10N-20N and 10E-10W) 5 years of data: Jun-Sept Underlying Surface Property Estimates Land Surface Temperature Anomalies (LSTA) (Proxi for soil moisture anomalies) Infrared MSG Resolution: ~3km Data available in clear sky conditions Soil Moisture Estimates ~1cm Microwave AMSR-E (AQUA) ……………… Resolution: ~40km ………………………… Data available in all atmospheric conditions…. Location of daytime MCS initiations Precise location of more than 3000 observed initiations of MCS Underlying surfaces characterised by two different products

Systems preferentially develop over particular surface properties? Land Surface Temperature Anomalies AMSR-E soil moisture MCS initiations more likely over drier surface presenting more variability at 1°x1° scale Initiation? yes no Initiation distribution Reference Distribution 1°x1° Dry More variability Warm (dry) More variability

At finer scale, influence of LSTA gradients? Computation of LSTA gradient along wind direction: Synoptic wind Synoptic wind + - negative gradients - in the environment -centered at initiation (I) 44 gradients sampled in the environment on a regular grid centered on initiation Sampling of the gradients at initiation location compared to its environment 2° 4° I

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

Initiations preferentially over the warm part of a cold-warm-cold transition High LFC: initiations increase with soil moisture variability. => Heterogeneities help the triggering of convection that would not happen otherwise Composite of LSTA centered on initiations Cross-wind distance (km) Along-wind distance (km) At finer scale, influence of LSTA gradients? Circulation opposite to synoptic wind direction induced by moisture gradient allows to overcome inhibition Variability of LSTA sorted as a function of LFC Low LFC High LFC= Less favourable for MCS initiation

1 MCS over 8 triggered over large negative gradients => Important triggering mechanism 1 MCS over 8 triggered over large negative gradients => Important triggering mechanism Adapted from Garcia-Carreras et al., 2010 Conclusions At 1°x1° scale: development of convection over dry surfaces presenting more variability (same results with 2 different products) At few tens of km scale: Development of MCS over negative gradients mesoscale circulation in opposite direction with synoptic wind that helps triggering of deep convection

Perspectives Need to parameterise this mechanism in numerical models? Is this mechanism observed in other parts of the world?

Thank you 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 numerical weather forecast models with satellite data Françoise Guichard, Christopher M. Taylor, Amanda Gounou, Fleur Couvreux, Richard J. Ellis, and Philip P. Harris