On The Large-scale Water Vapor And Vertical Wind Shear In Association With The Mesoscale Convective Systems Over The Indian Ocean

Mesoscale scale convective systems(MCSs)play important roles in the tropical energy and water cycle.However,their effects on the diabatic heating and the production of upper level ice clouds are not well embedded in cloud schemes of current climate models due to our insufficient knowledge.Thus better understanding on their behavior,characteristics and the large-scale environmental conditions with which they are associated is a foundation of future improvements on the representation of convection in climate models.This study aims to further our knowledge about MCSs by documenting the relationship between MCSs and their associated large-scale environmental moisture and wind shear field.A 6-year(2006-2011)deep convective dataset derived from MODIS and AMSR-E along with the TRMM 3B42,CMORPH,and ERA-Interim reanalysis are combined to investigate the association between mesoscale convective systems and large-scale environmental conditions including the tropospheric humidity and the vertical wind shear.To better discriminate the effect of large-scale convection.The omega500 are used to separate the large-scale convection to 4 phases based on the strength of large-scale upward motion.Thus the most active/suppressed large-scale convection is defined by the strongest/weakest quartile of upward motion(ie.the wettest/driest phases).Over Indian Ocean,it is found that the frequency of occurrence(FOC)of MCSs is highly modulated by the strength of large-scale upward motion.Large and merged systems tend to occur more frequently when the large-scale upward motion is stronger.At the occurrence time of MCSs,the middle troposphere(800-400hPa)humidity show large increases(?15%RH)when the large-scale convection changes from the suppressed to the active phases.The difference of the middle troposphere humidity between difference phases appears in a large area and reaches its maximum at 650-850km always from the center of MCSs.Meanwhile,smaller but statistically significant differences of the middle troposphere relative humidity are found among different categories and different sizes of MCSs.A more humid troposphere is found to occur within?650km around the center of merged and large MCSs even within the same phase.This distance is much larger than the size of any single MCSs.Further investigations on the temporal variation of the middle troposphere moisture show that similar spatial humidity differences appears at all phases 1-3 days before the MCSs occurs.Investigations on lower troposphere(1000-850hPa)moisture show that overall differences in lower troposphere relative humidity are much smaller than that of middle troposphere relative humidity.In all phases around all MCSs the oceanic boundary layer is always effectively moisturized(RH>90%).Temporal variations show that the lower troposphere relative humidity is dominated by diurnal cycles.Similar analyses are applied on the vertical wind shear.In phases that are dominated by upward motions,no clear differences across phases or systems of the wind shear are found.In all the cases they are always very large and very small low-level(1000-750hPa)wind speed difference and middle level(700-400hPa)wind speed difference occurring at large distance(>500km)away from MCSs.However,both the low-level and the middle level wind speed difference closely around the MCSs converge to moderate values of 3-4.2m/s and 5-7m/s,respectively.Small diurnal variations on wind shears are also founded.