Séminaire

The variability of lower-tropospheric humidity is a crucial feature of the tropical climate. Among the processes that impact local moisture budget, the vertical transport by turbulent mixing is generally overlooked. Using observations from CINDY/DYNAMO campaign, this is a first attempt to quantify it over the tropical ocean. Turbulent patches of ~100 m depth are observed in relation with large vertical gradients of specific humidity. Intense mixing is diagnosed within these intermittent patches. Large uncertainties on the corresponding eddy diffusivity coefficient however arise from parameters hard to experimentally constrain. In order to characterize the origin of this turbulence, we extend our analysis to a large set of soundings obtained in the Indian Ocean during 3 field campaigns (MISMO, CIRENE and CINDY/DYNAMO). We provide statistical characteristics of tropospheric Clear Air Turbulence (CAT) and its link with gravity wave (GW) activity. Above the mixed layer, CAT frequency varies from ~10% in the lower troposphere up to ~30% around 12km heights. GW are captured by their signature in horizontal wind, normalized temperature and balloon vertical ascent rate. These parameters emphasize different parts of the wave spectrum from longer to shorter wavelengths respectively. Composites are constructed in order to reveal the vertical structure of the waves and their link with CAT. The relatively longer wavelength GW described by their signature in temperature (GWT) are more active in the lower troposphere where they are associated with clear variations in moisture. For these waves, CAT is associated with a minimum in static stability and in wind shear, stressing the importance of the former and the possibility of convective instability. Conversely, the short waves described by their signature in balloon ascent rate (GWw) are detected primarily in the upper troposphere and their CAT is associated with a wind shear maximum suggesting the importance of dynamic instability. Furthermore, GWw appear to be linked with local convection whereas GWT are more active in suppressed and dry phases in particular of the Madden-Julian Oscillation. These waves maybe associated with remote sources such as organized convection or local fronts such as those associated with dry air intrusions.

Marie-Pierre.Lefebvre@lmd.jussieu.fr