17.4 A Lagrangian Perspective on Convective Downdrafts

Friday, 11 November 2016: 9:15 AM
Pavilion Ballroom (Hilton Portland )
Giuseppe Torri, Harvard University, Cambridge, MA; and Z. Kuang

Convective downdrafts are crucial components of deep convective systems and play an important role in maintaining organized mesoscale systems, such as squall lines. In certain cases, particularly over deep and dry boundary layers, downdrafts can pose a serious threat, among other things, to aviation by generating downbursts and microbursts. Downdrafts have also been invoked as responsible for balancing surface sensible and latent heat fluxes in the heat and moisture budget of tropical boundary layers. A method based on a Lagrangian particle dispersion model to investigate the dynamics of convective downdrafts will be presented, and results mainly from high-resolution simulations of cases in radiative-convective equilibrium will be discussed. In particular, it will be shown that, in such environments, downdrafts tend to have very low initial heights, most parcels originating within 1.5 km from the surface. Addressing the contribution of downdrafts to the flux of moist static energy at the top of the boundary layer, results will be discussed that indicate that this is of the same order of magnitude as the contribution due to convective updrafts, but much smaller than that due to turbulent mixing in the environment across the boundary layer top. Furthermore, considering the mechanisms driving the downdrafts, it will be shown that the work done by rain evaporation is smaller than that done by condensate loading.
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