The Effect of Vertical Wind Shear on Radiative–Convective Equilibrium States

Abstract

Using a three-dimensional cloud ensemble model, a systematic exploration is undertaken of radiative-convective equilibrium states as a function of the structure and magnitude of an imposed background flow with vertical shear. In such simulations, mesoscale organization appears naturally, independent of the particulars of the initial condition. As the magnitude of an imposed low-level shear increases, the convection becomes increasingly organized in lines or arcs, propagating broadly downshear, as predicted by earlier work. When the shear is very strong, the convection tends to organize into lines at an angle to the shear, such that the linenormal component is not far from its theoretical optimal value. Midlevel shear favors shear-parallel lines, but if it occurs in conjunction with sufficiently strong low-level shear, the convection can become very strongly organized into lines or arcs generally orthogonal to the low-level shear. Optimal organization occurs when the depth of the shear layer is comparable to that of the cold pools associated with the convective downdrafts. As the vertical shear is increased, the domain-averaged convective available potential energy (CAPE) at first increases but then decreases at stronger shear values. Associated with these changes, the lower to middle troposphere becomes drier at low shear values and more humid when the shear is strong. This relationship between humidity and CAPE is broadly consistent with recently developed CAPE theories. The authors also confirm previous work that shows that the transport of momentum by the simulated convection, though usually down the gradient of the background flow, is nonlocal in character. Finally, some simulations are performed with an imposed hodograph taken from a tropical cyclone. Convective arcs form with an orientation similar to observed outer spiral bands, but the simulated bands propagate more rapidly than observed, perhaps because of a dry middle troposphere in the simulations.

Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment

“…This is likely to influence moist convection (suppressing shallow cumulus, but giving the potential for intense downdraughts in deep cumulonimbus systems) and to impact the mean thermodynamic profiles (cf. Robe and Emanuel 2001). At the same time, in the lower part of the heat-low circulation, we find poleward advection of moist air towards the dry convecting region.…”

The formation of hurricaneHumberto (2001): The importance of extra-tropical precursors

“…Below about 5 km AMSL, the difference in the magnitude of shear is roughly a factor of two, approximately 6 m s −1 versus 3 m s −1 . This difference in shear is broadly consistent with a demarcation between shear-perpendicular convective lines and less organized convective systems (Barnes and Sieckman 1984;LeMone et al 1998;Robe and Emanuel 2001), although comparison with observations is complicated by differing thermodynamic environments and representativeness error of soundings. The less-organized system produced a weaker cyclonic vorticity-anomaly in its wake by 18 h (Fig.…”

Dependence of the precipitation intensity in mesoscale convective systems to temperature lapse rate

“…The cold pool produced by precipitation evaporation is an important ingredient for the organization and dynamics of squall lines Robe and Emanuel, 2001;Weisman and Rotunno, 2004;James et al, 2005). The intensity of the cold pool is therefore examined in relation to temperature lapse rate.…”

“…It is understood that the dynamical interaction between vertical shear and evaporatively induced cold pool is an important mechanism in controlling the strength and longevity of MCSs, specifically squall lines (Thorpe et al, 1982;Rotunno et al, 1988;Weisman et al, 1988;Fovell and Ogura, 1989;Robe and Emanuel, 2001;Weisman and Rotunno, 2004). Although there is an argument on how the shear affects the MCS structure and intensity (Lafore and Moncrieff, 1989;Rotunno et al, 1990;Weisman and Rotunno, 2004), it is recognized that the interaction between vertical shear and cold pool plays a significant role.…”