1.6
The Effect of the Diurnal Cycle on the Dynamics of Fronts
Michael J. Reeder, Monash University, Clayton, Vic., Australia; and K. J. Tory
Cold fronts are responsible for much of the significant weather outside the tropics, yet despite the central role they play in shaping extra-tropical weather patterns, few studies have investigated the effect of surface heating or cooling on their dynamics. Those that have been conducted have focused mainly on the role of sharp heating gradients associated with coastlines or on differential cloud cover, and none have have examined the nocturnal part of the diurnal cycle.
The effect of the diurnal cycle on the structure and evolution of the surface front that develops in the finite-amplitude baroclinic wave is investigated. The two central conclusions are that: (1) daytime homogeneous short-wave radiative heating of the surface is weakly frontolytic, although this result may be sensitive to the way in which the boundary layer turbulence is parameterized; and (2) nocturnal cooling through long-wave radiative emission from the surface is strongly frontogenetic. These conclusions are supported by recent observations of cold fronts taken in the arid regions of Central Australia.
While the prescribed flux of short-wave radiation at the surface during the day in the numerical experiements is independent of the horizontal coordinate, strong gradients in the surface sensible heat flux develop. This is because the sensible heat flux depends, among other things, on the temperature difference between the ground and the overlying air, and on the wind speed. Differential heating produced by horizontal variations in the mixed layer depth and the lapse rate above the mixed layer slightly weaken the front during the day. Strong cross-isobaric flow towards the trough acts to increase the convergence in the boundary layer, but the enhanced daytime turbulent stresses retards the low-level winds, which act to reduce the convergence. The net effect is that the frontogenetic effect of convergence is almost balanced the frontolytic effect of diabatic heating throughout most of the day. For this reason, the potential temperature gradient remains almost constant throughout the day, but increases rapidly only after the strong daytime turbulent mixing weakens late in the afternoon. At night, as the turbulence decays in the trough ahead of the surface front, and the cross-front flow accelerated down the pressure gradient towards the centre of the trough producing low-level surges. This enhanced nocturnal convergence rapidly strengthens the front. These results have implications for numerical weather prediction.
Session 1, Continued
Monday, 23 June 2003, 11:00 AM-1:30 PM
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