THE ROLE OF WINTERTIME RADIATION IN MAINTAINING AND DESTROYING STABLELAYERS

Strong stable layers are a common occurrence during western Colorado's winter. Analysis of radiosonde observations indicate wintertime bound ary layer heights are near 500 m. The terrain in this region consists of mountains that rise approximately 1500-2000 m above the ground to t he east, providing an effective blocking barrier. An experiment is des cribed to observe upwelling and downwelling, longwave and shortwave ra diative fluxes at two sites in western Colorado during January and Feb ruary 1992, for combinations of clear, cloudy, snow covered, and bare ground periods. Analysis of the observations and the surface energy bu dget for typical Bowen ratios provides a better understanding of the r ole of radiation in maintaining and destroying stable layers. During t he day, the surface received a net gain of energy from radiation, whil e at night there was a net loss. Over snow, the 24-hour net radiative flux was small and either positive or negative. Over bare soil, the 24 -hour net radiative flux was positive but still small. There is little difference in the net radiative flux between clear and cloudy days; t he reduction of the incident solar flux by clouds is nearly compensate d by the hindering of the longwave cooling. The cumulative effects of the 24-hour net radiative flux were negative over snow early in the ex periment. The 24-hour values shifted to near zero as the snow albedo d ecreased and were positive for bare ground. If the daytime net radiati ve flux is partitioned into sensible and latent heat flux using typica l Bowen ratios, the daytime sensible heat available for destroying bou ndary layers is small for the low solar angles of the winter season. W ith a Bowen ratio of 0.5, the daytime sensible heat flux available is only 0.3 to 1.2 MJ m(-2) over a snow surface and 1.4 to 2.3 MJ m(-2) o ver soil. These heat fluxes will not build a deep enough boundary laye r to break a typical wintertime inversion. The 24-hour sensible heat f lux was negative at both sites for the entire experiment with this Bow en ratio. The radiation observations and the use of typical Bowen rati os lead to the conclusion that the net radiation will sustain or stren gthen a stable atmosphere in the winter season in western Colorado. An alysis of the radiosonde observations confirm this result as the bound ary layer depths were less than 500 m early in the experiment and grew to only 700 m later in the experiment.