Deep Valley Radiation and Surface Energy Budget Microclimates. Part II: Energy Budget

Abstract

Surface energy budget measurements were made concurrently at five sites located on the valley floor, sidewalls and ridgetop of Colorado's 650-m deep Brush Creek Valley (39°32?N, 108°24?W) on the nearly clear day of 25 September 1984 using the Bowen ratio energy budget technique. Daily average surface heat flux values for a natural sagebrush ecosystem on the floor of the semiarid valley included an input of 109 W m?2 net all-wave radiation and 15 W m?2 ground heat flux, and a loss of 48 W m?2 latent heat flux and 76 W m?2 sensible heat flux. Significant differences in instantaneous, daily, and daytime fluxes occurred from site to site as a function of slope aspect and inclination angles and surface properties, including vegetation cover and soil moisture. Strong contrasts in instantaneous latent and sensible heat fluxes occurred between the opposing northeast-and southwest-facing sidewalls of the valley as solar insolation varied through the course of the day and as shadows propagated across the valley. This differential heating and moistening of the air above the opposing slopes produces cross valley circulations and the resulting moisture and heat transports observed by other investigators. The ridgetop site, with a nearly unobstructed view of the sky and the longest daytime period, received the highest daily total of net radiation (12.12 MJ m?2) and lost the highest sensible heat flux total (8.49 MJ m?2). The dry southwest-facing slope produced a nearly equivalent daily total sensible heat flux, despite the later sunrise and earlier sunset at this site, because of the dry soil, lack of vegetation, and intense afternoon radiation on the sloping surface. One of the valley floor sites, located in a wheatgrass meadow, produced a daily total latent heat flux (7.37 MJ m?2) over four times larger than the dry southwest-facing sidewall. Mean daytime Bowen ratios varied from 0.86 at the valley floor meadow site to 7.60 on the southwest-facing sidewall. Daily total sensible heat fluxes in the valley were much larger than required to destroy typical nocturnal temperature inversions, and the excess is available on clear fall days to grow deep convective boundary layers over the region. Hodographs show clockwise turning of the winds above the northeast-facing sidewall during the course of the day, counterclockwise turning on the southwest-facing sidewall, and clockwise turning on the floor of the narrow valley as the cycle of down-slope, down-valley, up-slope and up-valley winds is executed. The times of reversal of the slope and valley wind systems at the individual energy budget sites were closely related to the time of sign reversal of sensible heat flux, within the time resolution of the sensible heat flux data.