Characteristics of the atmospheric boundary layer at Summit, Greenland.

摘要

To determine impacts of snow photochemistry upon the composition of surface snow and its overlying atmosphere field measurements were made at Summit, Greenland. A detailed understanding of the processes involved in air-snow interactions is necessary for the development of reliable models of present and past atmospheric chemistry. This dissertation works towards that goal by utilizing data from the atmospheric boundary layer at Summit in pursuit of the following objectives: (1) to demonstrate that intermittent turbulence can be characterized using existing instrumentation; (2) to parameterize scalar transfers in the near-surface layer; and (3) to describe the annual energy budget and its impact on atmospheric conditions.; Spectral analyses of velocity and temperature during stationary runs show that order in the spectral domain does exist, which supports the use of similarity laws. This order is verified by comparisons of spectral peak frequencies to established empirical curves and model results. Cospectra of momentum follow surface layer similarity whereas vertical heat fluxes do not collapse onto expected −4/3 slopes in the inertial subrange. To capture almost all of the turbulent heat flux, sampling rates of no more than 15 Hz are necessary. An ensemble of nonstationary runs show that 50% of the total flux during a 30-minute interval is realized in less than 23% of the total time, while stationary fluxes take about 34% of the total time. The major controls on intermittence are wind speed and degree of stability during stable conditions.; Fluxes of momentum and sensible heat are constant with height (±20%). Analyses of turbulence data confirm that linear relationships for non-dimensional wind (&phis;_m) and temperature (&phis; _h) profiles exist in stable conditions. Because most unstable cases at Summit are near neutral, the choice of stability correction in these conditions is not critical. Functions derived in this study to correct fluxes in stable conditions perform better than those currently in use. These functions are used to calculate energy exchanges in all seasons. Importantly, fluxes of sensible heat directed away from the surface in summertime result in the formation of a shallow, mixed boundary layer, which in the absence of synoptic forcing extends to heights of 300 m. These near neutral (unstable) conditions are important in controlling katabatic forcing over the ice sheet.