Surface energy budget over the South Pole and turbulent heat fluxes as a function of an empirical bulk Richardson number

Abstract

Routine radiation and meteorological data at South Pole Station are used to investigate historical discrepancies of up to 50 W m -2 in the monthly mean surface energy budget and to investigate the behavior of turbulent heat fluxes under stable atmospheric temperature conditions. The seasonal cycles of monthly mean net radiation and turbulent heat fluxes are approximately equal, with a difference of 40 W m 2 between summer and winter, while the seasonal cycle of subsurface heat fluxes is only a few W m 2, For an 8-month period (the winter of 2001), we calculate two estimates of turbulent heat fluxes, one from Monin-Obukhov (MO) similarity theory and one as the residual of the surface energy budget (i.e., subsurface heat fluxes minus net radiation, where all fluxes toward the snow surface are positive). The turbulent fluxes from MO theory agree well with the residual of the energy budget under lapse conditions. However, under stable conditions MO theory underestimates turbulent fluxes by approximately 40-60%. The relationship between turbulent heat fluxes as a residual of the energy budget, temperature inversion strength, and wind shear as a function of the bulk Richardson number (Ri b) is examined under stable conditions (i.e., positive Ri b). The Ri b used here is calculated from 10-m wind speeds and 0- to 2-m temperature inversion strength. No critical value of Ri b is found where the turbulent heat fluxes drop to zero. However, a threshold (Ri b = 0.05) exists below which 70% of the turbulent energy fluxes can be explained by only the temperature inversion strength. For Ri b 0.05, the relationship between turbulent heat fluxes and temperature inversion strength decreases, while the importance of wind shear to turbulent heat transfer increases. Above Ri b = 0.05, a growing linear correlation also exists between atmospheric temperature inversion strength and wind shear. Thus, inversion strength and wind shear are not independent predictors of turbulent heat flux for extremely stable conditions. The exact values of the correlation coefficients and Ri b threshold are likely specific to the experimental conditions; however, their implications are probably valid for all stable flows. Knowledge of the time- varying surface characteristics would help to generalize these parameters. Copyright 2009 bv the American Geophysical Union.