Radiosonde Temperature Measurements in Strong Inversions: Correction for Thermal Lag Based on an Experiment at the South Pole

Abstract

Very steep shallow temperature inversions occur during most of the year in the near-surface layer on the Antarctic Plateau. A radiosonde carried by a balloon rising at a few meters per second does not measure such inversions accurately because the response time of the thermistor is several seconds. To quantify this error, the authors flew a radiosonde on a tethered kite on several occasions in winter at South Pole Station immediately prior to the routine launch of the same sonde on a weather balloon. In all cases, the equilibrated temperatures measured by the tethered sonde at a given pressure level were higher than those from the balloon-borne sonde throughout most of the inversion layer. Assuming that the tethered sonde data represent the true atmospheric temperature profile, a procedure can be developed to correct the temperature data from routine radiosonde soundings for the finite response time of the thermistor. The authors devise an accurate deconvolution method to retrieve the true atmospheric temperature profile from the radiosonde data when the thermistor response time is known. However, a simple technique of shifting the profile a few seconds back in time gives results that are nearly equivalent to the deconvolution. Additional temperature errors result at the South Pole because the radiosonde is launched immediately after being brought out of a warm room, making it necessary to further adjust data from the lowest few tens of meters. It is found that the temperature errors cause a 0.3 W m⁻² error in the computed downward longwave radiation flux in winter at the South Pole, most of which is in spectral regions dominated by emission from water vapor and carbon dioxide. This is similar to the 0.5 W m⁻² change induced by the increase in carbon dioxide concentration from preindustrial to present values. The thermal lag is shown to be significant also for winter profiles in Alaska. A correction for thermal lag is recommended for all situations where radiosondes are used to measure steep temperature gradients in the boundary layer: in polar regions throughout the year, at midlatitude continental stations in winter, and at the tops of subtropical marine stratocumulus clouds.