Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Von P. Walden x
  • Journal of Atmospheric and Oceanic Technology x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Ashwin Mahesh
,
Von P. Walden
, and
Stephen G. Warren

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.

Full access
Stephen R. Hudson
,
Michael S. Town
,
Von P. Walden
, and
Stephen G. Warren

Abstract

The response of radiosondes to an instantaneous change of environment was studied by taking the instruments from a warm building into the cold environment at South Pole Station. After being initialized inside, the radiosondes were carried outside and placed on the snow surface, where they were left until they reported stable values of temperature, pressure, and relative humidity. Three models of radiosondes were tested: Vaisala RS80, Atmospheric Instrumentation Research (AIR) 4A, and AIR 5A.

The reported temperature equilibrated to the outside conditions within 30 s. However, it frequently took 30 min before the relative humidity outside was accurately reported. Additionally, the reported pressure rose by several hectopascals over a 5-min period when the sonde was taken outside. In the RS80s this bias was as large as 10 hPa, and disappeared in about 30 min. In the AIR sondes, the maximum pressure bias was never much over 2 hPa, but seemed not to diminish with time.

The RS80s were also tested to see if, once equilibrated to the outside conditions, they could respond to smaller changes that would be encountered in flight. The results in this case indicate that, with some corrections for time lag, the RS80 can provide accurate data at low temperatures if allowed to equilibrate initially.

The results of these tests together indicate that the quality of upper-air data in cold regions could be improved if radiosondes are stored and prepared at ambient temperature or are given at least 30 min to equilibrate outside after being prepared inside.

Full access
Penny M. Rowe
,
Larry M. Miloshevich
,
David D. Turner
, and
Von P. Walden

Abstract

Middle to upper tropospheric humidity plays a large role in determining terrestrial outgoing longwave radiation. Much work has gone into improving the accuracy of humidity measurements made by radiosondes. Some radiosonde humidity sensors experience a dry bias caused by solar heating. During the austral summers of 2002/03 and 2003/04 at Dome C, Antarctica, Vaisala RS90 radiosondes were launched in clear skies at solar zenith angles (SZAs) near 83° and 62°. As part of this field experiment, the Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured downwelling spectral infrared radiance. The radiosonde humidity profiles are used in the simulation of the downwelling radiances. The radiosonde dry bias is then determined by scaling the humidity profile with a height-independent factor to obtain the best agreement between the measured and simulated radiances in microwindows between strong water vapor lines from 530 to 560 cm−1 and near line centers from 1100 to 1300 cm−1. The dry biases, as relative errors in relative humidity, are 8% ± 5% (microwindows; 1σ) and 9% ± 3% (line centers) for SZAs near 83°; they are 20% ± 6% and 24% ± 5% for SZAs near 62°. Assuming solar heating is minimal at SZAs near 83°, the authors remove errors that are unrelated to solar heating and find the solar-radiation dry bias of 9 RS90 radiosondes at SZAs near 62° to be 12% ± 6% (microwindows) and 15% ± 5% (line centers). Systematic errors in the correction are estimated to be 3% and 2% for microwindows and line centers, respectively. These corrections apply to atmospheric pressures between 650 and 200 mb.

Full access
Jinxue Wang
,
John C. Gille
,
Henry E. Revercomb
, and
Von P. Walden

Abstract

The Measurement of Pollution in the Troposphere (MOPITT) instrument is an eight-channel gas correlation radiometer selected for the Earth Observing System (EOS) Terra spacecraft launched in December 1999. Algorithms for the retrieval of tropospheric carbon monoxide (CO) profiles from MOPITT measurements have been developed. In this paper, validation studies of the MOPITT CO retrieval algorithm using observations by the Interferometric Monitor for greenhouse Gases (IMG) during the Winter Clouds Experiment (WINCE) conducted from 23 January to 13 February 1997 are described. Synthetic radiance spectra calculated by a line-by-line radiative transfer model, FASCOD3, using the retrieved CO profile agrees well with IMG-measured radiance spectra. Observations by the Moderate Resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS) from the NASA ER-2 platform during WINCE were successfully used to assist in the identification of clear and cloudy IMG observations.

Full access