Tropical Radiosonde Comparisons May Improve Past and Present Humidity Data
Westwater, E. R., University of Colorado/CET/ECE
Atmospheric Thermodynamics and Vertical Structures
Cloud Properties
Westwater, E.R., B.B. Stankov, D.Cimini, Y. Han, J.A. Shaw, B.M. Lesht, C.N. Long, 2003, Radiosonde Humidity Soundings and Microwave Radiometers during Nauru99, Journal of Atmospheric and Oceanic Technology, Vol. 21.
ARM's Nauru99 campaign provided a rare opportunity to compare original and corrected land-based radiosonde temperature and humidity measurements with those obtained at sea. (ARM photo)
Key Contributors: B. B. Stankov, D. Cimini, Y. Han, J. A Shaw, B. M. Lesht, C. N. Long
Along the equator in the Central Pacific, DOE's Atmospheric Radiation Measurement (ARM) Program maintains a field measurement site on the island of Nauru. One of three ARM sites in the Tropical Western Pacific (TWP), data collected at Nauru provide important information related to the effects of heat and moisture on the climate system. Large water vapor amounts in the TWP, or warm pool, provide an excellent environment for ARM's research focused on improving the accuracy of temperature and humidity profiles used in climate modeling.
In 1999, the ARM Program organized a campaign (Nauru99) to compare data collected from Nauru's ground-based measurement instrumentation against data from the surrounding ocean. Results from the Nauru99 campaign would help to verify, and perhaps improve, both microwave and infrared radiative transfer models for clear-air conditions. A more specific goal of the campaign was to evaluate the accuracy of a correction algorithm developed to adjust for dry bias in previous Vaisala radiosonde humidity soundings.
In collaboration with the National Oceanic and Atmospheric Administration, the research vessel (R/V) Ron H. Brown sailed from Australia to the Republic of Nauru in June and stayed at sea through July. The ship was equipped with a variety of in situ and remote sensors, many identical to those at the ARM site on Nauru. In addition, for three days in June, the Japan Marine Science and Technology Center ship, the R/V Mirai, was located immediately adjacent to Nauru, and launched the same type of radiosondes (though newer versions). The operation of essentially duplicate instruments in close proximity allowed the examination of quality and consistency among numerous data retrievals. The project team also used spectral radiance observations from an independently calibrated atmospheric emitted radiance interferometer to test scaling of radiosondes by microwave radiometer (MWR) data versus (1) the original sonde data, and (2) that sonde data corrected using an empirical technique supplied by Vaisala (the sonde manufacturers). Comparisons were made for both precipitable water vapor and calculated infrared radiation from the clear sky.
Because of the demonstrated quality of ARM's MWR instruments and data, the large scatter between measurements and calculations proved to be a result of radiosonde error in the older radiosondes. Nauru99 thus confirmed that the Vaisala correction algorithm greatly improved the accuracy of radiosonde measurements, and can be applied to both past and present data to reduce uncertainties in the radiosonde humidity measurements. It also demonstrated the importance of using MWR data to scale the results for even greater accuracy.
