Radiosonde Humidity Soundings and Microwave Radiometers during Nauru99
During June-July 1999, the NOAA R/V Ron H. Brown (RHB) sailed from Australia to the Republic of Nauru. On Nauru, the Department of Energy’s Atmospheric Radiation Measurement (ARM) Program has set up a long-term climate observing station. The purpose of the RHB cruise was to determine how well island measurements represent the surrounding ocean environment. During July, when the RHB was in close proximity to the island of Nauru, detailed comparisons of ship- and island-based instruments were possible. The data obtained during Nauru99 provided a rare opportunity to compare basic observations necessary for developing radiative transfer models. Essentially identical instruments were operated from the ship and the island’s Atmospheric Radiation and Cloud Station (ARCS-2). These instruments included simultaneously launched Vaisala radiosondes, the Environmental Technology Laboratory’s (ETL) Fourier Transform Infrared Radiometer (FTIR), and the Atmospheric Radiation Measurement Program’s (ARM) Atmospheric Emitted Radiance Interferometer (AERI), as well as cloud radars/ceilometers to identify clear conditions. The ARM Microwave radiometer (MWR) operating on Nauru provided another excellent data set for the entire Nauru99 experiment. The calibration accuracy was verified by a liquid nitrogen blackbody target experiment and by consistent high-quality tipping calibrations throughout the experiment. The data thus provide an excellent baseline for evaluation of the quality and consistency of RS-80H Vaisala radiosondes that were launched from the Nauru ARCS-2 and from the RHB. Comparisons were made for calculated clear sky brightness temperature (Tb) and for precipitable water vapor (PWV). Our results indicate that substantial errors, sometimes of the order of 20% in PWV, occurred with the original radiosondes. During Nauru99, the range of PWV, as measured by the original radiosondes, was 2.5 to 6.1 cm. When a Vaisala correction algorithm, which is a function of the age of the radiosondes and the observed relative humidity and temperature, was applied, calculated Tbs were in better agreement with the MWR than were the calculations based on the original data. Applications of the algorithm to data obtained under clear conditions resulted in, on the average, a 5 K change in Tb at 23.8 GHz and a 2.5 K change in Tb at 31.4 GHz. The range of PWV, determined from the corrected radiosondes, was now 2.8 to 6.4 cm. However, the improvement in Tb comparisons was noticeably different for different radiosonde lots and was not a monotonic function of radiosonde age. Three different absorption algorithms were compared: Liebe and Layton (1987) – L87, Liebe et al. (1993) – L93, and Rosenkranz (1998) – ROS. When the ROS or L87 absorption models were applied to newer radiosondes to calculate Tb, both from the RHB and the R/V Mirai, agreement with the ARM MWR Tb measurements was substantially better, again with a 4 to 5 K improvement. Comparisons of brightness temperature calculations showed that for the 23.8 GHz vaporsensitive channel, L87 and ROS differed by only 0.1 K in Tb comparisons, but each differed by about 3.0 K with L93. At 31.4 GHz, the difference was about 1.0 between L87 and ROS and 3.0 K between L87 and L93. It was also possible to scale radiosonde soundings using data derived from the MWR. Using 30 clear data sets, infrared spectral radiance calculated from scaled radiosonde data, using all of the microwave radiative transfer models, was compared with AERI observations from the ARCS-2 site; nearly equal agreement was obtained with the ROS and the L87 models. In addition, the radiosonde observations scaled by the MWR PWV (ROS or L87) agreed better with the relative differences in band-integrated radiance (from 750 to 950 cm-1) of AERI measurements (4.5%) than did: (a) the radiosondes corrected by the Vaisala proprietary algorithm (5.1%); (b) the radiosondes scaled by L93 (5.1%); or (c) the original radiosonde data (7.4%).
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15010643
- Report Number(s):
- PNNL-SA-39101; KP1201030
- Journal Information:
- Journal of Atmospheric and Oceanic Technology, 20(7):953-971, Journal Name: Journal of Atmospheric and Oceanic Technology, 20(7):953-971
- Country of Publication:
- United States
- Language:
- English
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