Evaluation of the WVSS-II moisture sensor using co-located in-situ and remotely sensed observations

The objective study has been initiated to assess the accuracy of WVSS-II humidity data by comparing it with radiosonde and ground-based remote sensing systems. Between 25 and 30 UPS B757 aircraft provided WVSS-II data through MDCRS for assessment during June 2005. In order to avoid the logistical complications of launching radiosonde in areas of congested air traffic near major airports, the tests will be conducted at the UPS hub in Louisville – where about 80% of the WVSS-II equipped planes land / take off daily.

The assessment was made using facilities provided by the UW mobile observing system and at CIMSS. Available intercomparison data sets included 1) an infrared AERI system (providing very high time frequency T/Q profiles in the boundary layer), 2) a surface GPS (providing a reference total atmospheric moisture content), 3) standard surface observations (Temp, Wind, Moisture, Ceiling), 4) a portable GPS rawinsonde system, and 5) geostationary and polar satellite data.

Approximately 2 weeks of collocated radiosonde and aircraft data were be collected with radiosonde launched 3 times nightly, immediately before, between and after periods of multiple aircraft arrivals/departures. More than 5 co-locations (with less than 1 hour time difference) were obtained daily. Measurements included the full complement of atmospheric state variables – Temperature, Wind Direction / Speed, and Humidity.

Because the accuracy of the B757 temperature and wind sensors is known, WVSS-II assessments focused primarily on the accuracy of water vapor measurements throughout the full depth of the troposphere. Comparisons available in near real time showed generally excellent agreement, especially in the lower troposphere. In addition to determining if additional engineering modifications need to be made to the test systems, ongoing statistical evaluations are being made of the performance of a variety of factors important for the optimal objective use of the aircraft data in combination with other data sources by assessing: 1) Similarity of reports from the different observing systems, 2) Biases that exist between ascent and descent reports from individual aircraft, 3) Variability in reports between different aircraft (to assess accuracy of individual instrument calibration and the effects of instrument aging), and 4) Capabilities of the systems to capture sharp moisture gradients accurately, including after an aircraft emerges from clouds. Initial results of these assessments will be presented at the meeting.

A second objective of this study is to gauge if aircraft moisture data can be used as an affordable means of providing sufficient numbers of high-resolution moisture data (both in time and space) to fill the gaps left between the 400 km and 12 hr spacing of the US rawinsonde network. In order to determine how many reports are needed – both in space and time, data sets of co-located high time frequency AERI moisture profiles and Profiler Wind data have been assembled at CIMSS. Analyses of the time-series data will be conducted to determine the observing frequency necessary to capture temporal changes in moisture structure critical to the development of a variety of different weather events. The humidity and moisture data will then be combined using time-to-space conversion techniques to provide guidance in determining minimal required observation spacing.