RAPID: Evaluation of the Impact of Near-surface Turbulence on the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS) Measurements

University Of Arizona, Tucson AZ

Investigators

Abstract

For more than ten years, the principal Investigator (PI) and his group have been developing an atmospheric remote sensing system called the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS). ATOMMS combines the best features of the Global Positioning System (GPS) radio occultation (RO) and the Microwave Limb Sounder (MLS) techniques by actively probing via radio occultation (a subset of) the absorption lines that MLS observes via passive emission. Analyses of available data shows that ATOMMS will profile tropospheric and middle atmosphere water vapor and middle atmosphere ozone to 1-5%, temperature to 0.5K, and geopotential heights to 10-20 m, all with ~200 m vertical resolution, in both clear and cloudy air. This unprecedented performance will improve significantly with averaging. Because the occultation signal source is observed immediately before or after each occultation, ATOMMS is self-calibrating, which eliminates long-term drift. These capabilities will fulfill crucial needs for climate change monitoring, research and policymaking. This research focuses on several critical, near-term research objectives associated with the ATOMMS development. The objectives of this project are to (1) complete the nearly finished first version of the ATOMMS instrument, (2) complete the instrument testing and performance evaluation in the laboratory, (3) use the instrument to measure for the first time the impact of near-surface turbulence on the ATOMMS signals and (4) assess that impact against recent theoretical predictions. The nearly completed, first version of the ATOMMS instrument provides two tones near 183 GHz which is the minimum number needed to make useful differential absorption measurements. Simplistically, one tone measures absorption plus unwanted amplitude variations due to other effects such as antenna pointing and the 2nd tone measures just the unwanted amplitude variations. As a result, the ratio of the amplitudes of the two tones largely removes unwanted relatively broadband effects like those caused by turbulent fluctuations of the real part of the index of refraction while retaining most of the desired gaseous absorption signature. A recent Ph.D. in the group identified a new error source: turbulent fluctuations in the imaginary part of the atmospheric index of refraction that will produce significant amplitude scintillations separate from and in addition to those due to fluctuations in the real part of the refractivity. The ground measurements will for the first time measure the magnitude of the scintillations ("twinkling of a star") of the ATOMMS signals caused by near-surface turbulent fluctuations in the real and imaginary parts of the index of refraction and provide critical information needed to isolate and better understand this new source of error. The four steps summarized above are critical in the overall evaluation and demonstration of the ATOMMS remote sensing capability and must be completed prior to ATOMMS progressing to its flight testing and demonstration phase that will proceed if the ATOMMS follow-on proposal is funded. These research objectives are important and well-justified scientifically even if the ATOMMS follow-on proposal is not funded because they will determine and document in the peer-reviewed literature the performance of a new RO climate instrument and an uncertain error source critical to the ultimate performance of the ATOMMS climate monitoring system. The intellectual merit of this project is to isolate and quantify a recently recognized key source of error. The turbulence testing will provide the information to isolate, quantify and reduce amplitude scintillations due to turbulent fluctuations in the imaginary and real refractivity. Without these measurements and subsequent analysis, it will not be possible to quantify this critical error leaving the assessment of ATOMMS incomplete and its ultimate performance uncertain. The long term goal is a constellation of a dozen or more small spacecraft making ATOMMS occultation measurements which will provide dense, global coverage with complete cloud-penetration and diurnal sampling every orbit. Given its unique and powerful combination of qualities critical to characterizing climate, ATOMMS should be a key element in the global climate observing system Global Climate Observing System (GCOS) which is a crucial component in solving the climate change problem.