Retrieval of Atmospheric Temperature from Airborne Microwave Radiometer Observations

Abstract

Atmospheric temperature is a key geophysical parameter associated with fields such as meteorology,climatology, or photochemistry. There exist several techniques to measure temperature profiles. In the case of microwave remote sensing, the vertical temperature profile can be estimated from thermal emission lines of molecular oxygen. The MTP (Microwave Temperature Profiler) instrument is an airborne radiometer developed at the Jet Propulsion Laboratory (JPL), United States. The instrument passively measures natural thermal emission from oxygen lines at 3 frequencies and at a selection of 10viewing angles (from near zenith to near nadir). MTP has participated in hundreds of flights, including on DLR's Falcon and HALO aircraft. These flights have provided data of the vertical temperature distribution from the troposphere to the lower stratosphere with a good temporal and spatial resolution. In this work, we present temperature retrievals based on the Tikhonov regularized nonlinear leastsquares fitting method. In particular, Jacobians (i.e. temperature derivatives) are evaluated by means of automatic differentiation. The retrieval performance from the MTP measurements is analyzed by using synthetic and real data. Besides, the vertical sensitivity of the temperature retrieval is studied by weighting functions characterizing the sensitivity of the transmission at different frequencies with respect to changes of altitude levels.