Measurement Characteristics of an airborne Microwave Temperature Profiler (MTP)

Abstract

Abstract. The Microwave Temperature Profiler (MTP), an airborne passive microwave radiometer, records radiances in order to estimate temperature profiles around flight altitude. From these data the state of the atmosphere can be derived and important dynamical processes (e.g. gravity waves) assessed. DLR has acquired a copy of the MTP from NASA-JPL, which was designed as a wing-canister instrument and is deployed on the German research aircraft HALO. For this instrument a thorough analysis of instrument characteristics has been made. This is necessary to correctly determine the accuracy and precision of MTP measurements, and crucial for a retrieval algorithm to derive vertical profiles of absolute atmospheric temperatures. Using a laboratory set-up, the frequency response function and antenna diagram of the instrument was carefully characterised. A cold-chamber was used to simulate the changing in-flight conditions and to derive noise characteristics as well as reliable calibration parameters for brightness temperature calculations, which are compared to those calculated from campaign data. Furthermore, using the radiative transfer model Py4CAtS, the sensitivity to the atmospheric layers around flight altitude was investigated. It was found that using the standard measurement settings, the DLR-MTP’s vertical range of sensitivity is limited to 3 km around flight altitude, but can be significantly increased by adjusting the standard measurement strategy, including slightly weaker oxygen absorption lines and a different set of viewing angles. Calibration parameters do clearly depend on the state of the instrument; using a built-in heated target for calibration may yield large errors in brightness temperatures, due to a misinterpretation of the measured absolute temperature. With here presented corrections to the calibration parameter calculations, the measurement noise becomes the dominant source of uncertainty and it is possible to measure the atmospheric temperature around flight level with a precision of 0.38 K. This is the first time such a thorough instrument characterisation of a MTP instrument is published. With the presented results, it is now possible to identify significant temperature fluctuation signals in MTP data and choose the best possible measurement strategy fitting the purpose of the measurement campaign.

Insight into Construction of Tikhonov-Type Regularization for Atmospheric Retrievals

“…As can be seen from both comparison cases, the retrieved temperature information at altitudes close to the aircraft altitude was more sensitive to the measurement information and less sensitive to the a priori information, while the retrieval at lower and higher altitudes (except for L 1 ) appeared to be insensitive to the measurement information and the a priori error turned out to be dominant. This limitation is associated with instrument characteristics, as confirmed in [46]. The difference from the retrieved profile to the true profile was larger at altitudes where the vertical distance to the aircraft was larger with a lower retrieval sensitivity.…”

“…The German Aerospace Center (DLR) has acquired a copy and designed as a wing-canister instrument that has been deployed on the High Altitude and Long Range Research Aircraft (HALO) [45]. For more details about measurement characteristics of DLR-MTP, see [46].…”