Long-Term Observations of Microwave Brightness Temperatures over a Metropolitan Area: Comparison of Radiometric Data and Spectra Simulated with the Use of Radiosonde Measurements

Abstract

Ground-based microwave radiometers are increasingly used in operational meteorology and nowcasting. These instruments continuously measure the spectra of downwelling atmospheric radiation in the range 20–60 GHz used for the retrieval of tropospheric temperature and water vapor profiles. Spectroscopic uncertainty is an important part of the retrieval error budget, as it leads to systematic bias. In this study, we analyze the difference between observed and simulated microwave spectra obtained from more than four years of microwave and radiosonde observations over Nizhny Novgorod (56.2° N, 44° E). We focus on zenith-measured and elevation-scanning data in clear-sky conditions. The simulated spectra are calculated by a radiative transfer model with the use of radiosonde profiles and different absorption models, corresponding to the latest spectroscopy research. In the case of zenith-measurements, we found a systematic bias (up to ~2 K) of simulated spectra at 51–54 GHz. The sign of bias depends on the absorption model. A thorough investigation of the error budget points to a spectroscopic nature of the observed differences. The dependence of the results on the elevation angle and absorption model can be explained by the basic properties of radiative transfer and by cloud contamination at elevation angles.

The estimation of seasonal variation of retrieval error in tropospheric temperature microwave profiling

“…It is suitable for radiative transfer calculations without the use of any cloud model. In [10] is show that the impact of 10-15km layer is significant and variable enough impact in the simulation. For that reason, the subset includes only those cases when radiosonde profile reaches 15km.…”

“…The threshold represents the LWP value that indicates the occurrence of raindrops [8]. The "clear sky" dataset is the dataset used in [10]. It is suitable for radiative transfer calculations without the use of any cloud model.…”

“…Thus we compiled the dataset of simultaneous microwave and radiosonde measurements. The dataset is well described in [10,11]. In brief the microwave profiles are averaged over 12-min intervals (6 min before and after a radiosonde launch).…”

“…Radiative transfer model is described in detail in [10]. In current work we limit ourselves to zenith measurement simulations.…”

“…where T 0 =2.725 K is cosmic microwave background temperature. Please refer to the works [10,12] about the details concerning the modelling of the influence of middle atmosphere, as well as the precision of pencil beam and monochromatic channels approximations. The work [13] discusses the accuracy of Rayleigh-Jeans approximation in (6) (see part 1.2.4).…”