New opportunities due to advancements in the techniques and methods of broadband resonator spectroscopy in the millimeter and submillimeter range (also known as the terahertz region) are considered. An upgraded variant of the spectrometer produced at IAP RAS (Nizhny Novgorod, Russia) possessing a sensitivity to the absorption coefficient of approximately 4 × 10 $^-9$ cm $^-1$ is presented. The spectrometer allows the continuous recording of gas spectra in the 45-500 GHz frequency range (6 waveguide sub-bands) at pressures of 10-1000 Torr. Fully automated spectrum recording within the waveguide sub-band reduces the overall time required for the experiment by almost an order of magnitude. Examples of recording the spectra of oxygen and water vapor resonance lines, as well as of continuum absorption, including the rotationally resolved water dimer spectrum, are presented. It is demonstrated that the uncertainty of measuring the broadening parameter of diagnostic atmospheric lines may be reduced to 0.1%, which meets the stringent present day requirements of remote sensing of the atmosphere. The physically attainable ultimate sensitivity of broadband microwave resonator spectrometers of various types is discussed. Prospects for further development of the spectrometer and its application for solving fundamental and applied problems of spectroscopy are analyzed.

In this work, we compare the values of 15 convective indices obtained from radiosonde and microwave temperature and water vapor profiles simultaneously measured over Nizhny Novgorod (56.2°N, 44°E) during 5 convective seasons of 2014–2018. A good or moderate correlation (with coefficients of ~0.7–0.85) is found for most indices. We assess the thunderstorm prediction skills with a lead time of 12 h for each radiosonde and microwave index. It is revealed that the effectiveness of thunderstorm prediction by microwave indices is much better than by radiosonde ones. Moreover, a good correlation between radiosonde and microwave values of a certain index does not necessarily correspond to similar prediction skills. Eight indices (Showalter Index, Maximum Unstable Convective Available Potential Energy (CAPE), Total Totals index, TQ index, Jefferson Index, S index, K index, and Thompson index) are regarded to be the best predictors from both the true skill statistics (TSS) maximum and Heidke skill score (HSS) maximum points of view. In the case of radiosonde data, the best indices are the Jefferson Index, K index, S index, and Thompson index. Only TSS and HSS maxima for these indices are close to the microwave ones, whereas the prediction skills of other radiosonde indices are essentially worse than in the case of microwave data. The analysis suggests that the main possible reason of this discrepancy is an unexpectedly low quality of radiosonde data.

We present the results of theoretical and experimental studies of the reflectivity of metals at cryogenic temperatures in the millimeter wavelength range. High-Q Fabry-Perot resonators operated at a temperature of 4-300 K in the frequency range 150-250 GHz are used for the experimental study. Silver, copper, gold, aluminum, and beryllium reflectors with different structures are examined. It has been shown that the reflection loss at cryogenic temperatures varies considerably (several times) depending on the structure of the sample surface and the presence of impurities. The obtained data make it possible to calculate the thermal noise of cooled reflectors of the antenna systems in millimeter and submillimeter telescopes.

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.