Ground-based Microwave Radiometry Research Articles

Prospects for Application of Ground-based Microwave Radiometry for Analysis of Atmospheric Fronts and Early Prediction of Severe Weather Events

Prospects for Application of Ground-based Microwave Radiometry for Analysis of Atmospheric Fronts and Early Prediction of Severe Weather Events

ПЕРСПЕКТИВЫ ПРИМЕНЕНИЯ МЕТОДОВ НАЗЕМНОЙ МИКРОВОЛНОВОЙ РАДИОМЕТРИИ ДЛЯ СИНОПТИЧЕСКОГО АНАЛИЗА АТМОСФЕРНЫХ ФРОНТОВ И ПРОГНОЗА ОПАСНЫХ ЯВЛЕНИЙ ПОГОДЫ

A brief description of the method of ground-based microwave radiometry for determining the characteristics of atmospheric moisture content is given. The results of experimental studies of total precipitable water and cloud liquid water content using ground-based microwave radiometers, in particular, during the development of severe weather events associated with the development of convective clouds, thunderstorm processes are reported. The capabilities of microwave radiometry in improving the diagnosis of synoptic and mesoscale structure of atmospheric fronts are discussed. The prospects for using radiophysical facilities of the Mozhaisky Military Space Academy Geophysical Observatory for experimental studies of the atmosphere, clouds, and precipitation in the Leningrad region, which are aimed at improving methods for forecasting severe weather events associated with the development of convective clouds, thunderstorms, and precipitation, are considered.

Integrated Water Vapor during Rain and Rain-Free Conditions above the Swiss Plateau

Water vapor column density, or vertically-integrated water vapor (IWV), is monitored by ground-based microwave radiometers (MWR) and ground-based receivers of the Global Navigation Satellite System (GNSS). For rain periods, the retrieval of IWV from GNSS Zenith Wet Delay (ZWD) neglects the atmospheric propagation delay of the GNSS signal by rain droplets. Similarly, it is difficult for ground-based dual-frequency single-polarisation microwave radiometers to separate the microwave emission of water vapor and cloud droplets from the rather strong microwave emission of rain. For ground-based microwave radiometry at Bern (Switzerland), we take the approach that IWV during rain is derived from linearly interpolated opacities before and after the rain period. The intermittent rain periods often appear as spikes in the time series of integrated liquid water (ILW) and are indicated by ILW ≥ 0.4 mm. In the present study, we assume that IWV measurements from radiosondes are not affected by rain. We intercompare the climatologies of IWV(rain), IWV(no rain), and IWV(all) obtained by radiosonde, ground-based GNSS atmosphere sounding, ground-based MWR, and ECMWF reanalysis (ERA5) at Payerne and Bern in Switzerland. In all seasons, IWV(rain) is 3.75 to 5.94 mm greater than IWV(no rain). The mean IWV differences between GNSS and radiosonde at Payerne are less than 0.26 mm. The datasets at Payerne show a better agreement than the datasets at Bern. However, the MWR at Bern agrees with the radiosonde at Payerne within 0.41 mm for IWV(rain) and 0.02 mm for IWV(no rain). Using the GNSS and rain gauge measurements at Payerne, we find that IWV(rain) increases with increase of the precipitation rate during summer as well as during winter. IWV(rain) above the Swiss Plateau is quite well estimated by GNSS and MWR though the standard retrievals are limited or hampered during rain periods.

Physical Retrieval of Rain Rate from Ground-Based Microwave Radiometry

Because of its clear physical meaning, physical methods are more often used for space-borne microwave radiometers to retrieve the rain rate, but they are rarely used for ground-based microwave radiometers that are very sensitive to rainfall. In this article, an opacity physical retrieval method is implemented to retrieve the rain rate (denoted as Opa-RR) using ground-based microwave radiometer data (21.4 and 31.5 GHz) of the tropospheric water radiometer (TROWARA) at Bern, Switzerland from 2005 to 2019. The Opa-RR firstly establishes a direct connection between the rain rate and the enhanced atmospheric opacity during rain, then iteratively adjusts the rain effective temperature to determine the rain opacity, based on the radiative transfer equation, and finally estimates the rain rate. These estimations are compared with the available simultaneous rain rate derived from rain gauge data and reanalysis data (ERA5). The results and the intercomparison demonstrate that during moderate rains and at the 31 GHz channel, the Opa-RR method was close to the actual situation and capable of the rain rate estimation. In addition, the Opa-RR method can well derive the changes in cumulative rain over time (day, month, and year), and the monthly rain rate estimation is superior, with the rain gauge validated R2 and the root-mean-square error value of 0.77 and 22.46 mm/month, respectively. Compared with ERA5, Opa-RR at 31GHz achieves a competitive performance.

Retrievals of soil moisture and vegetation optical depth using a multi-channel collaborative algorithm

Due to the success of the SMOS (Soil Moisture and Ocean Salinity) and SMAP (Soil Moisture Active Passive) missions, new satellite missions are on the horizon. The current and future missions can benefit from investigations that seek to improve retrieval algorithms that quantitatively map global soil moisture and vegetation optical depth (tau) from Earth's microwave emissions. In this study, we explore multi-angular and multi-frequency approaches for the retrieval of soil moisture and vegetation tau, considering the payload configurations of current and future satellite missions (such as the Copernicus Imaging Microwave Radiometer, the Water Cycle Observation Mission, and the Terrestrial Water Resources Satellite) using a new set of ground observations. Two ground-based microwave radiometry datasets collected in Inner Mongolia during the Soil Moisture Experiment in the Luan River from July to August 2017 (cropland) and August to September 2018 (grassland) are used for this study. The corn field, which covers an entire growth period, indicated that the degree of information increases linearly as the number of channels (in terms of the incidence angle and frequency) increases, and that the multi-frequency observations contain slightly more independent information than do the multi-angular observations under the same number of channels. The polarization difference in brightness temperature is sensitive to both soil moisture and vegetation water content, especially at L-band due to its penetrating ability. Soil moisture explains most of the variance in frequency differences of brightness temperature at adjacent frequencies (L- & C-bands, C- & X-bands), while the variance in incidence-angle differences of brightness temperature is mostly associated with the vegetation water content. A multi-channel collaborative algorithm (MCCA) is developed based on the two-component version of the omega-tau model, which utilizes information from collaborative channels expressed as an analytical form of brightness temperature at the core channel to rule out the parameters to be retrieved. Results of soil moisture retrieval show that the multi-angular approach used by the MCCA generally has a better performance, unbiased root mean square difference (ubRMSD) varying from 0.028 cm3/cm3 to 0.037 cm3/cm3, than the multi-frequency approach (ubRMSD from 0.028 cm3/cm3 to 0.089 cm3/cm3) for the corn field. This is attributed to the dependence of vegetation tau on the frequency being more significant than that on the incidence angle. Except for in the C- & X-band combination, the multi-frequency approach used by the MCCA performs better (ubRMSD from 0.018 cm3/cm3 to 0.023 cm3/cm3) than the multi-angular version (ubRMSD from 0.026 cm3/cm3 to 0.034 cm3/cm3) for the grass field due to reduced vegetation effects for this type of cover. It is affirmed that increasing the number of observation channels could make the soil moisture retrieval more robust, but might also limit the retrieval performance, as the probability that the model estimations will not match the observations is increased. This study provides new insights into the design of potential satellite missions to improve soil moisture retrieval. A satellite with simultaneous multi-angular and multi-frequency observation capabilities is highly recommended.

THE MICROWAVE MONITORING OF THE MIDDLE ATMOSPHERE OZONE ON KOLA PENINSULA DURING LAST THREE WINTERS

This work presents long-term investigation of a nature of the middle atmosphere ozone variability using a method ground-based microwave radiometry. Measurements were carried out with the help of mobile microwave ozonemeter (observation frequency 110836.04 MHz) which was established in Polar Geophysical Institute at Apatity (67N, 33E). The parameters of the device allow to measure a spectrum of the emission ozone line for time about 15 min with a precision of ~ 2%. The error of estimating the vertical distribution of ozone on the measured spectra by above described device does not exceed 10-15%. On the measured spectra were appreciated of ozone vertical profiles in the layer of 22 – 60 km which were compared to satellite data MLS/Aura and with the data of ozonesondes at station Sodankyla (67N, 27E). The analysis of the microwave data on behavior of polar mesospheric ozone in past winters shows, that sudden stratospheric warming (SSW) can cause significant and long influence on its diurnal variation which should be determined by photochemical processes.

Mesosphere Ozone and the Lower Ionosphere under Plasma Disturbance by Powerful High-Frequency Radio Emission

We present the results of experiments on the Earth’s lower ionosphere at mesospheric heights by creating artificial periodic irregularities (APIs) of the ionospheric plasma and simultaneous measurement of the atmospheric emission spectrum in the ozone line by ground-based microwave radiometry when the ionosphere was disturbed by powerful high-frequency radio emission from the midlatitude SURA heating facility (56.15° N; 46.11° E). The diagnostics of the ionosphere was carried out on the basis of measuring amplitudes and phases of signals scattered by periodic irregularities in the altitude range of 50–130 km. For each heating session lasting 30 min, two ozone spectra were measured. These spectra were compared with the measured spectra the periods when heating was turned off. During the heating session of the ionosphere, a decrease in the intensity of the microwave radiation of the atmosphere in the ozone line was observed. The lower ionosphere was characterized by intense dynamics. Rapid variations in the amplitude of the scattered signal and the relaxation time of artificial periodic irregularities were observed. The velocity of a regular vertical movement in the D-region of the ionosphere constantly varied direction with average minute values up to 4–5 m/s. We assume the decrease in the ozone emission spectrum at the altitude of 60 km can be explained by an increase in the coefficient of electron attachment to oxygen molecules during heating sessions. The lower boundary of the region enriched with atomic oxygen was estimated from the height profile of the API relaxation time.

Statistical Analysis of 1996–2017 Ozone Profile Data Obtained by Ground-Based Microwave Radiometry

Trends in the ozone layer remain among the major problems of the atmosphere physics; thus, results of measurements of the ozone altitude distribution (profile), carried out in the same place and via the same method, are very important. This paper presents the results of the statistical analysis of ensembles of ozone profiles obtained from ground-based microwave radiometry data acquired at the P.N. Lebedev Physical Institute over a period of two decades (1996–2017). The data collected show the significant difference between monthly mean statistical parameters of ozone profiles of the decades 1996–2006 and 2007–2017. The main and unexpected result is the drastic decrease in monthly root-mean-square (rms) variances of ozone profiles over Moscow above 30 km in cold months of the decade 2007–2017 (if compared to the variances in the decade 1996–2006) with the maximum fall by 46% at 39 km in February monthly mean variances. The decade change of variances obtained by averaging over all nine months in the analysis (from September to May) has the same decrease with maximum fall by 25% at 38 km. Additionally, significant decade changes were revealed in other monthly mean statistical parameters: probability density of ozone profile variances, inter-altitude covariance and correlation functions, and time covariance and correlation—as well as their frequency spectra. The decade change of the ozone profile obtained by averaging over the nine months appeared much less significant: the decrease by 5.7% at the altitude of 19 km (with 1.5% sampling error), minor decrease by 2.6% (with sampling error 1.5%) in the profile maximum at 37 km, and increases of 1.7% at 28 km and 2.5% at 47 km (with sampling errors 1.7%)—lower and higher of this maximum. In addition to that, the corresponding averaged mean total column (integral) ozone content above 20 km remained practically unchanged: 4.61 g/m2 for decade 1996–2006 as compared to 4.58 g/m2 for 2007–2017. Possible explanations of revealed offsets are proposed and discussed.

Ground-based Microwave Radiometry for measurements of Atmospheric Water Vapour and Cloud Liquid Water Contents

The method of ground-based microwave radiometry for determining the integral parameters of atmospheric moisture content, such as integrated water vapor and cloud liquid water content is considered. Some results of experimental studies of integrated water vapor and integrated cloud liquid in different regions by means of ground-based and ship-based of microwave radiometers are given. The microwave radiometric measurements of integrated water vapor were found to be in agreement with independent radiosonding data (rms is ∼1 kg/m2). Microwave radiometric measurements of integrated cloud liquid of stratified clouds are consistent with empirical models of cloudy atmosphere based on aircraft data sensing of clouds. The capabilities of the novel microwave water vapor radiometer for studying variations in atmospheric moisture content on micro and meso scales were demonstrated. Histograms of the moisture content are obtained, and an approximation for the temporal structural function of the integrated water vapor in a wide range of time scales is discussed.

A Study of the Density Variations of Mesospheric Ozone in the Case of Ionospheric Perturbation by The “Sura” Facility Radiation with Simultaneous Diagnostics of Ionospheric Plasma

We present the results of microwave observations of atmospheric radiation in the ozone line in the case of ionospheric perturbation by powerful high-frequency radio emission from the “Sura” facility in September 2016. The atmospheric-radiation spectrum in the ozone line was measured by the method of the ground-based microwave radiometry. The state of the lower ionosphere was simultaneously diagnosed in the altitude range 50–130 km using the method of resonant scattering of radio waves by the artificial periodic irregularities of ionospheric plasma. A decrease in the intensity of the microwave atmospheric radiation in the ozone line was observed. The results of measurements of the amplitude and phase of the signal scattered by artificial periodic irregularities demonstrated a perturbation of the parameters of the lower ionosphere when it was heated by high-frequency radiation.

Diurnal variation in middle-atmospheric ozone observed by ground-based microwave radiometry at Ny-Ålesund over 1 year

Abstract. We present an analysis of the diurnal ozone cycle from 1 year of continuous ozone measurements from two ground-based microwave radiometers in the Arctic. The instruments GROMOS-C and OZORAM are located at the AWIPEV research base at Ny-Ålesund, Svalbard (79° N, 12° E), and gathered a comprehensive time series of middle-atmospheric ozone profiles with a high time resolution. An intercomparison was performed with EOS MLS and ozone sonde measurements and simulations with SD-WACCM. The measured data sets were used to study the photochemically induced diurnal cycle of ozone in the stratosphere and mesosphere. Throughout the year the insolation in the Arctic changes drastically from polar night to polar day. Accordingly, the seasonal variations in the diurnal ozone cycle are large. In the stratosphere we found a diurnal cycle throughout the entire period of polar day with the largest amplitude in April. In the mesosphere a diurnal cycle was detected in spring and fall. SD-WACCM has been proven to capture the diurnal cycle well and was therefore used to analyse the chemical reaction rates of ozone production and loss at equinox and summer solstice. Furthermore GROMOS-C proved capable of measuring the tertiary ozone layer above Ny-Ålesund in winter.

Status and prospects of application of microwave radiometry of the atmosphere

The status of ground-based microwave radiometry in view of determining the air temperature and moisture profiles, atmospheric water vapor, liquid water in clouds, and rainfall intensity is discussed. Approaches to the solution of inverse problems of microwave radiometry of the atmosphere are described, as well as results of experimental studies of atmospheric water vapor and liquid water in clouds with a dual-frequency radiometer. Promising directions in the practical use of microwave radiometry of the atmosphere are formulated.

A compact receiver system for simultaneous measurements of mesospheric CO and O<sub>3</sub>

Abstract. During the last decades, ground-based microwave radiometry has matured into an established remote sensing technique for measuring vertical profiles of a number of gases in the stratosphere and the mesosphere. Microwave radiometry is the only ground-based technique that can provide vertical profiles of gases in the upper stratosphere and mesosphere both day and night, and even during cloudy conditions. Except for microwave instruments placed at high-altitude sites, or at sites with dry atmospheric conditions, only molecules with significant emission lines below 150 GHz, such as CO, H2O, and O3, can be observed. Vertical profiles of these molecules can give important information about chemistry and dynamics in the middle atmosphere. Today these measurements are performed at relatively few sites; more simple and reliable instrument solutions are required to make the measurement technique more widely spread. This need is urgent today as the number of satellite sensors observing the middle atmosphere is about to decrease drastically. In this study a compact double-sideband frequency-switched radiometer system for simultaneous observations of mesospheric CO at 115.27 GHz and O3 at 110.84 GHz is presented. The radiometer, its calibration scheme, and its observation method are presented. The retrieval procedure, including compensation of the different tropospheric attenuations at the two frequencies and error characterization, are also described. The first measurement series from October 2014 until April 2015 taken at the Onsala Space Observatory, OSO (57° N, 12° E), is analysed. The retrieved vertical profiles are compared with co-located CO and O3 data from the MLS instrument on the Aura satellite. The data sets from the instruments agree well with each other. The main differences are the higher OSO volume mixing ratios of O3 in the upper mesosphere during the winter nights and the higher OSO volume mixing ratios of CO in the mesosphere during the winter. The low bias of mesospheric winter values of CO from MLS compared to ground-based instruments was reported earlier.

Accuracy of retrieving temperature and humidity profiles by ground-based microwave radiometry in truly complex terrain

Abstract. Within the Innsbruck Box project, a ground-based microwave radiometer (RPG-HATPRO) was operated in the Inn Valley (Austria), in very complex terrain, between September 2012 and May 2013 to obtain temperature and humidity vertical profiles of the full troposphere with a specific focus on the valley boundary layer. In order to assess its performance in a deep alpine valley, the profiles obtained by the radiometer with different retrieval algorithms based on different climatologies are compared to local radiosonde data. A retrieval that is improved with respect to the one provided by the manufacturer, based on better resolved data, shows a significantly smaller root mean square error (RMSE), both for the temperature and humidity profiles. The improvement is particularly substantial at the heights close to the mountaintop level and in the upper troposphere. Lower-level inversions, common in an alpine valley, are resolved to a satisfactory degree. On the other hand, upper-level inversions (above 1200 m) still pose a significant challenge for retrieval. For this purpose, specialized retrieval algorithms were developed by classifying the radiosonde climatologies into specialized categories according to different criteria (seasons, daytime, nighttime) and using additional regressors (e.g., measurements from mountain stations). The training and testing on the radiosonde data for these specialized categories suggests that a classification of profiles that reproduces meaningful physical characteristics can yield improved targeted specialized retrievals. A novel and very promising method of improving the profile retrieval in a mountainous region is adding further information in the retrieval, such as the surface temperature at fixed levels along a topographic slope or from nearby mountaintops.

Signatures of the 2-day wave and sudden stratospheric warmings in Arctic water vapour observed by ground-based microwave radiometry

Abstract. The ground-based microwave radiometer MIAWARA-C recorded the upper stratospheric and lower mesospheric water vapour distribution continuously from June 2011 to March 2013 above the Arctic station of Sodankylä, Finland (67.4° N, 26.6° E) without major interruptions and offers water vapour profiles with temporal resolution of 1 h for average conditions. The water vapour time series of MIAWARA-C shows strong periodic variations in both summer and winter related to the quasi-2-day wave. Above 0.1 hPa the amplitudes are strongest in summer. The stratospheric wintertime 2-day wave is pronounced for both winters on altitudes below 0.1 hPa and reaches a maximum amplitude of 0.8 ppmv in November 2011. Over the measurement period, the instrument monitored the changes in water vapour linked to two sudden stratospheric warmings in early 2012 and 2013. Based on the water vapour measurements, the descent rate in the vortex after the warmings is 364 m d−1 for 2012 and 315 m d−1 for 2013.

Zeeman effect in atmospheric O<sub>2</sub> measured by ground-based microwave radiometry

Abstract. In this work we study the Zeeman effect on stratospheric O2 using ground-based microwave radiometer measurements. The interaction of the Earth magnetic field with the oxygen dipole leads to a splitting of O2 energy states, which polarizes the emission spectra. A special campaign was carried out in order to measure this effect in the oxygen emission line centered at 53.07 GHz. Both a fixed and a rotating mirror were incorporated into the TEMPERA (TEMPERature RAdiometer) in order to be able to measure under different observational angles. This new configuration allowed us to change the angle between the observational path and the Earth magnetic field direction. Moreover, a high-resolution spectrometer (1 kHz) was used in order to measure for the first time the polarization state of the radiation due to the Zeeman effect in the main isotopologue of oxygen from ground-based microwave measurements. The measured spectra showed a clear polarized signature when the observational angles were changed, evidencing the Zeeman effect in the oxygen molecule. In addition, simulations carried out with the Atmospheric Radiative Transfer Simulator (ARTS) allowed us to verify the microwave measurements showing a very good agreement between model and measurements. The results suggest some interesting new aspects for research of the upper atmosphere.

УТРОИТЕЛЬ ЧАСТОТЫ НА ДИОДАХ С БАРЬЕРОМ ШОТТКИ В ПРИЕМНИКЕ 3-ММ ДИАПАЗОНА ДЛЯ ИССЛЕДОВАНИЙ ЛИНИЙ ИЗЛУЧЕНИЯ АТМОСФЕРНЫХ ГАЗОВ

A Schottky diode frequency tripler for the 3-mm receiver system designed for observation of ozone and carbon monoxide spectral lines at frequencies 110.836 and 115.271 GHz, respectively, is developed. The used varactor parameters are investigated and its limiting values of frequency tripling efficiency in the 3-mm wavelengths calculated. A design of the multiplier, which allows to realize close to the calculated levels of frequency conversion efficiency is proposed. The characteristics of the developed multiplier are investigated within the frequency range 109-136 GHz. The output power of 8 mW is obtained at 132 GHz with about 62 mW pumping power. The tripler efficiency, output power and operating current at the desired frequency for atmospheric receiver of 112.5 GHz were measured. The highest efficiency (18 %) has been realized at this frequency with 15 mW input power. The values of the input power, operating current and reverse bias of the developed frequency tripler required for optimal receiver performance for observation of the emission lines of atmospheric gases have been determined. Key words: frequency tripler , receiver for investigation of atmospheric gases, frequency tripling efficiency, input power, output power Manuscript submitted 07.08.2014 Radio phys. radio astron. 2015, 20(1): 86-93 REFERENCES 1. FORKMAN, P., CHRISTENSEN, O. M., ERIKSSON, P., URBAN, J. and FUNKE, B., 2012. Six years of mesospheric CO estimated from ground-based frequency-switched microwave radiometry at 57° N compared with satellite instruments. Atmos. Meas. Tech. vol. 5, no. 11, pp. 2827–2841. DOI: https://doi.org/10.5194/amtd-5-3909-2012 2. RAFFALSK, I. U., BERG, H., HOCHSCHILD, G. and KOPP, G., 2002. 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Assessment of Ground-Based Microwave Radiometry for Calibration of Atmospheric Variability in Spacecraft Tracking

In a suggested radio propagation experiment using a deep space antenna, accurate calibration of the propagation delay through the Earth's atmosphere is essential. One or two microwave radiometers can be used for this purpose. Differences in precise locations of the radiometer(s) and antenna to be calibrated leave a residual wet path delay value. We computed the Allan Standard Deviation (ASD) of this residual, as well as the one resulting from different pointing positions in the plane of the sky, by simulations. Pointing offsets, e.g., to avoid solar radiation into the radiometer beam, lead in general to an increased ASD. However, for many observation geometries a deliberate pointing offset can compensate for the location differences. In the case studied we found a reduction of the ASD with up to 45% compared to the ASD obtained for a zero pointing offset. The size of the calculated ASD depends strongly on the model parameters used, e.g., the turbulence strength parameter Cn2, which has a significant natural variation over a year.

Mixing layer height retrievals by multichannel microwave radiometer observations

Abstract. The mixing layer height (MLH) is a key parameter for boundary layer studies, including meteorology, air quality, and climate. MLH estimates are inferred from in situ radiosonde measurements or remote sensing observations from instruments like lidar, wind profiling radar, or sodar. Methods used to estimate MLH from radiosonde profiles are also used with atmospheric temperature and humidity profiles retrieved by microwave radiometers (MWR). This paper proposes an alternative approach to estimate MLH from MWR data, based on direct observations (brightness temperatures, Tb) instead of retrieved profiles. To our knowledge, MLH estimates directly from Tb observations have never been attempted before. The method consists of a multivariate linear regression trained with an a priori set of collocated MWR Tb observations (multifrequency and multi-angle) and MLH estimates from a state-of-the-art lidar system. The proposed method was applied to a 7-month data set collected at a typical midlatitude site. Results show that the method is able to follow both the diurnal cycle and the day-to-day variability as suggested by the lidar measurements, and also it can detect low MLH values that are below the full overlap limit (~200 m) of the lidar system used. Statistics of the comparison between MWR- and reference lidar-based MLH retrievals show mean difference within 10 m, root mean square within 340 m, and correlation coefficient higher than 0.77. Monthly mean analysis for daytime MLH from MWR, lidar, and radiosonde shows consistent seasonal variability, peaking at ~1200–1400 m in June and decreasing down to ~600 m in October. Conversely, nighttime monthly mean MLH from all methods are within 300–500 m without any significant seasonal variability. The proposed method provides results that are more consistent with radiosonde estimates than MLH estimates from MWR-retrieved profiles. MLH monthly mean values agree well within 1 standard deviation with the bulk Richardson number method applied at radiosonde profiles at 11:00 and 23:00 UTC. The method described herewith operates continuously and is expected to work with analogous performances for the entire diurnal cycle, except during considerable precipitation, demonstrating new potential for atmospheric observation by ground-based microwave radiometry.

Diurnal variations in middle-atmospheric water vapor by ground-based microwave radiometry

Abstract. In this paper, we compare the diurnal variations in middle-atmospheric water vapor as measured by two ground-based microwave radiometers in the Alpine region near Bern, Switzerland. The observational data set is also compared to data from the chemistry–climate model WACCM. Due to the small diurnal variations of usually less than 1%, averages over extended time periods are required. Therefore, two time periods of five months each, December to April and June to October, were taken for the comparison. The diurnal variations from the observational data agree well with each other in amplitude and phase. The linear correlation coefficients range from 0.8 in the upper stratosphere to 0.5 in the upper mesosphere. The observed diurnal variability is significant at all pressure levels within the sensitivity of the instruments. Comparing our observations with WACCM, we find that the agreement of the phase of the diurnal cycle between observations and model is better from December to April than from June to October. The amplitudes of the diurnal variations for both time periods increase with altitude in WACCM, but remain approximately constant at 0.05 ppm in the observations. The WACCM data are used to separate the processes that lead to diurnal variations in middle-atmospheric water vapor above Bern. The dominating processes were found to be meridional advection below 0.1 hPa, vertical advection between 0.1 and 0.02 hPa and (photo-)chemistry above 0.02 hPa. The contribution of zonal advection is small. The highest diurnal variations in water vapor as seen in the WACCM data are found in the mesopause region during the time period from June to October with diurnal amplitudes of 0.2 ppm (approximately 5% in relative units).