Horizontal gradients in the wet path delay derived from four years of microwave radiometer data

We have analyzed four years of inferred wet path delay data from a microwave radiometer operating at 21.0 and 31.4 GHz. We have applied a four parameter gradient model to the wet delays, using different lengths of the time series for the gradient estimation. The mean gradient, averaged over 15 to 1440 minutes, varies between 0.9 and 0.4 mm End has a preferred direction towards the north-east. Increasing the averaging time causes the estimated gradient to decrease. The 15 minutes mean gradient is 1.3 mm for the summer months and 0.7 mm for the winter months. Structure function results are also presented

Trends in the AtmosphericWater Vapor Content From Ground-Based GPS: The Impact of the Elevation Cutoff Angle

We used 14 years of data from 12 GPS sites in Sweden and Finland to estimate trends in the atmospheric integrated water vapor (IWV) for 8 different elevation cutoff angles, from 5° to 40°, for the observations used in the analyses. These trends were compared to the corresponding trends obtained from radiosonde data at 7 nearby (< 120 km) sites. The results show a variation in the correlation of the trends between the two techniques for different elevation cutoff angles. The highest correlation coefficient of 0.88 is obtained for the 25° solution, whereas the smallest rootmean-square (RMS) differences between the IWV estimates themselves are obtained mainly for elevation cutoff angles of 10 and 15°. The results show that elevation-angle-dependent systematic errors vary with time. Therefore the elevation cutoff angle giving the best agreement between radiosonde and GPS for individual IWV estimates is not necessarily the optimum when estimating linear trends. The correlation between the trends from the two completely independent techniques is strong evidence that the two techniques provide information on the IWV trends although the true individual values are too small to be uniquely detected. In addition, we found that the choice of mapping functions is not critical for the IWV trend estimation

High temporal resolution wet delay gradients estimated from multi-GNSS and microwave radiometer observations

We have used 1 year of multi-GNSS observations at the Onsala Space Observatory on the Swedish west coast to estimate the linear horizontal gradients in the wet propagation delay. The estimated gradients are compared to the corresponding ones from a microwave radiometer. We have investigated different temporal resolutions from 5 min to 1 d. Relative to the GPS-only solution and using an elevation cutoff angle of 10 and a temporal resolution of 5 min, the improvement obtained for the solution using GPS, Glonass, and Galileo data is an increase in the correlation coefficient of 11 % for the east gradient and 20 % for the north gradient. Out of all the different GNSS solutions, the highest correlation is obtained for the east gradients and a resolution of 2 h, while the best agreement for the north gradients is obtained for 6 h. The choice of temporal resolution is a compromise between getting a high correlation and the possibility of detecting rapid changes in the gradient. Due to the differences in geometry of the observations, gradients which happen suddenly are either not captured at all or captured but with much less amplitude by the GNSS data. When a weak constraint is applied in the estimation of process, the GNSS data have an improved ability to track large gradients, however, at the cost of increased formal errors

On the use of water vapour radiometry for assessment of wet delay estimates from space geodetic techniques

During the development of the Mark III VLBI system in the seventies, water vapour radiometers (WVR) wereenvisaged to provide independent observations of the signal propagation delay due to water vapour along the lineof sight. The standard design of the WVR is to measure the atmospheric emission at two frequencies, close toand further away from the centre of the water vapour emission line at 22.2 GHz. These measurements are used toestimate two unknowns, the amount of water vapour, or the wet delay, and the amount of liquid water, along theline of sight. The main drawback of using a WVR is that the retrieval algorithm requires that any drops of liquidwater in the sensed volume of air are much smaller than the wavelength observed by the WVR, i.e. approximately1 cm. The algorithm therefore more or less breaks down during rain, meaning that the instrument cannot be reliedon for 100 % of time, unless it never rains on, or close to, the site. The method generally used to avoid usingWVR data with poor accuracy is to ignore observations obtained during rain and when the inferred liquid watercontent is above a specific threshold. However, there are a couple of difficulties with these procedures. (i) Theremay be rain drops in the sensed atmospheric volume in spite of the fact that no drops are detected at the groundon the site; (ii) there may still be drops of water on the WVR instrument, such as on the protective covers of thehorn antennas and the mirrors many minutes after the rain has stopped; (iii) a low density of large drops mayresult in a smaller liquid water content than many small drops.We have used WVR data from 2022 together, with rain observations, to study the retrieval accuracy bycomparing them to wet delay estimates from the GNSS station ONSA. We search for general rules of thumbsearching for periods when WVR and GNSS data offer the best agreement in the equivalent zenith wet delay,given the rain observations and the inferred liquid water content

Small scale atmospheric variations sensed with very short baseline interferometry (VSBI) and microwave radiometry

We have compared differential zenith wet delays, estimated between the 20 m telescope and the twin telescopes at Onsala, with linear horizontal gradients from a water vapour radiometer (WVR). The east and north gradients from the WVR are projected on to the baseline between the telescopes. The formal errors of the estimated differential zenith delays are comparable to the size of the estimated values.We obtain correlation coefficients for specific 24 h experiments in the range from 0 to 0.2, and the overall correlation is 0.1. Although the correlations are low, we use simulations to verify that they are in the expected range

Measurements of atmospheric water vapor with microwave radiometry

A dual-channel, ground-based microwave radiometer, working at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer, and radiosondes have been used for comparative measurements of the integrated amount of precipitable water vapor in the atmosphere over a period with zenith water vapor contents varying between 6 and 26 mm. The microwave radiometer was found to give comparable or better formal accuracy than the radiosondes, the absolute accuracy of which is believed to be about 1 mm. The rms difference of the integrated amount of water vapor in the zenith direction measured with the microwave radiometer and with radiosondes was 1.2 mm for all data, and 0.8 mm for a selected group of good weather data. These are lower formal errors than previously reported. It is shown that the simplified relation between the radiometer antenna termperatures and the integrated amount of water vapor in this case contributes with a formal error of about 0.3 mm. It is suggested that mean ground weather data can be used to adapt this relation to other sites and seasons

Diurnal variability of precipitable water in the Baltic region, impact on transmittance of the direct solar radiation

Diurnal variations in the Integrated Precipitable Water Vapour (IPWV) are studied from GPS observations acquired at 32 sites in the Baltic region during 1996-2005. The seasonal means for spring and summer show a diurnal sinusoidal pattern of the IPWV with the maximum value in the afternoon. The peak-to-peak (PtP) value of the average diurnal IPWV cycle was 0.5 mm in the spring and 0.6 mm in the summer. In the autumn and in the winter the diurnal variations in IPWV show no clear patterns and the average PtP values of the noise-like signal are only 0.2-0.3 mm. The diurnal IPWV cycle can only be estimated by averaging data from many years because the IPWV can show fast and large variations, reaching up to 5 mm/hour during several hours. These are explained exclusively by changes in the synoptic situation and substitution of airmasses above the location of observations; two case studies with analyses of the vertical humidity profiles are presented. The impact on the transmittance of the direct solar radiation is evaluated

Accuracy assessment of the two WVRs, Astrid and Konrad, at the Onsala Space Observatory

Two Water Vapour Radiometers (WVRs), Astrid and Konrad, have been operating at the Onsala Space Observatory during the time period 2013–2016. There are several data gaps due to different types of instrument failures and therefore we also use estimates of the equivalent zenith wet delay (ZWD) from the two GNSS reference stations: ONSA and ONS1. They providean almost continuous time series during the four years. ZWD root-mean-square differences are 0.38 cm between ONSA and ONS1, 0.92 cm between ONS1 and Astrid, and 0.75 cm between ONS1 and Konrad. For the horizontal linear gradients we see correlation coefficients of the order of 0.9 between ONSA and ONS1 and 0.5 between ONS1 and Konrad

Comparison of Atmospheric Gradients Estimated From Ground-Based GNSS Observations and Microwave Radiometry

Observations over four years from two nearby groundbasedGlobal Navigation Satellite System (GNSS) stationsand one microwave radiometer have been used toestimate linear horizontal gradients in the atmosphere.We find that gradients estimated by the radiometer havelarger amplitudes than those estimated using data fromthe Global Positioning System (GPS). One reason for thisis that they are estimated, every 15 min, independentlyof previous estimates, whereas the gradients from GPSare estimated every 5 min using constraints on their variability.We also find that the elevation cutoff angle has asignificant impact on the estimated GPS gradients. Decreasingthe cutoff angle results in smaller gradient amplitudes.The estimated gradients are not homogeneouslydistributed in all directions. When studying the largestgradients they all occur during the warmer period of theyear, beginning in April and ending in October. Specifically,for the 25 events with the largest gradient amplitudesfrom the GPS data, we find that the vast majority ofthem are associated with the passage of weather fronts