Diurnal cycle of short-term fluctuations of integrated water vapour above Switzerland

Abstract

Abstract. The TROpospheric WAter RAdiometer (TROWARA) continuously measures integrated water vapour (IWV) with a time resolution of 6 s at Bern in Switzerland. During summer, we often see that IWV has temporal fluctuations during daytime, while the nighttime data are without fluctuations. The data analysis is focused on the year 2010, where TROWARA has a good data quality without data gaps. We derive the spectrum of the IWV fluctuations in the period range from about 1 to 100 min. The FFT spectrum with a window size of 3 months leads to a serious underestimation of the spectral amplitudes of the fluctuations. Thus, we apply a band pass filtering method to derive the amplitudes as a function of period Tp. The amplitudes are proportional to Tp0.5. Another method is the calculation of the moving standard deviation with time window lengths from about 1 to 100 min. Here, we get similar results to those with the band pass filtering method. At all periods, the IWV fluctuations are strongest during summer, while they are smallest during winter. We derive the diurnal variation of the short-term IWV fluctuations by applying a moving standard deviation with a window length of 10 min. The daily cycle is strongest during the summer season, with standard deviations up to 0.22 mm at about 14:00 CET. The diurnal cycle disappears during wintertime. A similar seasonal behaviour is observed in the diurnal cycle of latent heat flux as provided by the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2 reanalysis) at Bern. Further, the 3 d averages of the latent heat flux and the magnitude of the short-term IWV variability show a strong correlation at Bern in 2010 (r=0.82 with a 95 % confidence interval from 0.75 to 0.87). Thus, we suggest that the diurnal cycle of short-term IWV fluctuations at Bern is mainly caused by large convective heating during daytime in summer.

Physical Retrieval of Rain Rate from Ground-Based Microwave Radiometry

“…R 2 and RMSE verifications of the Opa-RR estimation at 21 GHz are 0.43 (0.39) and 7.83 (6.75) mm/day, respectively. This is not surprising because the sensor at 31 GHz is less sensitive to water vapor than at 21 GHz [12]. Another finding worth noting in Figure 4 is that the daily rain rate using Opa-RR is significantly lower than rain gauges when rain rates are between 20 mm/day and 50 mm/day (heavy rain), since the linear regression fit line is located below the x = y line.…”

“…R 2 and RMSE verifications of the Opa-RR estimation at 21 GHz are 0.43 (0.39) and 7.83 (6.75) mm/day, respectively. This is not surprising because the sensor at 31 GHz is less sensitive to water vapor than at 21 GHz [12]. Figure 4 shows the verification scatter plots of the daily rain rate estimated by Opa-RR, and the comparison between ERA5 and rain gauges.…”

“…R 2 and RMSE verifications of the Opa-RR estimation at 21 GHz are 0.43 (0.39) and 7.83 (6.75) mm/day, respectively. This is not surprising because the sensor at 31 GHz is less sensitive to water vapor than at 21 GHz [12].…”

“…τ i is the zenith opacity. µ is the cosine of the zenith angle θ, i.e., µ = cos θ. T mean,i is the effective mean temperature of the atmosphere [12,13].…”

Trends of atmospheric water vapour in Switzerland from ground-based radiometry, FTIR and GNSS data

“…Furthermore, other factors than temperature might be responsible for IWV changes in winter, such as changes in dynamical patterns and the horizontal transport of humid air. Indeed, Hocke et al (2019) showed that evaporation of surface water plays a minor role in winter, with a latent heat flux that is in Bern six to seven times smaller than in summer, suggesting that horizontal transport of humid air is in winter more important than evaporation. We thus conclude that IWV in Bern changes as expected from temperature changes in early spring, late summer and autumn, but other processes might also be responsible for IWV changes, especially in winter.…”