Abstract
Cloud fraction (CF) is known as the dominant modulator of Earth’s radiative fluxes. Ground-based CF observations are useful to characterize the cloudiness of a specific site and are valuable for comparison with satellite observations and numerical models. We present for the first time CF statistics (relative to liquid clouds only) for Bern, Switzerland, derived from the observations of a ground-based microwave radiometer. CF is derived with a new method involving the analysis of the integrated liquid water distribution measured by the radiometer. The 10-year analyzed period (2004–2013) allowed us to compute a CF climatology for Bern, showing a maximum CF of 60.9% in winter and a minimum CF of 42.0% in summer. The CF monthly anomalies are identified with respect to the climatological mean values, and they are confirmed through MeteoSwiss yearly climatological bulletins. The CF monthly mean variations are similar to the observations taken at another Swiss location, Payerne, suggesting a large-scale correlation between different sites on the Swiss Plateau. A CF diurnal cycle is also computed, and large intraseasonal variations are found. The overall mean CF diurnal cycle, however, shows a typical sinusoidal cycle, with higher values in the morning and lower values in the afternoon.
Highlights
The importance of cloud fraction (CF) in the modulation of Earth’s radiative fluxes has been demonstrated both theoretically and experimentally
Several ground-based studies about CF have been conducted using observations at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site in Oklahoma, United States [2,3,5,6,7,8,9,10], a research facility characterized by a long series of measurements and the presence of multiple instruments [8]
We use the measurements provided by the Tropospheric Water Radiometer (TROWARA), a dual-channel microwave radiometer built by Peter and Kämpfer [19], capable of measuring vertically-integrated water vapor (IWV) and vertically-integrated cloud liquid water (ILW, known as liquid water path (LWP))
Summary
The importance of cloud fraction (CF) in the modulation of Earth’s radiative fluxes has been demonstrated both theoretically and experimentally. The Swiss Global Climate Observing System (GCOS) Office of the Federal Office of Meteorology and Climatology MeteoSwiss has published two studies comparing several years of CF observations from the Terra and Aqua satellites with CF observations at different Synop (synoptic) stations in Switzerland [15,16] The purpose of these studies was to improve and complement the long-term, high-quality ground-based observations of cloud properties in Switzerland through the use of satellite-based products. We present a 10-year CF climatology relative to liquid clouds for Bern, Switzerland, derived from the high-resolution observations (7–11 s) of a ground-based microwave radiometer for the period 2004–2013. This paper is organized as follows: in Section 2, we describe the instrument and the method used to derive the CF; in Section 3, we present the 10-year CF climatology, the CF monthly anomalies and the CF diurnal cycle; in Section 4, we summarize our findings and give an outlook on future work
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