Detection of Fog and Low Cloud Boundaries with Ground-Based Remote Sensing Systems

Abstract

The performance of the boundary determination of fog and low stratiform cloud layers with data from a frequency-modulated continuous-wave (FMCW) cloud radar and a Vaisala ceilometer is assessed. During wintertime stable episodes, fog and low stratiform cloud layers often occur in the Swiss Plateau, where the aerological station of Payerne, Switzerland, is located. During the international COST 720 Temperature, Humidity, and Cloud (TUC) profiling experiment in winter 2003/04, both a cloud radar and a ceilometer were operated in parallel, among other profiling instruments. Human eye observations (“synops”) and temperature and humidity profiles from radiosoundings were used as reference for the validation. In addition, two case studies were chosen to demonstrate the possibilities and limitations of such ground-based remote sensing systems in determining low clouds. In these case studies the cloud boundaries determined by ceilometer and cloud radar were furthermore compared with wind profiler signal-to-noise ratio time series. Under dry conditions, cloud-base and -top detection was possible in 59% and 69% of the cases for low stratus clouds and fog situations, respectively. When cases with any form of precipitation were included, performances were reduced with detection rates of 41% and 63%, respectively. The combination of ceilometer and cloud radar has the potential for providing the base and top of a cloud layer with optimal efficiency in the continuous operational mode. The cloud-top height determination by the cloud radar was compared with cloud-top heights detected using radiosounding humidity profiles. The average height difference between the radiosounding and cloud radar determination of the cloud upper boundary is 53 ± 32 m.

Simulating Z–LWC Relations in Natural Fogs with Radiative Transfer Calculations for Future Application to a Cloud Radar Profiler

“…Especially cloud radars that rely on the frequency modulated continuous wave technique (FMCW) are highly suitable for monitoring fog and low stratus layers because of their small near field. They enable the detection of clouds as low as about 30 m (CERMAK et al 2006;NOWAK et al 2008;BENNETT et al 2009a), which contrasts starkly with existing pulse cloud radars with larger near fields reaching up to about 500 m (e.g. LÖ HNERT et al 2001).…”

Behavior of cloud base height from ceilometer measurements

“…In addition, deficits in spatial (i.e. lack of data of some parts of the World) and temporal (i.e., soundings are launched two 20 times per day) hinder the broad use of sounding vertical profiles for climatic studies of cloud vertical structure (Poore et al 1995;Nowak, 2008). Figure 4 shows the retrieved monthly distributions of CBH for single layer cases, for some representative months (January, April, July and October, i.e.…”

Regional high‐resolution cloud climatology based on MODIS cloud detection data

“…cloud occurrence over a given location). Techniques used for their detection and parameterization range from simple manual observations (Warren et al, 2007), through digital photography (Long et al, 2006;Souza-Echer et al, 2006;Heinle et al, 2010), radars (Clothiaux et al, 1995;Danne et al, 1999), lidars (Nowak et al, 2008;Zhao et al, 2014), and radiometers onboard satellites orbiting the Earth (Stubenrauch et al, 1999). For an extensive review of cloud detection concepts and their accuracy see Tapakis and Charalambides (2013).…”