Fog and Low Cloud Frequency and Properties from Active-Sensor Satellite Data

Jan Cermak

doi:10.3390/rs10081209

Abstract

An analysis of fog and low cloud properties and distribution is performed using satellite-based LiDAR. Recent years have seen great progress in the remote sensing of fog and low clouds using passive satellite-based sensors. On this basis, maps of fog distribution and frequency as well as baseline climatologies have been constructed. However, no information on fog altitude and vertical extent is available in this way, and fog/low cloud below other clouds cannot be detected in most cases. In this study, ten years of observations by the LiDAR aboard the CALIPSO (Cloud-Aerosol LiDAR and Pathfinder Satellite Observations) platform are used to construct a map and statistical evaluations of fog/low cloud distribution and properties. For the purpose of evaluation, a comparison is made to an evaluation of fog/low cloud distribution in Europe, derived from Meteosat measurements using the Satellite-Based Operation Fog Observation Scheme (SOFOS). Both maps show good agreement in spatial patterns in this region with very diverse fog formation mechanisms. It is found that fog/low cloud layers display distinct spatial differences in terms of geometrical thickness and detection accuracy. The number of fog/low cloud instances missed by passive-sensor retrievals due to multi-layer cloud situations is considerable, with clear regional differences.

Highlights

Fog and low clouds (FLC) are phenomena that directly and indirectly impact on several human and natural systems: Air and ground-based traffic safety and volume are reduced, with economic impacts, and water is made available to a range of environments
While the Meteosat data set considers the situation in individual pixels, the Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations (CALIPSO) data set presents an aggregation in larger grid cells that do not have to be entirely covered by FLC to be counted
FLC patches are well represented in the CALIPSO data set, e.g., in Northern Spain; in other places local detail is lost e.g., in the Po valley of Northern Italy

Summary

Introduction

Fog and low clouds (FLC) are phenomena that directly and indirectly impact on several human and natural systems: Air and ground-based traffic safety and volume are reduced, with economic impacts, and water is made available to a range of environments. Bendix et al [1] produced a map of FLC frequency in Germany and surrounding regions based on data from the Advanced Very-High Resolution Radiometer (AVHRR) While this and similar maps include a large amount of spatial detail, they are not representative for the entire day, given that AVHRR is mounted on polar-orbiting satellite platforms with infrequent spatial coverage of any given region. No climatological evaluation of FLC presence and properties has been performed based on CALIPSO data despite the obvious potential for expanding present-day knowledge of FLC. On this basis, the following research questions are identified for the present study:. (1) Can CALIPSO data be used to plausibly represent FLC distribution? (2) Are there distinct differences in FLC properties between FLC regimes? (3) How frequent are FLC situations below other clouds?

Data and Methods

Results and Discussion

Conclusions and Outlook