A novel approach to fog/low stratus detection using Meteosat 8 data

A method is presented for fog and low stratus detection from daytime satellite imagery based on Meteosat 8 SEVIRI (Spinning-Enhanced Visible and Infra-Red Imager) data. With its excellent spatial, spectral and temporal resolutions, this imagery is an ideal basis for operational fog monitoring. The scheme utilizes a range of pixel-based and novel object-oriented techniques to separate fog and low stratus clouds from other cloud types. Fog and low stratus are identified by a number of tests which explicitly and implicitly address fog/low stratus spectral, spatial and microphysical properties. The scheme's performance is evaluated using ground-based measurements of cloud height over Europe. The algorithm is found to detect low clouds very accurately, with probabilities of detection (POD) ranging from 0.632 to 0.834 (for different inter-comparison approaches), and false alarm ratios (FAR) between 0.059 and 0.021. The retrieval of sub-pixel and temporal effects remain issues for further investigation.     

Temperature logs in deep wells—a useful tool for past climatic reconstruction

According to the theory of heat conduction in a semi-infinite body, temperature changes at the surface propagate into the subsurface with the amplitude attenuation and time delay that increase with depth. Temperature changes on the earth's surface, reflecting the past climatic history, can thus be evaluated by analysing the curvature they have caused in the present temperature-depth distribution. As a rule, temperature profiles to depths of 200–300 m record surface temperature trends accurately over the last two centuries or so; deeper holes may reveal climate history farther back but with decreasing resolution. We present several synthetic temperature-depth profiles to demonstrate the expected signature of past surface temperature changes in the subsurface, the analysis of which may help better identify the climate of the past. Examples of extracted climate recollections from holes in North America and Europe are discussed. While inconspicuous underground records may correspond to the postglacial warming 8–11 kyr ago, reasonably well-documented borehole logging data have confirmed climate excursions in the past millenium, namely the Little Climate Optimum and the Little Ice Age. Traces of recent warming are generally common in many temperature records, evidencing the temperature rise by 1–2 K over the past 100 years.     

Trends in Precipitation Variability: Prague (The Czech Republic)

Variability in precipitation at scales from 1 to 10 days was investigated with the use of the time series measured at Prague-Sporilov (the Czech Republic) between 1994 and 2001. Variability was detected by the method of absolute difference in precipitation between two adjacent discrete time periods. The results indicated a general increase in precipitation variability at all investigated scales within the 8-yr observational period. The variability patterns also showed quasi-seasonal variations. The summer wetter season proved to be the most variable. The analysis was accomplished by the investigations of precipitation variability at a monthly scale based on a century-long historical time record. On a longer time perspective, precipitation variability exhibits a general increase interrupted by quasi-decadal oscillations. The range of quasi-decadal variability has become more pronounced after about 1950, the fact that hints the possibility of further intensification of the hydrologic cycle. An obtained significant correlation between the North Atlantic Oscillation (NAO) activity and precipitation variability implies that the NAO may account for a large fraction of precipitation variability. Higher NAO-index values tend to be associated with low variability. The variability investigations may have a certain implication for climate change assessments both at the local scales as well as associated with the build-up of greenhouse gases.