Laboratory for
Climatology and Remote Sensing

Abstract:

Knowledge of the spatio-temporal precipitation distribution is of great value in agriculture, water engineering, climatology and risk management. So far, no adequate method existed for the detection and monitoring of precipitation at high temporal and spatial resolutions in most parts of the world where radar networks are not available. Due to spectral constraints, existing retrieval techniques rely on a relationship between rainfall probability and intensity and the cloud top temperature measured in an infrared channel. These techniques show considerable drawbacks concerning precipitation processes in the mid-latitudes. Improved techniques for rain area identification based on spectral enhancements of new generation satellite systems used to be only available on polar orbiting platforms with poor temporal resolutions. Furthermore, these algorithms are only applicable during day-time. With the advent of Meteosat Second Generation (MSG) Spinning-Enhanced Visible and InfraRed Imager (SEVIRI) in 2004, a geostationary satellite system with significantly improved spectral and spatial resolutions has become available. The central aim of the present study therefore was to develop a novel method for operational precipitation detection during day- and night-time based on MSG SEVIRI data. The focus of the newly developed scheme lies on precipitation processes in the mid-latitudes in connection with extra-tropical cyclones. It is therefore not only applicable to convectively dominated rain areas but also to precipitating cloud areas of advective-stratiform character. The newly developed rainfall retrieval scheme based on the advanced second-generation GEO system MSG SEVIRI rests upon the following conceptual model: • Precipitating cloud areas are characterized by a sufficiently high cloud water path and ice particles in the upper part. • Cloud areas with higher rainfall intensities are characterized by a higher cloud water path and a higher amount of ice particles in the upper part. • Convective clouds with very high rainfall intensities are characterized by a large vertical extension and a high rising cold cloud top. Based on this conceptual design, the new retrieval scheme consists of an entirely new methodology compiling novel and innovative algorithms and approaches. The following three components are the focal parts of the novel technique: • A new algorithm for the identification of the rain area during day- and night-time was developed for SEVIRI. The method allows not only a proper detection of mainly convective rain areas but also enables the detection of advective-stratiform precipitation (e.g. in connection with mid-latitude frontal systems). It is based on information about the CWP and the cloud phase in the upper cloud regions. • An infrared retrieval technique appropriate for convective precipitation processes in the mid-latitudes was successfully transferred and adapted to MSG SEVIRI. The phenomenon of positive brightness temperature differences between the WV and IR channels (dTWV-IR), which enables the detection and classification of convectively dominated raining cloud areas was investigated for the WV and IR channels of SEVIRI. Based on radiative transfer calculations, which revealed the existence of positive ΔTWV-IR for all SEVIRI WV-IR differences, the dTWV technique could be applied and transferred to SEVIRI. • A new technique for precipitation process and rainfall intensity separation was developed for SEVIRI. The process separation and the further subdivision relies on information about the cloud top height, the cloud water path and the cloud phase in the upper parts. The subdivision is realized in a stepwise manner. In a first step the rain area is separated into the subareas of convective and advective-stratiform precipitation processes. In the following both separated process areas are divided into subareas of differing rainfall intensities. The process separation and the subdivision of the convective precipitation area relies on information about the cloud top height. The subdivision of the advective-stratiform precipitation area is based on information about the CWP and the particle phase in the upper parts of the cloud. The rain area and the process-oriented rainfall intensities detected and classified by the newly developed retrieval technique were validated against corresponding ground-based radar data of Germany, representative for mid-latitude precipitation processes. The results of the validation study indicate persuading performance of the new algorithm concerning rain area identification as well as process and intensity differentiation and indicate the stability of the introduced conceptual design.

Abstract:

Automated detection of fog and low stratus in nighttime satellite data has been implemented on the basis of numerous satellite systems in past decades. Commonly, differences in small-droplet emissivities at 11?m and 3.9?m are utilized. With Meteosat SEVIRI, however, this method cannot be applied with a fixed threshold due to instrument design: The 3.9?m band is exceptionally wide and overlaps with the 4?m CO2 absorption band. Therefore, the emissivity difference varies with the length of the slant atmospheric column between sensor and object. To account for this effect, the new technique presented in this paper is based on the dynamical extraction of emissivity difference thresholds for different satellite viewing zenith angles. In this way, varying concentrations of CO2 and column depths are accounted for. The new scheme is exemplified in a plausibility study and shown to provide reliable results.

Abstract:

An evaluation of precipitation fields for four selected months simulated by the regional climate model and provided by the satellite retrieval method is presented. As reference, observations at 5 km resolution on a daily and monthly basis are used. We applied conventional verification tools (root mean square error, grid-point based categorical error scores, etc.) as well as the new error score SAL, which separately considers aspects of the structure, amplitude and location of the precipitation field in a predefined area. We also discussed the advantages and disadvantages of each of the scores. The aim of our evaluation was to unfold the strengths and weaknesses of and to calculate daily and monthly high resolution precipitation. As a result we found that the catchment averaged monthly mean precipitation is simulated with an acceptable accuracy by both methods. The spatial pattern of the monthly precipitation (typically with a precipitation maximum in the alpine foreland) can only be reproduced by . Regarding the daily precipitation, our evaluation revealed that both methods still need improvement. The deviations to the observations increase with decreasing precipitation amount resulting in large uncertainties in case of very dry conditions. Overall, we can conclude that is better suited to simulate precipitation at 5 km resolution on a daily basis than .

Abstract:

The scientific community that includes meteorologists, physical scientists, engineers, medical doctors, biologists, and environmentalists has shown interest in a better understanding of fog for years because of its effects on, directly or indirectly, the daily life of human beings. The total economic losses associated with the impact of the presence of fog on aviation, marine and land transportation can be comparable to those of tornadoes or, in some cases, winter storms and hurricanes. The number of articles including the word ‘‘fog’’ in Journals of American Meteorological Society alone was found to be about 4700, indicating that there is substantial interest in this subject. In spite of this extensive body of work, our ability to accurately forecast/nowcast fog remains limited due to our incomplete understanding of the fog processes over various time and space scales. Fog processes involve droplet microphysics, aerosol chemistry, radiation, turbulence, large/small-scale dynamics, and surface conditions (e.g., partaining to the presence of ice, snow, liquid, plants, and various types of soil). This review paper summarizes past achievements related to the understanding of fog formation, development and decay, and in this respect, the analysis of observations and the development of forecasting models and remote sensing methods are discussed in detail. Finally, future perspectives for fog-related research are highlighted.

Abstract:

A scheme for the detection of fog and low stratus over land during daytime based on data of the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument is presented. The method is based on an initial threshold test procedure in the MODIS solar bands 1–7 (0.62–2.155µm). Fog and low stratus detection generally relies on the definition of minimum and maximum fog and low stratus properties, which are converted to spectral thresholds by means of radiative transfer calculations (RTC). Extended sensitivity studies reveal that thresholds mainly depend on the solar zenith angle and, hence, illumination-dependent threshold functions are developed. Areas covered by snow, ice and mid-/high-level clouds as well as bright/hazy land surfaces are omitted from the initial classification result by means of a subsequent cloud-top height test based on MODIS IR band 31 (at 12 µm) and a NIR/VIS ratio test. The validation of the final fog and low stratus mask generally shows a satisfactory performance of the scheme. Validation problems occur due to the late overpass time of the TERRA platform and the time lag between SYNOP and satellite observations. Apparent misclassifications are mainly found at the edge of the fog layers, probably due to over- or underestimation of fog and low stratus cover in the transition zone from fog to haze.

Abstract:

Abstract Flowering and fruiting as phenological events of 12 tree species in an evergreen tropical mountain rain forest in southern Ecuador were examined over a period of 3– 4 years. Leaf shedding of two species was observed for 12 months. Parallel to the phenological recordings, meteorological parameters were monitored in detail and related to the flowering and fruiting activity of the trees. In spite of the perhumid climate of that area, a high degree of intra- and inter-specific synchronisation of phenological traits was apparent. With the exception of one species that flowered more or less continuously, two groups of trees could be observed, one of which flowered during the less humid months (September to October) while the second group started to initiate flowers towards the end of that phase and flowered during the heavy rains (April to July). As reflected by correlation coefficients, the all-time series of meteorological parameters showed a distinct seasonality of 8–12 months, apparently following the quasi-periodic oscillation of precipitation and related cloudiness. As revealed by power spectrum analysis and Markov persistence, rainfall and minimum temperature appear to be the only parameters with a periodicity free of long-term variations. The phenological events of most of the plant species showed a similar periodicity of 8–12 months, which followed the annual oscillation of relatively less and more humid periods and thus was in phase or in counterphase with the oscillations of the meteorological parameters. Periods of unusual cold or dryness, presumably resulting from underlying longer-term trends or oscillations (such as ENSO), affected the homogeneity of quasi-12- month flowering events, fruit maturation and also the production of germinable seeds. Some species show underlying quasi-2-year-oscillations, for example that synchronise with the development of air temperature; others reveal an underlying decrease or increase in flowering activity over the observation period, influenced for instance by solar irradiance. As Ecuador suffers the highest rate of deforestation in South America, there is an urgent need for indigenous plant material for reforestation. A detailed knowledge of the biology of reproduction in relation to governing external factors (mainly climate) is thus required.

Abstract:

The diurnal precipitation dynamics in an east-west-oriented valley that connects the Amazon lowlands and the inter-Andean basin of southern Ecuador (Rio San Francisco valley) is investigated by means of a K-band rain-radar profiler (located at the ECSF research station, latitude: 3° 58'S, longitude: 79° 4?W) and additional remotely sensed data. A pre-dawn/dawn (5:30–6:30 LST) maximum of rainfall is found and a secondary peak is observed after noon (14:30–15:30 LST). Although the frequency distribution of rain rates reveals that a great portion of rainfall is of stratiform character, vertical profiles of rain rate and droplet concentration points to the important contribution of embedded convection and/or showers produced by local heating for the overall amount of rainfall. Specific differences in stratification and process dynamics could be found for both peak times. The pre-dawn maximum can be related to mesoscale instabilities over the Peruvian Amazon close to the south Ecuadorian border. Extended cold air drainage flow from the Andes and low-level confluence due to the concavity of the Andean chain in this area leads to convective instability in the nocturnal Amazonian boundary layer, which is extended to the study area by the predominant easterlies in the mid-troposphere. Rain clouds with at least embedded shallow convection can overflow the bordering ridges of the San Francisco valley providing rains of higher intensity at the ECSF research station. On the contrary, the afternoon convective precipitation can be caused by locally induced thermal convection at the bordering slopes (up-slope breeze system) where the ECSF station profits from precipitation off the edge of these local cells due to the narrow valley.