Observed Cloud Cover Research Articles

Drought and Associated Cloud Fields over the Canadian Prairie Provinces

Little is known about clouds during drought. From 1999 to 2005 the Canadian Prairies experienced one of the most severe and prolonged droughts in the historical record. This study characterizes clouds during drought in the Canadian Prairie provinces with a particular focus on this recent drought. Drought severity was determined using the Standardized Precipitation Index (SPI) based on monthly precipitation on a 1° × 1° grid. Cloud fields from the National Aeronautics and Space Administration/Global Energy and Water Experiment's (NASA/GEWEX) Surface Radiation Budget database were used to examine overall cloud amount, optical thickness, and top-of-the-atmosphere albedo. Anomalies in monthly precipitation in the satellite record from 1984 to 2004, with an emphasis on the recent drought from 1999 to 2004, were related to anomalies in cloud fields. During drought, a decrease in cloud amount was observed. During the spring and summer months of the 1999–2004 drought, for example, the observed cloud cover fractio...

A Methodological Approach for the Definition of the Urban Heat Island in Rome and Possible Use for the Evaluation of the Impact of Heat waves on Mortality

ISEE-0666 Background: High temperatures have an impact on health however, few studies have addressed the heterogeneity in the impact within urban areas. Ambient temperatures vary considerably within cities and this phenomenon is typically known as urban heat island (UHI). The objective of the present study was to define the UHI in Rome using satellite data and ambient temperatures during summer. The UHI indicator can be used to assess the spatial variation in the impact of heat on health within the urban area of Rome. Methods: AATSR satellite data for the period 2003–2006 were used to define the UHI in Rome with a spatial resolution of 1 km × 1 km. The urban area of Rome covers about 300km2. Cloud-free days were derived combining satellite data and cloud cover observations. A regression model was developed to derive air temperatures from land surface temperatures (LST) for all the urban area. The UHI intensity was calculated as difference between rural and urban temperature values during the evening. Results: The UHI phenomenon identified in Rome was similar in all the 60 cloud-free days considered, suggesting the robustness of our indicator. The warmest areas were concentrated around the urban core extending to the east of the city, with LST values on average +2–3°C higher than the rural area. The UHI indicator accurately discriminated the cooler vegetated areas within the city. This indicator was used to analyze the differential impact of heat on mortality during heat waves in Rome for the period 2003–2006. Mortality and socio-economic status data, by census tract of residence, are considered to indentify “hotspot” which are more at risk during heat waves due to the UHI effect. Air pollution data and dispersion models were also considered. Conclusion: Results from this analysis can be used in public health to intensify prevention measures in at risk areas during heat waves.

Trends in aerosol radiative effects over China and Japan inferred from observed cloud cover, solar “dimming,” and solar “brightening”

This study examines multidecadal changes in surface downward shortwave (SW) radiation flux, total cloud cover, SW cloud effect, and related parameters over China and Japan during 1960–2004 using monthly gridded data from the Global Energy Balance Archive, synoptic cloud reports, and the International Satellite Cloud Climatology Project. We use the concept of cloud cover radiative effect, defined as the change in downward SW flux produced by a change in cloud cover, to quantify and remove the impact of cloud cover anomalies on surface solar flux. This will allow radiative effects of long‐term changes in anthropogenic aerosol to be more clearly distinguished from natural weather and climate variability. As fit by a linear trend between 1971 and 1989, surface solar flux decreased by a statistically significant 11 W m−2 per decade over China and decreased by a nonsignificant 1 W m−2 per decade over Japan. The small decline over Japan can be entirely explained by an increase in cloud cover, but changes in cloud cover made negligible contribution to the 1971–1989 solar flux trend over China. Between 1990 and 2002, surface solar flux increased by a statistically significant 8 W m−2 per decade over Japan and increased by a nonsignificant 4 W m−2 per decade over China. Half of the 1990–2002 solar flux trend over China and one third of the trend over Japan can be attributed to a reduction in cloud cover. The 1971–1989 decrease in surface solar flux over China and the 1990–2002 increase in surface solar flux over Japan are both spatially widespread and exhibit consistent sign across seasons.

Total and partial cloud amount detection during summer 2005 at Westerland (Sylt, Germany)

Abstract. The detection of cloudiness is investigated by means of partial and total cloud amount estimations from pyrgeometer radiation measurements and visible all-sky imager observations. The measurements have been performed in Westerland, a seaside resort on the North Sea island of Sylt, Germany, during summer 2005. An improvement to previous studies on this subject resulting in the first time partial cloud amounts (PCAs), defined as cloud amounts without high clouds calculated from longwave downward radiation (LDR) according to the APCADA algorithm (Dürr and Philipona, 2004), are validated against both human observations from the National Meteorological Servive DWD at the nearby airport of Sylt and digital all-sky imaging. The aim is to establish the APCADA scheme at a coastal midlatitude site for longterm observations of cloud cover and to quantify errors resulting from the different methods of detecting cloudiness. Differences between the resulting total cloud amounts (TCAs), defined as cloud amount for all-cloud situations, derived from the camera images and from human observations are within ±1 octa in 72% and within ±2 octa in 85% of the cases. Compared to human observations, PCA measurements, according to APCADA, underestimate the observed cloud cover in 47% of all cases and the differences are within ±1 octa in 60% and ±2 octa in 74% of all cases. Since high cirrus clouds can not be derived from LDR, separate comparisons for all cases without high clouds have been performed showing an agreement within ±1(2) octa in 73(90)% for PCA and also for camera-derived TCA. For this coastal mid-latitude site under investigation, we find similar though slightly smaller agreements to human observations as reported by Dürr and Philipona (2004). Though limited to daytime, the cloud cover retrievals from the sky imager are not really affected by cirrus clouds and provide a more reliable cloud climatology for all-cloud conditions than APCADA.

Estimation of solar radiation and its application to crop simulation models in Greece

Lack of data on site-specific daily solar radiation (Rg) is a significant impediment for most crop modeling applications. For this reason, 5 methods for estimating Rg were tested: the Angstrom-Prescott equation (AP), the Supit - van Kappel formula (SK) and 3 temperature-based methods: Campbell-Donatelli model, Hargreaves equation, and piecewise multiple linear regres- sions with a breakpoint (PLR). To overcome the lack of long and continuous time series of Rg mea- surements at multiple sites, satellite-derived Rg from the HelioClim-1 database were tested against observations from 2 stations, and then interpolated for 12 additional stations. When sunshine duration data were available, the AP equation was best, because it (1) produced intercepts and slopes closest to zero and unity, respectively and (2) had the lowest relative RMSE (9 to 18.6%). When cloud cover observations and data on maximum and minimum temperatures were available, the SK equation was equally effective as AP in most assessment metrics. When the only data for a site were daily maxi- mum and minimum temperatures, the PLR approach with a breakpoint, which reflects the value at which the response of daily Rg changes as a function of the extraterrestrial solar radiation RA and the diurnal temperature range, performed best. The mean relative RMSE of the PLR approach was <3.7% higher than that of AP. The SK equation provided the most suitable simulation of measured Rg for the CERES-Wheat crop model, while among the temperature-based methods PLR produced the smallest yield errors. Future validation efforts should explore the validity of the PLR model in other regions and under regimes with greater availability of Rg data

The EU‐CLOUDMAP project: Cirrus and contrail cloud‐top maps from satellites for weather forecasting climate change analysis

The EU‐CLOUDMAP project took place between 1997–2000 as a collaboration between five university and government research groups in the UK, Germany, the Netherlands and Switzerland. The original scientific motivation of the EU‐CLOUDMAP Project was to improve the measurement and characterization of cirrus and contrail cloud properties. IPCC (Penner et al.1999) demonstrated that contrail clouds could play a small but significant role in changing the radiative balance of the atmosphere based on work from one of the CLOUDMAP partners (DLR) using AVHRR data over Europe. However, the scope was broadened to include properties of clouds at all altitudes as (Cess et al.1993) had shown that depending on how cloud processes are parameterised can lead to an order of magnitude difference in predictions of surface temperature due to changes in CO2 radiative forcing. This error is by far the largest uncertainty in making accurate forecasts of global warming. The primary technological motivation of the Project was to develop new techniques for deriving cloud‐top properties (cloud‐top height, amount, microphysics and winds) from a new series of meteorological sensors based on the use of either cloud‐top stereo (ATSR‐2 and MISR) or Oxygen A‐band (MOS) and their application to the generation of new cloud climatology products. A secondary goal was to develop an automated technique, based on fuzzy logic, to detect contrails in non‐thermal imagery where contrails can only be detected through their unique spatial characteristics. Validation of cloud products was perceived as a crucial central issue to any adoption of these products by the Numerical Weather Prediction (NWP) and climate forecasting communities. This presents unique challenges as validation must be conducted simultaneously with satellite data acquisition. Ground‐based remote sensing techniques exploiting continuously operating radars and lidars were used and a new technique based on visible stereo digital cameras for retrieving cloud‐base properties was pioneered. A technology spin‐off from the CLOUDMAP work at UCL was the development of an uncooled thermal IR fish‐eye lens camera for continuously monitoring cloud cover from the ground to replace existing manual observations of cloud cover (Chapman et al. 2007). Eight papers in this issue show some of the highlights of the project including fundamental aspects of the algorithms for retrieval of cloud‐top height using stereo photogrammetry from ATSR‐2 (Muller et al. 2007, Denis et al. 2007) and MOMS (Drescher 2007) and Oxygen A‐band from MOS (Preusker et al. 2007). Seiz et al. (2007) describe the development of a novel technique for the automated retrieval of cloud‐base height from stereo visible digital cameras. Naud et al. (2007) describe the validation of cloud‐top heights from ATSR‐2 and MOS using ground‐based radar and lidar. Hetzheim (2007) shows how fuzzy logic systems can be employed for contrail detection when thermal imagery is not available using MOS and ATSR‐2 to demonstrate his techniques. Finally, Meyer et al. (2007) show results from applying the thermal IR contrail detection technique to AVHRR over Asia.

Trends in aerosol radiative effects over Europe inferred from observed cloud cover, solar “dimming,” and solar “brightening”

We examine multidecadal changes in surface downward shortwave (SW) radiation flux, total cloud cover, SW cloud effect, and related parameters over Europe during 1965–2004 using monthly gridded data from the Global Energy Balance Archive (GEBA), synoptic cloud reports, and the International Satellite Cloud Climatology Project (ISCCP). One key issue is distinguishing the effects of natural cloud variability from long‐term anthropogenic aerosol influences on surface SW flux. Accordingly, we introduce the concept of cloud cover radiative effect (CCRE), defined as the change in downward SW flux produced by a change in cloud cover. The correlation between pan‐European time series of CCRE anomalies and GEBA solar radiation anomalies is 0.88, indicating that cloud cover variability and associated changes in cloud albedo dominate SW radiation variability on monthly to decadal timescales. After these weather‐related cloud effects are removed by subtracting CCRE anomalies from GEBA solar radiation anomalies via linear regression, a distinct decreasing trend followed by a distinct increasing trend remain in the residual time series. Depending on the method of trend calculation, pan‐European residual flux declined by a statistically significant 2.7–3.5 W m−2 per decade during 1971–1986 and rose by a statistically significant 2.0–2.3 W m−2 per decade during 1987–2002. The fact that independent grid boxes exhibit mostly negative trends in the earlier period and mostly positive trends in the later period demonstrates that these long‐term variations in SW flux are real and widespread over Europe. Changes in cloud cover cannot account for the trends in surface SW flux since cloud cover actually slightly decreased during 1971–1986 and slightly increased during 1987–2002. The most likely explanation is changes in anthropogenic aerosol emissions that led to more scattering and absorption of SW radiation during the earlier period of solar “dimming” and less scattering and absorption during the later period of solar “brightening.”

Validation of an 8-14 μm cloud monitor using visual observations of antarctic cloud cover

A cloud monitor instrument, sensing 8-14 microm infrared radiation, has been installed near McMurdo, Antarctica, in order to provide supporting cloud cover information for a high-resolution sky-viewing Fabry-Perot spectrometer experiment. Validation of the cloud monitor observations was performed by a 1 year long comparison with the available 6 h visual cloud observations, expressed in octas (obscured eighths of the sky). The agreement between visual observations and those of the cloud monitor is good--the variation being slightly over one step, or octa--except at the clear sky extreme, which the subjective visual observations tend to overestimate during dark and moonless conditions. Both visual observations and the cloud monitor measurements show an austral winter-summer difference with the winter being clearer.

Assessing land-surface-atmosphere coupling in the ERA-40 reanalysis with boreal forest data

This analysis uses long time-series of daily mean data from the three BERMS flux sites in central Saskatchewan to explore biases in ERA-40 at a close-by grid-point, and to study the relationships between surface variables and fluxes and cloud cover in the observed and model data sets. On the seasonal timescale the biases in ERA-40 of temperature and humidity are small, but the model has a high bias in evaporation, and except in mid-summer a low bias of reflective cloud, which gives a high bias in the surface downward net shortwave flux. In summer, on days with high observed cloud cover, the model has too little cloud, and a corresponding high sensible heat flux and a warm, dry bias; while on summer days when observed cloud is lower, the model biases are generally reversed. The internal relationships between near-surface relative humidity, linked to the mean lifting condensation level, cloud cover and the surface radiation fluxes are however very similar in model and data. Although cause and effect cannot be determined from these biases alone, given the complexity of the interactions, they suggest possible errors in both the land-surface model and the atmospheric cloud parameterizations.

Dry season clouds and rainfall in northern Central America: Implications for the Mesoamerican Biological Corridor

The proposed Mesoamerican Biological Corridor (MBC) is an ambitious effort to stem the erosion of biodiversity in one of the world's biologically richest regions. The intent is to connect large existing parks and reserves with new protected areas by means of an extensive network of biological corridors within Mesoamerica/Central America to create an environment which provides better prospects for the long-term survival of native species while also addressing the region's socioeconomic needs. While the forest types in northern Central America generally receive some dry season rainfall, in the proposed protected regions, however, it is unclear whether current rainfall has been altered by regional land-use change. Based upon climatological rainfall records at 266 stations in Guatemala and adjacent areas, dry season rainfall in March is markedly lower in deforested areas than in forested areas of the same life zone for each of the widespread life zones. In general, dry season deforested habitats have higher daytime temperatures, are less cloudy, have lower estimated soil moisture and lower values of normalized difference vegetation index (NDVI) than do forested habitats in the same life zone. The result is hotter and drier air over deforested regions, with lower values of cloud formation and precipitation. Rainfall is predicted from the correlation of raingauge measurements and observed cloud cover; moreover, March rainfall deficiencies > 25 mm are found for several Holdridge life zones. The data suggest that deforestation is locally intensifying the dry season, increasing the risk of fire, especially for the long corridor connecting regions. In addition, forest regeneration in some parts of the MBC may not result in second-growth forest that is characteristic of that life zone but rather in forest regeneration more typical of drier conditions. The extent to which this would influence the conservation utility of any given corridor depends upon the ecological requirements of the organisms concerned.

Cloud Parameterizations in SUNYA Regional Climate Model for the East Asia Summer Monsoon Simulations

East Asia summer monsoon (EASM) simulations are conducted to evaluate three schemes which determine cloud properties used in the radiation calculation of the State University of New York at Albany (SUNYA) regional climate model (RCM). Scheme-Ⅰ uses diagnostic cloud cover and cloud water while Scheme-Ⅱ uses prognostic cloud water along with overcast sky; both schemes are commonly employed in RCMs. In Scheme-Ⅲ, cloud cover is determined by diagnostic formula, but the cloud water is calculated by the weighted means of its diagnosed and prognosed values. Therefore, Scheme-Ⅲ considers the subgrid-scale clouds as Scheme-Ⅰ and maintains consistent cloud properties in radiation calculation and microphysical processes as Scheme-Ⅱ. Cloud radiative forcing (CRF) which provides a quantification of the cloud-radiation-climate interaction is adopted to compare the three schemes in simulating the 1991 EASM, characterized by large amounts of cloud and persistent rainfall over Yangtze-Huai River valley. With these three cloud schemes, the SUNYA RCM is capable of simulating the intra-seasonal variations of observed cloud cover and longwave CRF. The transition of shortwave CRF is not properly simulated due to the constantly presented low-level clouds. Mostly the magnitude of CRF is overestimated by 13-22 W m^(-2) for shortwave CRF and by 12-16 W m^(-2) for longwave CRF. It is also found that the surface temperature biases are highly correlated (with correlation coefficient greater than 0.8) to the shortwave CRF biases. Therefore, Scheme-Ⅲ resulting in less low-level cloud water and the least shortwave CRF biases simulates surface temperatures in better agreement with observations. Analyses of surface energy balance components indicate that the CRF changes dominate the surface temperature responses and the consequent surface latent heat and sensible heat flux feedbacks significantly offset them. Finally, comparisons of the diurnal variations of simulated cloud water among the three schemes show that Scheme-Ⅲ provides consistency between cloud microphysics and radiation.

Baseline cloudiness trends in Canada 1953–2002

Baseline cloudiness trends in Canada for the 1953 to 2002 time period were assessed using raw hourly observations of total cloud cover from eighty‐five airport stations with at least forty years of data. Trends in the count of the number of hours when more than half of the sky dome was observed to be covered by clouds (Mainly Cloudy conditions) increased significantly by several hundreds of hours during the fifty‐year period at numerous stations along the Canada‐U.S border and the few stations located in the Mackenzie Basin. Trends in average annual overall, daytime, night‐time, and seasonal total cloud cover were also computed. The general spatial pattern was similar as in the case of Mainly Cloudy conditions, however, the exact areal extent and significance of the trends would vary from one case to another. In particular, the area of negative trends in British Columbia and the Prairies, gained in significance and extent in the case of daytime cloudiness, while it reversed to weak, mostly positive, trends in the case of night‐time cloudiness. This could correspond to the unprecedented increases in both daily temperature maximum and minimum in these locations during the second half of twentieth century. Significant positive seasonal trends were seen at some stations in southern Canada during spring (coasts) and summer (centre of the country) and the Mackenzie Basin during summer and fall; while significant negative trends were seen at stations in northern Alberta and British Columbia during winter and north‐eastern Canada during summer. The quality of the observations was given careful consideration and potential issues that could affect the continuity of the records of observed total cloud amount are discussed at length. The complete monitoring system was examined: training, affiliation and workload of the observers; automation of the observations in the 1990s by the introduction of the Automated Weather Observing Systems (AWOS); numerous changes in data archiving procedures especially prevalent in the 1990s; underestimation of night‐time cloudiness on nights with insufficient illumination, etc. It was determined that observations, although to a certain degree subjective and not particularly accurate for cloud amounts near 5/10, should be reasonably consistent in the case of human observers (no systematic biases); underestimation of night‐time cloudiness, although relevant if the actual amount is required for certain purposes, probably is not critical in studies concerning cloud trends. Automation, possibly along with the changes in data processing in the 1990s, unequivocally emerged as the least studied subject, however, it is likely to have a considerable impact on the continuity of cloud observations at some stations; in this study about 25% of the stations use AWOS in some way.

Increasing Evapotranspiration from the Conterminous United States

Abstract Recent research suggests that evapotranspiration (ET) rates have changed over the past 50 years; however, some studies conclude ET has increased, and others conclude that it has decreased. These studies were indirect, using long-term observations of air temperature, cloud cover, and pan evaporation as indices of potential and actual ET. This study considers the hydrological cycle more directly and uses published precipitation and stream discharge data for several large basins across the conterminous United States to show that ET rates have increased over the past 50 years. These results suggest that alternative explanations should be considered for environmental changes that previously have been interpreted in terms of decreasing large-scale ET rates.

Long‐term (1936–2003) ultraviolet and photosynthetically active radiation doses at a north Norwegian location in spring on the basis of total ozone and cloud cover

We have reconstructed a homogeneous and consistent 68‐year time series of ultraviolet (UVR) and photosynthetically active radiation (PAR) doses for the spring period at a marine location in northern Norway. The time series is simulated mainly on the basis of the Tromsø total ozone series and routine meteorological observations of cloud cover. All other relevant input parameters were fixed at realistic values determined by local measurements and climatology. Our approach is applicable to a wide range of locations worldwide where routine ozone measurements and cloud observations are done. For the whole period 1936–2003, we find trends of +4.5%, +2.8% and +1.3% per decade for the monthly UVB doses in March, April and May, respectively. UVA and PAR doses, which are not affected by total ozone, reveal smaller trends (+1–2% per decade) in March and April, while the May trend is comparable to UVB. At shorter term, however, the radiation trends were very variable.

Objective method for classifying air masses: an application to the analysis of Buenos Aires' (Argentina) urban heat island intensity

¶During recent years, numerous studies have examined the Buenos Aires urban climate, but the relationship between large-scale weather conditions and the Buenos Aires urban heat island (UHI) intensity has not been studied. The goal of this paper is to apply an objective synoptic climatological method to identify homogeneous air masses or weather types affecting Buenos Aires during winter, and to relate the results to the UHI intensity. A K-means clustering method was used to define six different air masses considering the 03:00, 09:00, 15:00 and 21:00 LT surface observations of dry bulb temperature, dew point, cloud cover, atmospheric pressure and wind direction and velocity at Ezeiza, the most rural meteorological station of the Buenos Aires metropolitan area (Fig. 1). Results show that the mean UHI intensity is at its maximum (2.8 °C) a few hours before sunrise when conditions are dominated by cold air masses associated with cold-core anticyclones, weak winds and low cloud cover. Inverse heat islands are found during the afternoon for all air masses indicating that surface processes are not dominant at that time. The relatively infrequent and warmest air mass is the only one that presents a mean negative urban-rural temperature difference (−0.1 °C) during the afternoon with the smallest diurnal cycle of the UHI intensity probably due to the prevailing high humidity and cloudy sky conditions. The paper provides an insight into the Buenos Aires urban–rural temperature difference under a variety of winter weather types and results could be useful to improve local daily temperature forecasts for the metropolitan area of Buenos Aires on the basis of the routine forecasts of weather types.

A comprehensive model inter-comparison study investigating the water budget during the BALTEX-PIDCAP period

A comparison of 8 regional atmospheric model systems was carried out for a three-month late summer/early autumn period in 1995 over the Baltic Sea and its catchment area. All models were configured on a common grid using similar surface and lateral boundary conditions, and ran in either data assimilation mode (short term forecasts plus data assimilation), forecast mode (short term forecasts initialised daily with analyses) or climate mode (no re-initialisation of model interior during entire simulation period). Model results presented in this paper were generally post processed as daily averaged quantities, separate for land and sea areas when relevant. Post processed output was compared against available analyses or observations of cloud cover, precipitation, vertically integrated atmospheric specific humidity, runoff, surface radiation and near surface synoptic observations. The definition of a common grid and lateral forcing resulted in a high degree of agreement among the participating model results for most cases. Models operated in climate mode generally displayed slightly larger deviations from the observations than the data assimilation or forecast mode integration, but in all cases synoptic events were well captured. Correspondence to near surface synoptic quantities was good. Significant disagreement between model results was shown in particular for cloud cover and the radiative properties, average precipitation and runoff. Problems with choosing appropriate initial soil moisture conditions from a common initial soil moisture field resulted in a wide range of evaporation and sensible heat flux values during the first few weeks of the simulations, but better agreement was shown at later times.

Mesoscale modeling of katabatic winds over Greenland and comparisons with AWS and aircraft data

Summary Simulations of the katabatic wind system over the Greenland ice sheet for the two months April and May 1997 were performed using the Norwegian Limited Area Model (NORLAM) with a horizontal resolution of 25 km. The model results are intercompared and validated against observational data from automatic weather stations (AWS), global atmospheric analyses and instrumented aircraft observations of individual cases during that period. The NORLAM is able to simulate the synoptic developments and daily cycle of the katabatic wind system realistically. For most of the cases covered by aircraft observations, the model results agree very well with the measured developments and structures of the katabatic wind system in the lowest 400 m. Despite NORLAM’s general ability of reproducing the four-dimensional structure of the katabatic wind, problems occur in cases, when the synoptic background is not well captured by the analyses used as initial and boundary conditions for the model runs or where NORLAM fails to correctly predict the synoptic development. The katabatic wind intensity in the stable boundary layer is underestimated by the model in cases when the simulated synoptic forcing is too weak. An additional problem becomes obvious in cases when the model simulates clouds in contrast to the observations or when the simulated clouds are too thick compared to the observed cloud cover. In these cases, the excessive cloud amount prevents development of the katabatic wind in the model.

Cloud attenuation modelling for SHF and EHF applications

AbstractTropospheric propagation impairments that affect earth–space communication signals increase in severity with the increase of frequency. Introduction of satellite services using higher frequency bands such as Ka‐ and V‐band requires the characterization of propagation factors that are normally considered negligible at lower frequency bands. Cloud attenuation is considered one such factor. Clouds are present during a large fraction of an average year and cloud attenuation, together with gaseous absorption, will determine the system performance under non‐rainy conditions. A cloud attenuation model that has global applicability is presented in this paper. The model is based on average properties of four cloud types and their occurrence probabilities. The occurrence probabilities are derived from long‐term observations of cloud cover carried out by several thousand meteorological stations throughout the world. Predictions made with the model are compared with cloud attenuation data gathered using Ka‐band beacon signals and radiometers. Copyright © 2001 John Wiley &amp; Sons, Ltd.

PARAMETERIZATION OF INCOMING LONGWAVE RADIATION IN HIGH-MOUNTAIN ENVIRONMENTS

Some ecological applications and energy budget calculations of the earth's surface require accurate estimates of incoming longwave radiation. As cloud cover observations are not conducted frequently in high-mountain environments, a new model for the parameterization of daily mean incoming longwave radiation is proposed based on global radiation instead of cloud cover. Besides global radiation, the new model requires data for air temperature, relative humidity, and an estimate of daily mean cloudless global radiation. The model was calibrated with data from high-mountain and lowland stations and the results are compared with existing models. The new model yielded consistent results under all cloud-cover conditions, for different sites, and for all seasons. For the conditions tested, the absolute mean bias error was generally less than 10 Wm-2 and the root mean square error was always between 11 Wm-2 and 16 Wm-2. Of the other models tested, some did not perform well under cloudless conditions and others yielded large errors under overcast conditions or were not applicable to high mountain sites. The new model is a viable alternative to the existing longwave parameterization models, especially for high-mountain environments, and it can be applied without the resource-consuming observation of cloud cover. [Key words: longwave radiation, global radiation, cloud cover, high mountains, model.]

Combination of ground-based and satellite cloud observations on a routine basis

A Cloud Detection System (CDS) is designed, installed and operated for a two year period (1995/1996). In the ground-based network ten clouds and radiation stations are distributed over a 120 x 120 km 2 area. Every station consists of a lidar ceilometer, an IR-radiometer and a pyranometer. The ground based observations are combined with the observations from the operational meteorological satellites Meteosat and NOAA/AVHRR. The CDS was intended to provide a data set for the evaluation of atmospheric model output. From an analysis of the data for a two month period (January/February 1995) it is shown that the synoptical cloud cover observations are reproduced with a standard deviation of two octa for a single station and one octa for the cDs-area averaged values. The CDS data allow for the derivation of the following cloud parameters: both height and temperatures of cloud base and top, optical depth and emissivity. In this paper an example is given of the derivation of the cloud emissivity in the spectral region of the IR-radiometers. In this article the set-up of the CDS is described, together with a description of the algorithms which are used to retrieve the cloud parameters. The usefulness of the CDS data is illustrated with several examples. The CDS data set contains a two years time series of objective cloud data that can be used for model evaluation and for the validation of satellite retrievals. The CDS concept is widely applicable. Worldwide there is a large number of potential ground based stations. This has not been fully exploited yet.