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Cloud climate investigations in the Nordic region using NOAA AVHRR data
Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Norrköping.
1997 (English)In: Theoretical and Applied Climatology, ISSN 0177-798X, Vol. 57, no 3-4, 181-195 p.Article in journal (Refereed) Published
Abstract [en]

A method to estimate monthly cloud conditions (monthly cloud frequencies) from multispectral satellite imagery is described. The operational cloud classification scheme SCANDIA (the SMHI Cloud ANalysis model using DIgital AVHRR data), based on high resolution imagery from the polar orbiting NOAA-satellites, has been used to produce monthly cloud frequencies for the entire year of 1993 and some additional months in 1991, 1992, 1994 and 1995. Cloud analyses were made for an area covering the Nordic countries with a horizontal resolution of four km. Examples of seasonal, monthly and diurnal variation in cloud conditions are given and an annual mean for 1993 is presented. Comparisons with existing surface observations showed very good agreement for horizontal cloud distributions but approximately 5% smaller cloud amounts were found in the satellite estimations. The most evident problems were encountered in the winter season due to difficulties in identifying low-level cloudiness at very low sun elevations. The underestimation in the summer season was partly fictious and caused by the overestimation of convective cloud cover by surface observers. SCANDIA results were compared to ISCCP (International Satellite Cloud Climatology Project) cloud climatologies for two selected months in 1991 and 1992. ISCCP cloudiness was indicated to be higher, especially during the month with anticyclonic conditions where a cloudiness excess of more than 10% were found. The regional variation of cloud conditions in the area was found to be inadequately described by ISCCP cloud climatologies. An improvement of the horizontal resolution of ISCCP data seems necessary to enable use for regional applications. The SCANDIA model is proposed as a valuable tool for local and regional monitoring of the cloud climatology at high latitudes. More extensive comparisons with ISCCP cloud climatologies are suggested as well as comparisons with modelled cloudiness from atmospheric general circulation models and climate models. Special studies of cloud conditions in the Polar areas are also proposed.

Place, publisher, year, edition, pages
1997. Vol. 57, no 3-4, 181-195 p.
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-23043DOI: 10.1007/BF00863612ISI: A1997XM12100005OAI: oai:DiVA.org:su-23043DiVA: diva2:189962
Available from: 2006-11-18 Created: 2006-11-18 Last updated: 2013-12-10Bibliographically approved
In thesis
1. The use of a satellite-derived cloud climatology for studying cloud-aerosol processes and the performance of regional cloud climate simulations
Open this publication in new window or tab >>The use of a satellite-derived cloud climatology for studying cloud-aerosol processes and the performance of regional cloud climate simulations
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The entry of satellite-derived decadal cloud datasets with homogeneous coverage in time and space enables studies not possible before. This thesis presents two such applications. The first study deals with cloud-aerosol processes and the second with an evaluation of cloud simulations from a regional climate model.

The first part of the thesis describes the used satellite-derived dataset based on imagery from the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting NOAA satellites. A method for cloud retrieval and the compilation of a 1991-2000 Scandinavian cloud climatology are described.

The second part reveals an intriguing anti-correlation between monthly mean satellite-derived cloudiness and the concentration of the cosmogenetic isotope Beryllium-7 in near-surface aerosol samples for three measurement sites in Sweden. Large-scale transport processes are suggested as the most likely physical mechanism for this behaviour but more complex relations to cloud microphysical processes are not ruled out.

The final part presents a thorough evaluation of cloud simulations of the SMHI Rossby Centre regional atmospheric model (RCA3). Several model-to-satellite adaptations are applied to avoid artificial biases of results. The study stresses the necessity to account for initial differences between observed and modelled clouds caused by satellite cloud detection limitations. Results show good agreement of modelled and observed cloud amounts while the vertical distribution of clouds appears largely different. RCA3 underestimates medium-level clouds while overestimating low- and high-level clouds. Also, the current use of the Maximum cloud overlap approach in the radiation scheme and an indicated excess of cloud condensate in modelled clouds appear to create excessive cloud optical thicknesses with serious implications for the surface radiation budget.

Future applications are outlined based on greatly enhanced satellite-derived cloud and radiation budget datasets.

Place, publisher, year, edition, pages
Stockholm: Meteorologiska institutionen (MISU), 2006. 142 p.
Keyword
Clouds and climate, satellite cloud climatology, validation of clouds, clouds and aerosols
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-1364 (URN)91-7155-353-3 (ISBN)
Public defence
2006-12-18, C 609, Arrheniuslaboratoriet, Stockholm, 10:00
Opponent
Supervisors
Available from: 2006-11-18 Created: 2006-11-18Bibliographically approved

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