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AVI 080 Project "ACROSS" |
"Analyzed Climatology of Rainfall Obtained from Satellite and Surface data
for the Mediterranean Basin"
END OF FIRST YEAR REPORT
The ACROSS project aims at producing a unified satellite and raingauge climatology of rainfall over the Eastern Mediterranean Region, in order to permit a much more satisfactory evaluation and interpretation than has been possible hitherto of over-land and over-water anomalies. The study covers the period of available microwave satellite observations from 1978 to 1994 and a study area including the Eastern Mediterranean sea and land areas. The partners of this project are the Hydraulics Institute of the University of Genova (I), the Centre for Remote Sensing of the University of Bristol (UK), the Civil Engineering Department of the Middle East Technical University (Turkey), the Department of Geology of the University of Jordan (Jordan). The International Center for Agricultural Research in the Dry Area (ICARDA - Syria) has joined the Consortium since June 1996.
The work undertaken during the first year of the project activity was mainly devoted to the acquisition of the basic data resources needed for the eventual development of dedicated research studies, and to the generation of initial overwater products based on SMMR satellite data (1978-87). In particular the collection, collation and analysis of raingauge data for suitable and available stations in the study area were addressed, in order to produce maps of average rainfall for months, seasons, years, plus maps, graphs and statistics for rainfall variability and departures from the norm. At the same time the collection and geo-registering of SMMR (1978-1987) and SSM/I (1987-1994) images was undertaken prior to their preliminary analysis for over water rainfall within the study area to complement the above mentioned products, and so complete a regional picture for the Eastern Mediterranean. The definition of suitable graphical tools for presentation of project results was also addressed and the acquisition of ancillary data completed. The two data-sets were implemented within a relational database and a hydrologically oriented Geographycal Information Systems (GIS).
The 30-seconds Digital Elevation Model of the Mediterranean region, obtained from USGS, was selected as the basic information over which both satellite and raingauge data are represented. A large number of raingauge and meteo-climatic stations were identified in the study area and the acquisition of data for the period 1978-1994 started. In particular daily rainfall series from the NOAA / NCDC dataset were acquired and complemented with sparse data from the national networks (provided by partner institutions) in order to achieve the information density of about one raingauge per 625 km2 (25 x 25 km grid).
Microwave satellite images were collected for the period 1978-1994, both from SSMR and SSM/I sensors. In particular data from the SMMR were obtained in the form of Temperature Calibrated Tapes (TCTs) for the entire instrument operation period from 25 October 1978 to 20 August 1987 from the US National Space Science Data Center. SSM/I images for the period 1987-1994 were obtained from the US Defence Meteorological Satellite Programme (DMSP) via the Marshall Space Flight Center of NASA.
The objectives envisaged for the first period of research were those of Tasks 1.1 to 1.4 of the work programme, and in particular:
These were addressed through the following steps:
In this preliminary phase the two components of this project, namely the ground-based and the satellite perspective, were therefore expected to process in parallel as the acquisition, collection and collation of basic material were actually the main objectives envisaged.
In the next paragraphs, therefore, we will consider separately the research activities conducted under Tasks 1.1, 1.4 and 2.2 (ground-based perspective and data presentation issues) and those for Tasks 1.2, 1.3 and 2.1 (satellite perspective).
Tasks 1.1, 1.4 and 2.2
As for the raingauge information, it was agreed by the consortium that daily time series would be collected at a suitable number of raingauge stations within the study area, for selected countries, and in particular from those stations located along the coast or very near to the sea. Stations along the coast - and especially those located over small islands - are indeed very important in that they allow integration between the two datasets - traditional and satellite derived - and thus calibration and verification of rainfall estimates on the sea areas. Specific attention is thus devoted to the coastal areas where the two datasets must fit each other so as to provide a final unified rainfall climatology of the region. Hourly data from selected raingauge stations are collected in the case of precipitation extremes for a series of case studies, when these are detected by the analysis of daily data. Raingauge data are therefore stored in the ACROSS archive as hourly/daily time series associated with a given location in space which is definitively characterised by two geographical coordinates, in a linear latitude/longitude reference system, and one elevation value. The format and storing strategies for such data have been defined under Task 1.1. Also, the development of automated procedures able to support the handling of data archived in a hydrological and climatic database integrated within a Geographical Information System was addressed under Task 2.2.
Tasks 1.2, 1.3 and 2.1
Data from the SMMR were obtained in the form of Temperature Calibrated Tapes (TCTs) for the entire instrument operation period from 25 October 1978 to 20 August 1987 from the US National Space Science Data Center (NSSDS), Greenbelt, Maryland. The raw radiometric readings were corrected for actual antenna patterns including sidelobe effects, and the vertical and horizontal polarization components of the brightness temperature were separated out. SSM/I data were obtained in three formats: a) the Wentz Format - directly obtained from Frank Wentz of Remote Sensing Systems, Santa Rosa, California - b) the NESDIS 1B Format - obtained from the Marshall Space Flight Center (MSFC) at Hunsville, US - and c) the FNMOC Format - obtained from the MSFC also. Software is available in the CRS to process all data formats. Due to the great volume of data involved in ACROSS, including both SSM/I and SMMR, an automatic quality control procedure has been devised and incorporated into the data processing. The SMMR data were acquired and processed as described in Report 1.4. The quality control procedure required extensive fine-tuning to screen out all bad data. A projection was established using an equal area Universal Transverse Mercator (UTM) projection of 25km resolution. The rainfall algorithm used was the 37ghz Polarisation Difference algorithm described in Report 1.2. This was chosen in preference to the H37 algorithm to minimise the influence of surface and atmospheric temperature variations and to ensure a unique relationship of algorithm and rain rates. Estimates of rainfall were produced at the UTM projection and then re-projected to a linear latitude/longitude projection for display purposes.
Tasks 1.1, 1.4 and 2.2
The 30 seconds Digital Elevation Model of the Mediterranean region, obtained from USGS, was selected as the basic information over which both satellite and raingauge data are represented. A large number of raingauge stations were identified in the study area. The National Authorities in charge of the meteorological and hydrological services were contacted in France, Italy, Croatia, Albania, Greece, Turkey, Jordan and Syria. The acquisition of rainfall data for the period 1978-1994 started. In particular daily rainfall series from the NOAA / NCDC dataset were acquired and complemented with sparse data from the national networks (provided by each partner institution for its own country and neighbouring ones) in order to achieve a suitable information density. In particular the following raingauge densities have been decided: a) the NCDC available density for the whole study region and, b) about one raingauge per 625 km2 in the 50 km wide coastal strip of the Spanish, French, Croatian, Albanian and Greek territory, and in the whole territory of Italy, Turkey, West Jordan and Syria. A GIS-based system for the storing, georeferencing and integration of satellite derived information and ground-based data has been designed and implemented using the Intergraph MGE software products.
Tasks 1.2, 1.3 and 2.1
The first result was the complete acquisition and preparation of satellite imagery in order to enable the eventual production of rainfall estimates from remotely sensed data. This was made for the whole study period (1978-1994) both from the Scanning Multichannel Microwave Radiometer (SMMR) and from the Special Sensor Microwave/Imager (SSM/I). Data processing was also performed so as to apply quality control routines which discard bad data, using the available automated codes in the case of SSMR images and incorporating a manual quality control scheme in the case of SSM/I data where corrupt scan lines leading to errors in rainfall estimates are removed without affecting the non-corrupted data. The calibration and validation of selected algorithms for estimation of rainfall over water using passive microwave satellite data was carried out, using radar rainfall maps as validation data sets. Finally, passive microwave algorithms for retrieval of land surface information (soil moisture, vegetation index) were analysed in order to propose a suitable strategy for monitoring of surface characteristics.
Estimates of rainfall using SMMR data were produced for the following quantities:
The estimates were integrated over the following periods:
Tasks 1.1, 1.4 and 2.2
The main difficulties encountered in the collection of rainfall data are those related to the lack of data in some of the North African and Middle Eastern countries. A second difficulty is due to the fact that the contract with the partner in Syria has only recently been signed, and so data collection has only just started in that country. It is envisaged that the collection of rainfall data may not be completed in the next 9-12 months, with some delay affecting the project timetable. This may have major consequences on the project development. Also data from countries in North Africa will, except for the NCDC-datasets, aren't available. Due to the delay in the completion of the rainfall archive, the supporting GIS system was designed and developed without any direct access to the database. Indeed, the process of data storing and digitising is still under way and the connection will be set up when all the available data are definitively input to the system.
Tasks 1.2, 1.3 and 2.1
From the analysis and interpretation of satellite data on the sea areas it appears that within the region closest to the land areas the estimate of rainfall amounts is affected by large uncertainties. These are due to the fact that - because of physical reasons linked to the microwave emission characteristics of the land and sea areas - algorithms are available that perform well on the sea and others that perform well on the land. A narrow strip along the coast is therefore the boundary where both such algorithms produce uncertain results. It was therefore decided that along this strip (about 50 km wide) no satellite estimate of rainfall will be provided. Rainfall amounts in this region will have to be derived from joint observation of satellite and raingauge data by use of appropriate procedures. A careful "fine-tuning" of the quality control procedure was required to remove as much of the corrupt data as possible.
A window region with boundaries at 12°W, 29°N and 45°E, 48°N - covering the entire Mediterranean basin from the Atlantic in the west to the Middle East countries and extending north to include the Black Sea - has been chosen for the ACROSS project. This area is larger than that specified in the original contract, for the whole Mediterranean Basin can be covered for little extra processing time (at least as far as satellite data are concerned) than the eastern region alone. Increasing the study region will produce a more valuable product, particularly in light of possible applications of the final products and future follow-on projects.
For display of the rainfall products a linear latitude/longitude projection was chosen for ACROSS, in consultation among the project partners, for this is simple both to compute from the latitude/longitude information associated with each satellite footprint, and to utilise with raingauge data also. Furthermore, the linear latitude/longitude projection facilitates simple remapping to other projections. One feature of such a projection is that its pixels have non-equal areas, with pixel size on the ground decreasing with increasing latitude. However this effect is not serious in middle latitudes.
A WWW Server for ACROSS has been developed by the project coordinator and is now operating on the Internet. The available pages contain information about the project objectives, the partners of the ACROSS consortium, a synthesis of the current status of the project and abstracts of the relevant documents.
University of Genova (Coordinating Institution):
Middle East Technical University:
University of Jordan:
