PRECIPITATION MEASUREMENTS FROM SPACE Vincenzo Levizzani CNR - Institute of Atmospheric Sciences and Climate, via Gobetti 101, I-40129 Bologna, Italy, Email: v.levizzani@isac.cnr.it ABSTRACT Measuring precipitation from space is a long standing issue of meteorology, hydrology and climatology. Since the launch of the first meteorological satellites in the 60s several visible/infrared/microwave techniques for “inferring”, rather than “measuring” rainfall intensity from space were conceived, but seldom reached operational application. Algorithms have greatly evolved and now offer an acceptable quality level when products are averaged over suitable time and space scales. Daily, monthly and yearly products have become important inputs for climate studies, but their quality significantly lowers when the algorithms are applied to estimate instantaneous rainrates. More recent methods go back to the basic physical principles of precipitation formation and evolution. The re-examination of the physical content of the algorithms is driven by new insights on precipitation formation mechanisms now available together with new observational tools from space and from the ground. The current status of precipitation measurements from space will be addressed with an eye on what is available for applications such as those of telecommunications. The focus will be on advanced algorithms, satellite missions, activities and efforts of international organizations, including generation of high resolution precipitation products for propagation applications. 1. RECENT METHODS Methods have tried to exploit visible (VIS), infrared (IR), near infrared (NIR), water vapour (WV), passive microwave (PMW), and radar data. A complete review of satellite rainfall estimation methods is out of the scope of the present paper. The reader will find reviews and comparisons, among others, in the articles by Adler et al. (2001, global products), Kidd (2001, climate), Levizzani et al. (2001, 2002, generic), and Petty (1995, over land methods). A state-of-the-art collection of papers on the advances in the field is now in press (Levizzani et al. 2007). We will here concentrate on the most recent methods, i.e. 1) PMW physical-statistical methods, and 2) combined algorithms for global products. 1.1 Passive microwave methods At PMW frequencies precipitation particles are the main source of attenuation of the upwelling radiation and thus PMW techniques are physically more direct than those based on VIS/IR radiation. The emission of radiation from atmospheric particles results in an increase of the received signal while at the same time the scattering due to hydrometeors reduces the radiation stream. Type and size of the detected hydrometeors depend upon the frequency of the upwelling radiation. Scattering and emission happen at the same time with radiation undergoing multiple transformations within the cloud column in the sensor’s field of view. At different frequencies the radiometers observe different parts of the rain column and this principle is behind the choice of observing channels. Precipitation drops strongly interact with MW radiation and are detected by radiometers without the IR strong biases. The biggest disadvantage is the poor spatial and temporal resolution, the first due to diffraction, which limits the ground resolution for a given satellite MW antenna, and the latter to the fact that MW sensors are consequently only mounted on polar orbiters. The matter is further complicated by the different radiative characteristics of sea and land surfaces underneath. While the sea surface has a relatively homogeneous emissivity, land surfaces have a high and variable emissivity, close to that of precipitation, and low polarization. The emissivity depends upon the characteristics of the surface including vegetation and moisture content. Rainfall over land will increase the upwelling radiation stream but at the same time will absorb radiation introducing considerable difficulties in the identification of rain areas. Scattering is thus the key to the PMW rainfall estimation techniques over land. Several approaches have been developed in time from relatively simple threshold-based algorithms to the most recent and complex physical-statistical techniques. Some of these rather simple threshold methods were used also to produce global estimates such as that of the National Oceanic and Atmospheric Administration (NOAA), which has just recently been reconsidered trying to lower the existing biases (McCollum and Ferraro 2003). Products are available at http://www.orbit.nesdis.noaa.gov/corp/scsb/mspps/main .html PMW frequencies have always shown an obvious tendency to perform better for convective precipitation while stratiform rain or, more generally, low rainrates are poorly detected. An example is shown in Fig. 1 where the same precipitation system is simultaneously _____________________________________________________ Proc. ‘EuCAP 2006’, Nice, France 6–10 November 2006 (ESA SP-626, October 2006)