IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 45, NO. 7, JULY 2007 2287 Information Content of Millimeter-Wave Observations for Hydrometeor Properties in Mid-Latitudes Mario Mech, Susanne Crewell, Member, IEEE, Ingo Meirold-Mautner, Catherine Prigent, and Jean-Pierre Chaboureau Abstract—For future remote sensing applications the potential of the millimeter wavelength range for precipitation observations from geostationary orbits is investigated. Therefore, a database consisting of hydrometeor profiles from various mid-latitude pre- cipitation cases over Europe and corresponding simulated bright- ness temperatures at 18 microwave frequencies was built using the cloud resolving model Méso-NH and the radiative transfer model MicroWave MODel. The information content of the database was investigated by applying simple statistical methods, as well as developing first-order retrieval approaches. The results show that, particularly for snow and graupel, the total column content can be retrieved accurately with relative errors smaller than 25% in dominantly stratiform precipitation cases over land and ocean surfaces. The performance for rain-water path is similar to the one for graupel and snow in light precipitation cases. For the cases with higher precipitation amounts, the relative errors for rain-water path are larger particularly over land. The same behavior can be seen in the surface rain rate retrieval with the difference that the relative errors are doubled in comparison to the rain-water path. Algorithms with reduced number of frequencies show that window channels at higher frequencies are important for the surface rain rate retrieval because these are sensitive to the scattering in the ice phase related to the rain below. For the frozen hydrometeor retrieval, good results can be achieved by retrieval algorithms based only on frequencies at 150 GHz and above which are suit- able for geostationary applications due to their reduced demands concerning the antenna size. Index Terms—Hydrometeor, millimeter wave radiometry, re- mote sensing, retrieval, satellite applications. I. I NTRODUCTION P RECIPITATION results from a complex chain of processes implicating a high variability in space and time. Therefore, the observation of precipitation with a sufficient temporal and spatial resolution for many applications, such as numerical weather forecast models or hydrological purposes, is still a challenging task. Manuscript received June 6, 2006; revised April 4, 2007. This work was supported by EUMETSAT under Contract EUM/CO/04/1311/KJG (“Simu- lation study of precipitating clouds from geostationary orbits with passive microwaves”). M. Mech and S. Crewell are with the Institute for Geophysics and Meteorology, University of Cologne, 50923 Cologne, Germany (e-mail: mech@meteo.uni-koeln.de). I. Meirold-Mautner and C. Prigent are with the LERMA, l’Observatoire de Paris, 75014 Paris, France. J.-P. Chaboureau is with the Laboratoire d’Aérologie, Université Paul Sabatier and CNRS, 31400 Toulouse, France. Digital Object Identifier 10.1109/TGRS.2007.898261 Traditional precipitation observation systems such as rain gauges measure precipitation at single points, mostly as accu- mulated daily sums with an unsatisfactory coverage particularly over ocean. In well-developed countries ground-based radar networks provide precipitation observations with a temporal resolution of about 5 min over large areas with high spatial resolution. Relating the measured reflectivities of the radar sampling volume to the surface rain rate can lead to substantial errors of the order of a factor of 2 [1], particularly at distances larger than 100 km. Due to the different error factors involved in the measurement process, a homogeneous quality radar precipitation composite will not be available in the near future, even for Europe. With satellite-based observations in the lower microwave regions promising results were accomplished over ocean from polar orbiters (see [2] and [3]). Their algorithms for retrieving precipitation rates use frequencies below 40 GHz and are based on the brightness temperature increase through emission by the precipitation layer over the radiatively cold ocean. An algo- rithm applicable over ocean surfaces, including the emission signal and the information contained in the signal influenced by scattering by frozen hydrometeors above the precipitating layer, was developed by Bauer and Schlüssel [4]. By using window channels, the surface rain rate and also the hydrometeor profiles were retrieved (see [5] or [6]). Over land, the emission- based approach is not useful, since land surface emissivity is too large and heterogeneous within the radiometer footprint. Therefore, to reduce the sensitivity to surface emissivity, higher frequencies are applied. In the millimeter wavelength region, information can be gained by examining the signal originating in the precipitating layer and modulated by scattering by large frozen hydrometeors above. Since the formation of precipita- tion in the mid-latitudes mostly follows the cold rain process in the solid form as snow and graupel which melts to rain at lower altitudes, there is a relation between the microwave signal originating in the upper atmospheric ice layer and the surface rain rate. This indirect approach was implemented for the 85.5-GHz window channel in various algorithms (see [7]–[12]). The potential of millimeter (30–300 GHz) and submillimeter-wave (300+ GHz) channels for passive space- borne remote sensing of the troposphere and lower stratosphere has been numerically investigated by Gasiewski [13]. He found that high-frequency window channels (340 or 410 GHz) show a potential for observing clouds with very low liquid water contents and that these high frequencies can map thick clouds 0196-2892/$25.00 © 2007 IEEE