AUGUST 2006 T O URNADRE 1131 Improved Level-3 Oceanic Rainfall Retrieval from Dual-Frequency Spaceborne Radar Altimeter Systems J. TOURNADRE Département d’Océanographie Physique et Spatiale, IFREMER, Plouzané, France (Manuscript received 15 March 2005, in final form 28 November 2005) ABSTRACT Since the launch of the Ocean Topography Experiment (TOPEX)/Poseidon in 1992, several studies have demonstrated that dual-frequency altimeter measurements cannot only accurately detect rain events but can also be used to infer quantitative values. The main problems with these techniques are the limited time and space sampling of nadir-looking instruments and the uncertainty in the height of the freezing level necessary to infer the surface rain rate from the measured signal attenuation. In addition to radar altimeters, altimetric satellites carry microwave radiometers designed to correct for atmospheric water effects. Using a radiative transfer model and simplified rainy atmospheres, a method of inversion of the microwave brightness temperatures in terms of freezing level is presented. The surface rain rate is then computed from the altimeter attenuation and the radiometer freezing level. The rain climatology is computed for the three altimeters currently in operation using a mixed lognormal distribution. Comparison with the Global Pre- cipitation Climatology Project and Special Sensor Microwave Imager (SSM/I) climatologies shows that the use of freezing level greatly improves the altimeter climatology, which is of the same quality as that of the SSM/I for annual mean. The merging of the three altimeters is investigated. The resulting monthly mean rain rates are comparable to those derived from SSM/I. The high along-track resolution of altimeters also allows the determination of the length of rain events. The mean length is close to the SSM/I footprint size in the Tropics, but at higher latitude 80% of the rain has length scales smaller than 10 km, which might explain the relative underestimation of the mean rain rate by SSM/I. 1. Introduction the limited time and space sampling and the other is the uncertainty in the height of the melting layer. The melt- The determination of accurate rain-rate estimates ing-layer height, necessary to infer the surface rain rate over the global ocean has been a challenging task for from the altimeter attenuation that is integrated over over a decade. In situ rain measurements at sea are the atmospheric path, was fixed either to a constant difficult and not always reliable. Because rain is by its value (Chen et al. 1997, 2003) or to a climatological very essence a transitory phenomenon, only satellite- mean, varying with latitude and season (Quartly et al. borne instruments can provide appropriate coverage. 1999). As shown by Quartly et al. (1999), these approxi- Since the launch of the Ocean Topography Experiment mations can lead to large errors in the surface rain-rate (TOPEX)/Poseidon, the first dual-frequency altimeter estimates, especially outside of the tropical regions. to be flown in space, it has been demonstrated by vari- However, altimetric mission satellites carry, in addi- ous studies (Tournadre and Morland 1997; Quartly et tion to radar altimeters, microwave radiometers de- al. 1999; Chen et al. 1997, 2003; Cailliau and Zlotnicki signed primarily to correct the altimetric measurements 2000; McMillan et al. 2002) that this kind of instrument for the effects of atmospheric water (path delay, attenu- cannot only accurately detect rain events but also can ation, etc.). These radiometers measure brightness tem- be used to yield quantitative values. However, there are peratures (Tbs) at two or three frequencies between 18 still two main problems with these techniques—one is and 37 GHz. These frequencies are close to the ones used by the Special Sensor Microwave Imager (SSM/I) for which several algorithms have been developed to Corresponding author address: J. Tournadre, IFREMER, infer both freezing-level altitude and rain rate (Wilheit Technopole Brest-Iroise, 29280 Plouzane´, France. et al. 1977, 1991). In this study, we present a method E-mail: [email protected] that combines altimeter and radiometer data to simul- © 2006 American Meteorological Society Unauthenticated | Downloaded 10/02/21 05:19 AM UTC JTECH1897 1132 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 23 taneously estimate the melting-layer altitude and rain 1) TOPEX/POSEIDON rate. The TOPEX/Poseidon satellite was developed by the Currently, three altimeter missions are in operation, National Aeronautics and Space Administration Jason TOPEX/Poseidon, , and the Environmental Sat- (NASA) and the French Space Agency [Centre Na- ellite (Envisat). These satellites have different charac- tional d’Etudes Spatiales (CNES)]. It was launched on teristics and orbits, and thus different time and space 10 August 1992, and up to now it has provided more samplings of the global ocean. The simultaneous avail- than 12 yr of data. TOPEX/Poseidon is dedicated to ability of these three data sources gives a good oppor- ocean altimetry, and its orbit and satellite bus and pay- tunity to investigate the potential of multisatellite rain load were optimized to map the ocean surface. The climatology. The data merging requires the intercom- satellite carries two altimeters, one developed by CNES parison and the intercalibration of the freezing-level and the other by NASA. The Poseidon-1 altimeter de- and the rain-rate estimates from the different satellite veloped by CNES is a single-frequency, solid-state ex- systems. perimental instrument intended to demonstrate new A key advantage of the altimeter systems is their high technology, and it operates approximately 10% of the along-track resolution (ϳ6 km), which allows the study time. The NASA radar altimeter (NRA), which oper- of the length scales of the rain events over the global ates at 13.6 (Ku band) and 5.3 (C band) GHz simulta- ocean. The rain cell size distribution and its geographi- neously, is the primary sensor of the mission. Only the cal variability are significant and informative character- dual-frequency NRA data are considered in this study. istics for the description of rain fields (Sauvageot et al. Depending on the sea state, the altimeter footprint var- 1999). The information obtained from altimeters would ies from 5- to 10-km radius. The satellite samples the complement those obtained on land using meteorologi- ocean surface between 66°S and 66°N at a 1-s interval cal radars. The joint analysis of length scales and radi- (corresponding to a 5.8 km ground distance) for each of ometer measurements can also give insight into the ef- the 254 passes that make up a 9.9156-day repeat cycle. fect of partial beam filling and explain the differences A detailed description of the NRA instrument and data between rain estimates from the altimeter and radiom- processing is given in Zieger et al. (1991) and Marth et eter. al. (1993). In section 2, the three altimeter missions currently The satellite also carries the TOPEX/Poseidon mi- available, that is, TOPEX/Poseidon, Jason, and En- crowave radiometer (TMR), which was primarily de- visat, and the auxiliary data used for validation are signed to provide corrections for the altimeter range briefly described. In section 3, the method to infer the errors (the so-called path delay) induced by the highly freezing level and the rain rate is presented. The freez- variable atmospheric water vapor content (Ruf et al. ing-level estimates are validated by comparison to 1994; Janssen et al. 1995; Keihm et al. 1995). TMR is a those of the National Centers for Environmental Pre- modified version of the Special Sensor Microwave Ra- diction (NCEP) NWP model and SSM/I in section 4. diometer (SMMR) that flew on Seasat and Nimbus-7.It Section 5 presents the mean rain fields derived from the operates at three frequencies of 18, 21, and 37 GHz in three altimeters and also presents a comparison with a nadir-viewing direction with a footprint diameter of the ones estimated from SMM/I and the Global Pre- 43.4, 36.4, and 22.9 km, respectively (Ruf et al. 1994). cipitation Climatology Project (GPCP). The merging TMR is temporally and spatially coaligned with the of the three altimeters is also tackled in this section. TOPEX/Poseidon altimeter in order to accommodate The analysis of the distribution of the rain cell length its mission requirements. In addition to water vapor and its influence on the mean rain rate is presented in retrieval, TMR also provides useful data of cloud liquid section 6. water. 2) JASON 2. Data The CNES–NASA Jason mission is designed to en- sure the continuity of the observation and monitoring a. Dual-frequency altimeter missions: of the ocean provided by TOPEX/Poseidon and it has TOPEX/Poseidon, Jason, and Envisat basically the same characteristics. It was launched on 7 The data from the archive of the three altimeter mis- December 2001. Its main instrument is the Poseidon-2 sions, which use dual-frequency radar altimeter, have altimeter, which is derived from the experimental been used in this study. The instruments and data are Poseidon-1 altimeter. It is a compact, low-power, low- briefly presented in this section. mass instrument offering a high degree of reliability. Unauthenticated | Downloaded 10/02/21 05:19 AM UTC AUGUST 2006 T O URNADRE 1133 The dual-frequency Poseidon-2 operates at the same level is flagged as missing. If multiple zero crossings are frequencies as the NRA, that is, at 13.6 (Ku band) and found, resulting from temperature inversions, only the 5.3 (C band) GHz. A detailed description of the Posei- lowest Z0 value is kept. don-2 altimeter is given in Ménard and Fu (2001). The Jason microwave radiometer (JMR), a Jet Pro- 2) RAIN CLIMATOLOGY pulsion Laboratory instrument of TMR heritage, is a Two types of rain climatology are used for compari- passive receiver that collects radiation emitted by the son in this study.