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S. L. Durden, L. Li, E. Im, and S. H. Yueh

Abstract

The operational algorithm for rainfall retrieval from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar data requires a measurement of the path-integrated attenuation (PIA) as a constraint. This constraint is derived via the surface reference technique, whereby a measurement of the ocean surface in a raining area is compared with the ocean surface backscatter in a neighboring clear area. This method assumes that the surface backscatter difference is due only to the presence of rain, although variation in surface winds could also cause differences in the reference and rain measurements. An alternative surface reference method is to use a measurement of the surface winds and a backscatter model to predict the rain-free, or reference, cross section. Such an approach is developed here for airborne Doppler radar measurements in hurricanes. This approach provides an independent measurement of the reference backscatter, which is compared with the standard clear-air reference. The mean difference between the standard and Doppler-derived PIA is less than or equal to 1 dB; the rms difference is in the range 0.9–2.6 dB. In deriving the model function for backscatter estimation from wind measurements, the authors also find that the dependence of ocean backscatter on wind appears to saturate at high wind speeds at 25° incidence.

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S. M. Hristova-Veleva, P. S. Callahan, R. S. Dunbar, B. W. Stiles, S. H. Yueh, J. N. Huddleston, S. V. Hsiao, G. Neumann, B. A. Vanhoff, R. W. Gaston, E. Rodriguez, and D. E. Weissman

Abstract

Scatterometer ocean surface winds have been providing very valuable information to researchers and operational weather forecasters for over 10 years. However, the scatterometer wind retrievals are compromised when rain is present. Merely flagging all rain-affected areas removes the most dynamic and interesting areas from the wind analysis. Fortunately, the Advanced Earth Observing Satellite II (ADEOS-II) mission carried a radiometer [the Advanced Microwave Scanning Radiometer (AMSR)] and a scatterometer, allowing for independent, collocated retrievals of rain. The authors developed an algorithm that uses AMSR observations to estimate the rain inside the scatterometer beam. This is the first in a series of papers that describe their approach to providing rain estimation and correction to scatterometer observations. This paper describes the retrieval algorithm and evaluates it using simulated data. Part II will present its validation when applied to AMSR observations. This passive microwave rain retrieval algorithm addresses the issues of nonuniform beam filling and hydrometeor uncertainty in a novel way by 1) using a large number of soundings to develop the retrieval database, thus accounting for the geographically varying atmospheric parameters; 2) addressing the spatial inhomogeneity of rain by developing multiple retrieval databases with different built-in inhomogeneity and rain intensity, along with a “rain indicator” to select the most appropriate database for each observed scene; 3) developing a new cloud-versus-rain partitioning that allows the use of a variety of drop size distribution assumptions to account for some of the natural variability diagnosed from the soundings; and 4) retrieving atmospheric and surface parameters just outside the rainy areas, thus providing information about the environment to help decrease the uncertainty of the rain estimates.

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N. Reul, B. Chapron, E. Zabolotskikh, C. Donlon, A. Mouche, J. Tenerelli, F. Collard, J. F. Piolle, A. Fore, S. Yueh, J. Cotton, P. Francis, Y. Quilfen, and V. Kudryavtsev

Abstract

Wind radii estimates in tropical cyclones (TCs) are crucial to helping determine the TC wind structure for the production of effective warnings and to constrain initial conditions for a number of applications. In that context, we report on the capabilities of a new generation of satellite microwave radiometers operating at L-band frequency (∼1.4 GHz) and dual C band (∼6.9 and 7.3 GHz). These radiometers provide wide-swath (>1,000 km) coverage at a spatial resolution of ∼40 km and revisit of ∼3 days. The L-band measurements are almost unaffected by rain and atmospheric effects, while dual C-band data offer an efficient way to significantly minimize these impacts. During storm conditions, increasing foam coverage and thickness at the ocean surface sufficiently modify the surface emissivity at these frequencies and, in turn, the brightness temperature (Tb) measurements. Based on aircraft measurements, new geophysical model functions have been derived to infer reliable ocean surface wind speeds from measured Tb variations. Data from these sensors collected over 2010–15 are shown to provide reliable estimates of the gale-force (34 kt), damaging (50 kt), and destructive winds (64 kt) within the best track wind radii uncertainty. Combined, and further associated with other available observations, these measurements can now provide regular quantitative and complementary surface wind information of interest for operational TC forecasting operations.

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