Simultaneous measurements of ku- and ka-band sea surface cross sections by an airborne Radar

The dual-frequency Airborne Precipitation Radar-2 (APR-2) was deployed during the Wakasa Bay Experiment in 2003, for validation of the Advanced Microwave Scanning Radiometer-EOS. Besides providing extensive observations of diverse precipitating systems, this Ku-(13.4 GHz) and Ka-band (35.6 GHz) cross-track scanning radar measured sea surface backscatter simultaneously. While the characteristics of the normalized sea surface cross section /spl sigma//sup 0/ at Ku-band are well understood and widely published, the existing experimental data concerning /spl sigma//sup 0/ at Ka-band are scarce and results are inconsistent. In this letter, the Ku/Ka-band /spl sigma//sup 0/ measurements collected by APR-2, together with the estimated uncertainties, are discussed. In general, the measured /spl sigma//sup 0/ at Ka-band at around 10/spl deg/ incidence angle appears to be close to that at Ku-band /spl sigma//sup 0/, and Ka-band exhibits a nonnegligible difference in wind dependence with respect to Ku-band for moderate to high winds.     

On the role of “hot towers” in tropical cyclone formation

Summary The probabilistic approach to tropical cyclogenesis is advanced here by examining the role of convection in the early stages. The development of hot towers, that is tall cumulonimbus towers which reach or penetrate the tropopause, and their role in tropical cyclogenesis is investigated in two well-documented cases of formation. namely hurricane Daisy (1958) in the Atlantic and Tropical Cyclone Oliver (1993) in the Coral Sea. The hot towers in Daisy had been intensively studied by Malkus and Riehl three decades ago but remained mainly unpublished. The dynamics of Oliver genesis by merging mesoscale vortices has been recently reported, but much of the aircraft data remained. This paper adds the evolving contribution of cumulus-scale events and their associated electrification, which was made possible by the addition of an electric field mill, a numerical cloud model and other remote sensors.In their genesis stages, Daisy and Oliver appeared very different because Daisy resulted from a deepening tropical wave in the Atlantic and the pre-Oliver vortex emerged eastward from the Australian monsoon trough. However, the vertical profiles of _E in the rain areas were nearly identical, with the characteristic concave shape showing substantial midlevel minima. Therefore, both required increasing upflux of high _E subcloud air in order to accomplish the formation stage, with about two hot towers each in the nascent eyewall. In both cases, partial eyewalls developed at the edge of the convection, permitting subsidence in the forming eye, which was shown to contribute to the pressure fall. The probabilistic concept proposes that any contribution to early pressure fall raises the probability of success. When the incipient storm goes through those fragile phases more rapidly, the risk of death by the onset of unfavorable large-scale factors such as wind shear or upper-level subsidence is reduced. Daisy developed in an inactive, moist environment with light, variable winds throughout the troposphere while in Oliver, strong divergent upper outflow apparently outweighed strong wind shear, although the latter was responsible for a slow and messy development of a closed, circular eye.In both storms, the hot towers in the major rainband were taller and stronger than those in the early eyewall. Onedimensional time-dependent model runs were used to simulate both in Oliver with two important results: 1) the taller rainband clouds permitted greater high level heating, if it could be retained; and 2) greater electrification and more lighting occurred in the rainband although the partial eyewall clouds also showed strong electrification. Airborne radar, electrification measurements and models are fitted together to understand their relationship. An important result is the clear inference that fairly deep mixed phase regions existed in both eyewall and rainband, in which the DC-8 aircraft experienced liquid water at temperatures colder than –40°C below freezing. These results show that the claims of no supercooled liquid water in tropical cyclones require re-examination with the proper measurements of electricification that are now feasible.With 13 Figures     

Large-scale managed retreat and structural protection following the 2011 Japan tsunami

Natural Hazards - On March 11, 2011, a magnitude MW 9.0 thrust earthquake ruptured the Japan Trench along the northwest coast of Honshu and generated a tsunami that killed 15,894 people, left 2585...     

A physical radar cross-section model for a wind-driven sea with swell

A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the0deg-70degrange. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies (Lband) and near normal incidence but can be nearly eliminated by using higher frequencies (K_{u}band) and large angles of incidence (approx 50deg).