Wednesday, 6 August 2003
Estimation of effective raindrop shape model from 35 GHz attenuation and differential attenuation
Rainfall measurements using polarimetric radar observables exploit their sensitivity to the shape-size dependence of raindrops. The equilibrium shape-size relationship of raindrops is well established. For radar applications raindrops are modeled as oblate spheroids with size dependent axial ratios. The relationship between size and axial ratio may be altered by oscillations. Furthermore, the drop shape projected on the plane perpendicular to the radar line-of-sight direction is also affected by drop canting. It is of interest to determine whether or not there is a deviation from the equilibrium shape-size model and to estimate an effective shape model for raindrops from measurements during a rainfall event. For this purpose we propose the use of specific attenuation Ah and specific differential attenuation ƒ´A=Ah - Av at 35 GHz, where h and v denote horizontal and vertical polarization. It is shown that raindrop canting and oscillation affect Ah negligibly and ƒ´A significantly. Simulations using DSDs from ground based disdrometer measurements indicate that the relationship between Ah and ƒ´A is sensitive to the raindrop shape model used. For example, power law relationship of the form Ah=a(ƒ´A)b (with both Ah and ƒ´A having units of dB/km) have the following coefficients for an equilibrium drop shape model (a=6.78, b=0.889) and two different oscillation models (a=8.25, b=0.873; and a=8.91, b=0.887). It is suggested that simultaneous measurements of Ah and ƒ´A could be used together with simulated Ah - ƒ´A relationships for determining the presence of drop oscillations and canting and for estimating an effective raindrop shape model during a given rainfall event.