A Feasibility Study for Simultaneous Estimates of Water Vapor and Precipitation Parameters Using a Three-Frequency Radar

Abstract

The radar return powers from a three-frequency radar, with center frequency at 22.235 GHz and upper and lower frequencies chosen with equal water vapor absorption coefficients, can be used to estimate water vapor density and parameters of the precipitation. A linear combination of differential measurements between the center and lower frequencies on one hand and the upper and lower frequencies on the other provide an estimate of differential water vapor absorption. The coupling between the precipitation and water vapor estimates is generally weak but increases with bandwidth and the amount of non-Rayleigh scattering of the hydrometeors. The coupling leads to biases in the estimates of water vapor absorption that depend primarily on the phase state and the median mass diameter of the hydrometeors. For a downlooking radar, path-averaged estimates of water vapor absorption are possible under rain-free as well as raining conditions by using the surface returns at the three frequencies. Simulations of the water vapor attenuation retrieval show that the largest source of error typically arises from the variance in the measured radar return powers. Although the error can be mitigated by a combination of a high pulse repetition frequency, pulse compression, and averaging in range and time, the radar receiver must be stable over the averaging period. For fractional bandwidths of 20% or less, the potential exists for simultaneous measurements at the three frequencies with a single antenna and transceiver, thereby significantly reducing the cost and mass of the system.

Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles

“…For example, prior studies have proposed to observe surface pressure using channels in the 60 GHz oxygen absorption complex scattered off of the Earth surface (Lin and Hu 2005;Millán et al 2014) and some initial observations have been made with an airborne prototype (Lawrence et al 2011). Use of the 22 GHz absorption line has been proposed to observe water vapor within precipitation echoes (Meneghini et al 2005). Additionally, exploiting the differences in absorption by water vapor in the continuum has been proposed to retrieve water vapor (Ellis and Vivekanandan 2010;Tian et al 2007).…”

The feasibility of water vapor sounding of the cloudy boundary layer using a differential absorption radar technique

“…Prior study of DAR techniques have focused on use of frequencies near the 60 GHz oxygen absorption complex for surface pressure sounding (Flower and Peckham, 1978;Lawrence et al, 2011;Lin and Hu, 2005;Millán et al, 2014). The use of a differential radar signal to retrieve water vapor has previously been explored theoretically using a triplet of frequencies centered on the 22 GHz water vapor absorption line (Meneghini et al, 2005). Observed dual-frequency radar reflectivities using S/Ka band (Ellis and Vivekanandan, 2010) or Ku/W band (Tian et al, 2007) have also been used to infer water vapor profiles.…”

Water vapor estimates using simultaneous dual-wavelength radar observations

“…Padmanabhan et al [2009] use tomographic inversion of microwave brightness temperatures to retrieve high spatial and temporal resolution three-dimensional water vapor Figure 1. Illustration of three different radar based humidity estimates: the black arrows represent the path integrated dual-wavelength humidity retrievals proposed in this paper, the white arrows represent the triple wavelength retrieval through cloud proposed by Meneghini et al [2005], and the black dashed line represents the near-surface refractivity based retrieval of Fabry [2004].…”