How well can ARM measure liquid water path during precipitation?

Poster PDF

Description

Column integrated liquid water path (LWP) measurements are a critical parameter for cloud and precipitation studies. On one end of the spectrum (thin clouds), accurate measurements of low LWP values are needed to accurately assess the radiative effect of clouds and parameterize their propensity to precipitate (e.g., autoconversion schemes). A number of retrieval techniques that combine active (e.g., dual-wavelength radar) and passive (infrared and microwave) techniques have been developed to improve our ability to measure low values of LWP. On the other end of the spectrum (deep precipitating clouds with high liquid water amount), the ARM program upgraded its radar facilities with the intent to measure precipitation. A number of radar-based techniques that can provide estimates of the rain water path and in-situ ground-based sensors that measure rainrate and raindrop size distributions at the surface are available. However, our ability to measure the LWP when a measurable amount of precipitation reaches the ground is limited. During precipitation microwave radiometer (MWR) measurements become unreliable due to wetting of the sensor. In addition, non-Rayleigh effects can introduce biases to the measurements. The lack of LWP measurements will hinder our ability to conduct closure studies and investigate precipitation efficiency at the ARM sites. Here, first, we investigate at what point, the MWR LWP measurements become unreliable. Second, we investigate the potential of radar-based techniques to provide an estimate of the LWP using Path Integrated Attenuation (PIA) techniques based on multi-wavelength radar observations that account for the wet radar radome effects. Furthermore, a preliminary analysis of the feasibility of using the calibrated sky brightness noise observed at every time profile by the Ka-band ARM Zenith-pointing Radar (KAZR) to estimate LWP during precipitation is presented.