Stratocumulus Liquid Water Content from Dual-Wavelength Radar

Abstract

A technique is described to retrieve stratocumulus liquid water content (LWC) using the differential attenuation measured by vertically pointing radars at 35 GHz and 94 GHz. Millimeter-wave attenuation is proportional to LWC and increases with frequency, so LWC can be derived without the need to make any assumptions on the nature of the droplet size distribution. There is also no need for the radars to be well calibrated. A significant advantage over many radar techniques in stratocumulus is that the presence of drizzle drops (those with a diameter larger than around 50 µm) does not affect the retrieval, even though such drops may dominate the radar signal. It is important, however, that there are not significant numbers of drops larger than 600 µm which scatter outside the Rayleigh regime at 94 GHz. We use a lidar ceilometer to locate cloud base in the presence of drizzle falling below the cloud. An accuracy of around 0.04 g m −3 is achievable with averaging over one minute and 150 m (two range gates), but for the previously suggested frequency pair 10 GHz and 35 GHz, the corresponding accuracy would be considerably worse at 0.34 g m −3 . We first simulate the retrieval of LWC using aircraft-measured size spectra taken from a profile through marine stratocumulus. Results are then presented from two case studies, one using two cloud radars at Chilbolton in Southern England, and another using the Cloud Profiling Radar System at the Atmospheric Radiation Measurement site in Oklahoma. Liquid water path from the technique was found to be in good agreement with the values obtained from microwave radiometers, with the difference between the two being close to the accuracy of the radiometer retrieval. In the case of wellmixed stratocumulus the profiles were close to adiabatic.

G band atmospheric radars: new frontiers in cloud physics

“…The IPT then applies different Z-LWC relationships depending on the precipitation conditions in the cloud, though LWC uncertainties can nevertheless be larger 50 % and the retrieval of cloud droplet size is not possible in the presence of precipitation. Hogan et al (2005) proposed the use of dual-wavelength reflectivity (DWR) methods to profile liquid water clouds. In this case the accuracy is limited by the relatively small amount of dual-wavelength attenuation obtained when using frequencies at 35 and 94 GHz (the differential mass attenuation coefficient is roughly 4 dB km −1 per g m −3 ), which can be difficult to measure and usually requires significant averaging.…”

“…potential complete attenuation of the radar signal in rain after a few kilometres) it can also be exploited to provide water content profiles by using dual-frequency approaches. Hogan et al (2005) demonstrated water content profiling capabilities at a vertical resolution of 150 m for stratocumulus clouds with an accuracy of 0.04 g m −3 by employing the 35-94 GHz (8.6-3.2 mm) pair (when dwelling times longer than one minute are adopted). G band frequencies have the advantage of producing even larger dual-wavelength attenuation, with the possibility of more accurate profiling and of targeting thinner boundary layer clouds.…”

“…DWR is defined as DWR = 10 log 10 (Z i /Z j ) where Z i and Z j are the radar reflectivities measured at frequencies i and j where i < j . As noted in Hogan et al (2005) there are three error sources in the dual-wavelength absorption technique: (1) random errors associated with the reflectivity measurement; (2) errors in the temperature profile which affect uncertainties in the gas absorption profile; (3) the presence of non-Rayleigh targets within the radar backscattering volume. These three errors sources are now discussed separately.…”

“…The same procedure followed by Hogan et al (2005) is reproduced to show that random errors of 10 mg m −3 for 150 m vertical resolution and 1 min integration time are achievable when including the 35-220 GHz pair (Fig. 5) in correspondence to targets with the signal to noise ratio exceeding 0 dB.…”

Tomographic retrieval of cloud liquid water fields from a single scanning microwave radiometer aboard a moving platform – Part 2: Observation system simulation experiments

“…On the other hand, active remote sensing techniques like cloud radar (Frisch et al, 1995;Hogan et al, 2005) with rapid scanning capability provide a less direct measurement of cloud LWC (since LWC is proportional to the third moment of cloud drop size distribution but radar reflectivity is proportional to the sixth moment) and also would likely be much more costly than passive methods.…”