Liquid-water content retrieval techniques

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Differential attenuation and dual-band techniques-continue

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study because it is believed to regulate the thermody-

5.3.1.5 Liquid-water content retrieval techniques.

namic structure and radiative coupling of the marine

5.3.1.5.1 Single-band retrieval techniques. Frisch et

boundary layer (Frisch et al. 1995). Lhermitte (1987)

al. (1995) develop theory and demonstrate, using the

developed theory for W-band radar and demonstrated

NOAA ETL radar (Martner and Kropfli 1993), the

some of its capabilities as a Doppler radar, but he did not

retrieval of drizzle and cloud droplet parameters with a

measure cloud liquid-water content.

Ka-band radar and a radiometer for measuring integrated

A University of Massachusetts 95-GHz dual-polarized

liquid water. Doppler techniques were used to measure

radar for ground-based and airborne use flies on the

the drizzle drop-size spectra, and drizzle and cloud

University of Wyoming King Air research aircraft (Mead

liquid-water content were measured from reflectivity.

et al. 1994). It has a demonstrated range of 0.1 to 2.9 km

Cloud liquid-water content was computed by using

and has observed 1- to 2-mm graupel with some rimed,

Doppler velocities of drizzle and cloud to parse the two

branched crystals, crystal aggregates up to 4 mm diame-

liquid-water contents. Overall, by using Ka-band radar,

ter, and needles up to 1 mm diameter. In a study with

Doppler features, and a simple drizzle model, the

this radar, a 30-km segment of shallow stratus produc-

authors were able to extract drop number, size distribu-

ing freezing drizzle was flown. The radar beam, point-

tion, liquid-water content, and mean liquid-water flux.

ing vertically, observed detailed cloud structure at 30-m

They indicate that there is a potential for ground-based

resolution, and radar backscatter was compared to cloud

remote sensors to do long-term monitoring of cloud and

parameters measured with in-situ instruments. The radar-

drizzle parameters, such as at airports. This radar sys-

enhanced interpretation of in-situ measurements was not

tem in a scanning mode, together with a RASS for

itself used to measure specific cloud physical parame-

measuring temperature profiles, may be an adequate

ters. A similar radar, built by the University of Massa-

airport-based system.

chusetts and the NASA Jet Propulsion Lab, the Airborne

Liao and Sassen (1994) have also developed a tech-

Cloud Radar, flies on a NASA DC-8. More than 50 hours

nique, although only as modeled theory, that allows

of testing has occurred in cirrus, stratus, and cumulus

extraction of cloud liquid-water content by linking

clouds, and melting layers have been observed (GEWEX

liquid-water content and reflectivity, assuming a drop

1996).

concentration of 100 cm3. The model applies for esti-

In other W-band applications, Klugmann and Judasch-

mating liquid-water content in nonprecipitating cumulus

ke (1996) have developed a W-band Doppler radar in

and stratocumulus clouds. They also developed theory

Germany to measure vertical velocities within clouds by

for extracting ice water contents from clouds with Ka-

tracking drop speeds. Clothiaux et al. (1995) also

band reflectivity.

explored the use of W-band radar combined with other

remote sensors. They indicate that, when pointed at

5.3.1.5.2 Differential attenuation and dual-band tech-

niques. Combining two radar frequencies and analyz-

zenith, W-band radar is valuable for mapping cloud base

ing cloud liquid-water content using differential atten-

and top, but base is often indicated as too low because of

uation has become a preferred method of measuring

precipitation. Though methods were not developed to

range-resolved cloud liquid water. According to Mart-

compute cloud liquid-water content from the radar, in-situ

ner et al. (1993a), using two radar wavelengths with

measurements of liquid-water content were compared

significantly different liquid-water attenuation coeffi-

with the radar calibration, and calibrations were consis-

cients to measure cloud water and ice content was first

tent with in-situ measurements.

theorized by Atlas (1954). Martner et al. (1991) and

Sassen and Liao (1996) developed theory for meas-

Gosset and Sauvageot (1992) independently developed

uring the contents of ice and water clouds from W-band

field tests and additional theory, concluding that the best

radars. They indicate that for most cloud drops, Rayleigh

radar wavelengths were in the X and Ka bands. Sand

scattering applies in W band, and they provide algorithms

and Kropfli (1991) patented the concept. According to

for computing cloud liquid and ice content from reflec-

Martner et al. (1993a), as the radar beams enter a cloud,

tivities. Fox and Illingworth (1997a,b) computed W-band

the Ka band is attenuated more rapidly than the X band.

reflectivity and cloud liquid-water content from more

Assuming Rayleigh attenuation, the range derivative

than 4000 km of flight in-situ drop spectra measurements

of the difference of the reflectivities is proportional to

by aircraft. They computed the probability of detecting

the liquid-water content. Both water and ice contribute

various values of liquid-water content as a function of

to the reflectivity. However, according to Martner et al.

radar sensitivity. Computations were complicated by the

(1993b), only the liquid water generates the differen-

predominance of drizzle drops in marine stratocumulus.

tial attenuation needed to compute liquid-water content.

They concluded that a highly sensitive space-based ra-

Thus, the attenuation due to ice is the same for both

dar could detect 100% of all marine and stratocumulus

bands--very small.

clouds, but it was not clear if cloud liquid-water content

According to Gosset and Sauvageot (1992), the dual-

could be directly measured.

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