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<br />VOLUME 23
<br />
<br />1260
<br />
<br />JOURNAL OF CLIMATE AND APPLIED METEOROLOGY
<br />
<br />TABLE 1. Example values of Rand Z, for rain and snow.
<br />
<br />This would be an expression appropriate for estimating
<br />snowfall rates from radar measurements.
<br />Table 1 compares equivalent radar reflectivity fac-
<br />tors calculated for precipitation rates of 1 and 10
<br />mm h-1 for rain, using the Marshall-Palmer relation-
<br />ship
<br />Ze = 200R1.6, (15)
<br />
<br />and for snow, using (14). One should remember that
<br />here the precipitation rates for snow have to be
<br />expressed in terms of melted water equivalents. At
<br />R = 1 mm h-I, the Ze value for snow is 3 dB higher
<br />that that for rain. Two main factors contribute to the
<br />difference, in opposite senses. One is that ice is a
<br />weaker dielectric than water, which tends to reduce
<br />the reflectivities for snow. But the fall speeds of
<br />snowflakes are lower than those of raindrops, so the
<br />other factor is that larger sizes or greater concentra-
<br />tions of snowflakes are needed to achieve the same
<br />precipitation rate; that tends to increase the reflectiv-
<br />ities. At 1 mm h-l, the latter factor is evidently
<br />dominant.
<br />The more rapid increase of Ze for snow between 1
<br />and 10 mm h -I reflects the tendency for increased
<br />precipitation rates in snow to be associated with
<br />aggregation into larger flakes. In fact, the snowflake
<br />number concentration tends to decrease as R increases,
<br />whereas the opposite is true for the raindrop concen-
<br />tration. The larger aggregates have correspondingly
<br />greater radar cross sections because of the D6 factor
<br />in (2). That leads to an exponent higher in (12) than
<br />in (15), so that Ze increases more rapidly with R for
<br />snow than for rain.
<br />Statements are frequently made to the effect that
<br />radar echoes from snow are weaker than those from
<br />rain. Sometimes the difference is attributed to the
<br />weaker dielectric properties of ice. The foregoing
<br />discussion shows that the extent to which such state-
<br />ments are true must reflect a tendency for the precip-
<br />itation rates to be generally lower in snow more than
<br />any factor related to the scattering properties of the
<br />individual hydrometeors.
<br />
<br />4. Concluding.remarks
<br />
<br />This note is intended to aid in comparing radar
<br />and particle-size observations of snow or ice particles,
<br />or in using radar to measure snowfall. The main
<br />objective has been to clarify the differences between
<br />
<br />Precipitation rate R
<br />(mm h-')
<br />
<br />10
<br />
<br />Z, (rain) - dBz
<br />Ze (snow) - dBz
<br />
<br />23
<br />26
<br />
<br />39
<br />48
<br />
<br />Z and Ze for those situations, so that the calculations
<br />based on particle-size data can be correctly performed.
<br />The results apply for dry snowflakes or small ice
<br />particles only; for wet particles or sizable hailstones,
<br />the Marshall and Gunn argument mentioned in
<br />Section 1 is not valid. The treatment of those situa-
<br />tions is more complicated.
<br />
<br />Acknowledgments. This work was carried out under
<br />U.S. Bureau of Reclamation Contract 8-07-83-V0009
<br />and Grants ATM-8025598 and ATM-83 11 145 from
<br />the Division of Atmospheric Sciences, National Sci-
<br />ence Foundation. The author would like to acknowl-
<br />edge helpful discussions with A. S. Dennis, and useful
<br />comments from an anonymous reviewer regarding
<br />Section 1.
<br />
<br />REFERENCES
<br />
<br />Battan, L. J., 1973: Radar Observation of the Atmosphere. University
<br />of Chicago Press, 324 pp.
<br />Bohren, C. F., and L. J. Battan, 1980: Radar backscattering by
<br />inhomogeneous precipitation particles. J. Atmos. Sci.. 37,
<br />1821-1827.
<br />-, and -, 1982: Radar backscattering of microwaves by
<br />spongy ice spheres. J. Atmos. Sci., 39, 2623-2628.
<br />de Loor, G. P., 1983: The dielectric properties of wet materials.
<br />IEEE Trans. Geosci. Remote Sens.. GE-21, 364-369.
<br />Evans, S., 1965: Dielectric properties of ice and snow-a review.
<br />J. Glacial., S, 773-792.
<br />Gunn, K. L. S., and T. W. R. East, 1954: The. microwave properties
<br />of precipitation particles. Quart. J. Roy. Meteor. Soc., 80,
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<br />-, and J. S. Marshall, 1958: The distribution with size of
<br />aggregate snowflakes. J. Meteor.. IS, 452-466.
<br />Hendry, A., and Y. M. M. Antar, 1981: Ice crystal observations at
<br />1.8 cm wavelength using a polarization diversity radar. Pre prints
<br />20th Corif. Radar Meteor., Boston, Amer. Meteor. Soc., 579-
<br />585.
<br />Marshall, J. S., and K. L. S. Gunn. 1952: Measurement of snow
<br />parameters by radar. J. Meteor.. 9, 322-327.
<br />Sekhon, R. S., and R. C. Srivastava, 1970: Snow size spectra and
<br />radar reflectivity. J. A/mos. Sci.. 27,299-307.
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