AS SNOW CHANGES TO RAIN: UNDERSTANDING THE VICISSITUDES OF
ELECTROMAGNETIC SCATTERING THROUGH THE MELTING LAYER (FROM
ABOVE AND BELOW)
Kenneth Sassen
Geophysical Institute, University of Alaska Fairbanks
Fairbanks, Alaska 99775
Email: ksassen@gi.alaska.edu
1. INTRODUCTION
An unexpected consequence of
the transformation of hydrometeors
undergoing the phase change in the
melting layer is what World War II
microwave radar operators called the
bright band. Literally, a bright signal
band sometimes appeared on the radar
set oscilloscope displays of their time
during rainfall. It was soon recognized
by the new breed of post-war radar
meteorologists that this phenomenon
was
attributable
to
certain
characteristics of melting snowflakes,
which briefly enhanced radar returns.
Relatively simple models that treated
the changes in hydrometeor size,
fallspeed (and thus concentration), and
ice/water content were able to
reproduce the salient features of the
radar bright band (Battan 1973; Dennis
and Hitchfield 1990; Fabry and
Zawadski 1995). Because of the large
increase in the dielectric constant (and
hence
Rayleigh
backscattering)
between ice and water particles, the
influence of the melt water was
considered to be a major factor, along
with the gradual decrease in particle
concentrations as snowflakes changed
to faster-falling raindrops. Doppler radar
vertical velocity profiles through the
melting layer soon confirmed this basic
conception. Laboratory and field studies
of the composition of the melting layer
were also spurred by attempts to better
understand bright bands (e.g., Stewart
et al. 1984; Mitra et al. 1990; Oraltay
and Hallett 2005).
At least this was the early view
when
predominantly
centimeterwavelength weather radars were used
to probe precipitation. Since the 1950s,
however, new research tools have
discovered
new
electromagnetic
scattering features associated with the
melting region. For example:
2. LIDAR ANALOGS
Early lidar studies detected a
bright band analog (i.e., a relatively
narrow signal spike) under some melting
layer conditions that was attributed to
strong optical backscattering and
overwhelming attenuation in the larger
snowflakes- this is simply a particle
density effect, not a dielectric one
(Sassen 1977a).
Like polarization
radars, lidars found variations in
depolarization in the melting layer due to
special nonspherical particle scattering
effects (Sassen 1975).
Lidars later
discovered a pronounced dark band
near the bottom of the melting layer
(Sassen and Chen 1995), apparently a
result of the backscattering behavior of
mixed-phase raindrops with ice blocking
the central retro-reflected internal ray
path (Sassen 1977b). (Wet lidars were
something to be avoided, previously!)
3. MILLIMETER-WAVE RADAR DARK
AND BRIGHT BANDS
The situation at millimeter radar
wavelengths is, in comparison to
weather radars, chaotic. Measurements
of rain at K-band (~10-mm) radar only
occasionally show a bright band, while
those at W-band (3.2 mm) may never
and sometimes even detect a weak dark
band at the top of the melting layer
(Sassen et al. 2005). Clearly, nonRayleigh scattering effects at these
wavelengths are coming into play in a
major way because snowflakes (and
many raindrops) are too large to behave
as Rayleigh scatterers. Theories to
explain the still-debated W-band radar
dark band can be divided into two
distinct groups. The first believes that
the
well-known
Mie
theory
backscattering oscillations for particles
with sizes of about the incident
wavelength
(due
to
scattering
resonance effects) cause depressions in
radar
reflectivity
from
growing
snowflakes of just the right size
(Lhermitte 2002; Kollias and Albrecht
2005). A related W-band radar dim
band was attributed to a combination of
this effect with a specified snowflake
density-versus-size relationship that
strongly limited radar reflectivities in the
Mie regime (Heymsfield et al. 2008),
although this affect is not directly tied to
the melting layer. The other approach
to account for the W-band radar dark
band involves treating barely-wet
snowflakes as concentric water-coated
ice spheres, with the backscattering
reductions coming from the reverse
dielectric effect predicted by Mie theory
(Sassen et al. 2005).
4. THE VIEW FROM SPACE
With the advent of spaceborne
radar observations of precipitation
(TRMM, Simpson et al. 1996) and
clouds (CloudSat, Stephens et al. 2003),
melting layer effects are being examined
from
the
top-down.
Although
conventional TRMM radar bright bands
are commonly observed, surprisingly so
are apparent CloudSat W-band radar
bright bands, but only in observations
from above (Sassen et al. 2007).
Analogous to the lidar bright band, this
feature has been attributed to increasing
microwave backscatter followed by
strong extinction, not in the snow above,
but the rain below in this case (Matrosov
2007).
5. CONCLUSIONS
These findings at several
radar/lidar wavelengths have put new
constraints
on
melting
layer
microphysical and scattering theories,
which will be discussed to see if our
current understanding of the bright and
dark bands are consistent with the
microphysics of precipitation (and vice
versa), particularly with regard to the
evolution of the particle size distribution.
We suggest that an additional tool that
should be applied to completing our
understanding
of
melting
layer
microphysics
and
scattering
are
scanning
polarization
lidar
measurements (Roy and Bissonnette
2001), which will reveal through
backscattering anisotropy further details
of the evolution of hydrometeor shape
and orientation in the melting region.
Acknowledgements. This research is
supported by NSF grant ATM-0630506.
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