Influence of the ice phase on Z-R modes
Marco Clemens, Felix Ament, Gerhard Peters
Meteorologisches Institut
Universität Hamburg
Bundesstraße 55
D-20146 Hamburg
Recent studies with high-resolution micro-rain radars during the Intensive Observing Period
of the German AQUARadar project show almost homogeneous periods with respect to the
empirical relationship between radar reflectivity factor Z and rain rate R, so called Z-R
modes. These modes change instantaneously from one state into another one (Clemens et
al. 2008). While many authors associated Z-R modes with the existence or nonexistence of a
brightband being a criterion for stratiform or convective rainfall (Nzeukou et al. 2004, Amitai
2000, Tokay and Short 1996, Rosenfeld et al. 1995), the signature of the brightband is not a
sufficient criterion to explain these observed changes. Rather accompanying modifications of
the spectral shapes of the drop size distributions indicate variations in the precipitation
forming microphysical processes.
There is common consensus that water in the ice phase above the brightband has a
determining influence on the forming processes of precipitation. We therefore hypothesize
that the behavior of Z-R modes is largely controlled by ice particles in the upper part of a
cloud. Deeper insight in these mechanisms will allow for an identification of modes and will
consequently enable better retrievals of surface rain rate by weather radar systems, which
require stable Z-R relations.
Hence, it is necessary to gain more knowledge about the vertical distribution as well as the
temporal behavior of the microphysical properties of atmospheric water in its solid phase
above, as well as in the transition to the liquid phase within the melting layer, as they
describe the seeding mechanisms for precipitation. Considering the ice phase, the so-called
secondary production process is still not well understood: The shedding of splinters during
riming ice crystals in a temperature range of -3 to -8°C, known as the Hallett-Mossop process
(Hallett and Mossop 1973), seems to be the most commonly invoked mechanism especially
in cumulus and stratus clouds, as reported by Bower et al. (1996). A secondary maximum in
the production of small ice crystals in high concentrations was observed by Mason (1998). It
is attributed to the transport of small ice splinters produced in the Hallet-Mossop zone that
grow at small ice supersaturations in weak updraughts between -8°C and -12°C levels, and
thereafter at water saturation in convective cells between -12°C and -15°C. As a
precondition for the initiation of the Hallet-Mossop process, supercooled cloud liquid water is
needed, which is also considered as a significant indicator of precipitation potential having
relevance for weather modifications (Sassen et al. 1990).
Consequently, the proposal has one general and one specific objective: We want to
contribute to a better understanding of processes related to the ice phase and precipitation,
and we wish to analyse specifically the relation of the ice phase and Z-R modes.
To achieve these objectives, vertically pointing instruments using different measuring
techniques (frequencies) for the remote sensing of the cloud parameters are needed. Here
the vertically measuring 24.1 GHz micro-rain radar in combination with cloud radar (35.5
GHz) permits an entire vertical coverage from ground level up to 10 km height with a fine
temporal, spatial as well as spectral resolution. Data of co-located micro-rain radar and cloud
radar obtained during a time period of several years, accompanied by ceilometer data and by
temperature, wind and humidity profiles throughout the troposphere are available at the
Richard Assmann Observatory (RAO) Lindenberg of the German Meterological Service
(DWD) (Lindenberger Säule). We plan to expand this data set by simultaneous observations
of polarimetric scattering properties in the melting layer using pairs of tilted micro-rain-radars,
by monitoring horizontal inhomogeneities with a second cloud radar deployed at the
experimental site in Falkenberg and by continuous scanning of rain fields in the profiler
environment with an X-band local area weather radar.
Planned Work packages
 Statistical analysis of relation between ice phase and Z-R modes (UHH)
Time series of Z-R modes and their abrupt changes can be derived from MRR
observations (Clemens et al 2008). These time series will be used as a reference
to identify signals in the cloud radar observations in the melting layer and above,
which are highly correlated with the evolution of the Z-R relations. In this respect,
a novel target classification algorithm, based on a multipeak analysis of the
retrieved microphysical characteristics by the cloud radar (type Metek MIRA 36)
will be used. Preliminary application of this multipeak analysis (Melchionna et al.
2008) suggests the possibility of separate detection of co-existing particles in a
liquid and an ice fraction above the melting layer. Both fractions can be identified
by their particular vertical structure of the falling velocities as pointed out by
Zawadzki et al.( 2001): While ice particles show increasing velocities passing the
melting layer due to phase changes, the velocities of the liquid particles remain
nearly constant
 Statistical analysis of ice processes (UHH)
The comprehensive data set of simultaneous retrievals of particles sprectra from
radar observations and of ambient conditions (temperature, humidity and wind
profile) will allow an evaluation of existing parameterizations of ice phase
processes, like e.g. the Hallet and Mossop effect.
 Providing cloud radar and MRR measurements (UHH)
We will provide observational data – if needed - from the cloud radar and MRRs
to other groups of the Forschergruppe along with assistance in data interpretation.
 Exploration of advanced measurement techniques (UHH + partners)
(to be refined) While the differential reflectivity at 24 GHz shows nearly no signal
in the pure liquid phase, recent polarimetric measurements with tilted micro-rain
radars revealed a significant signature in the melting band (to be published). The
interpretation in terms of particle size, shape and phase-composition needs input
on scattering theory from other co-projects of this proposal.
 Z-R mode detection based on polarimetric C-Band weather radar data (UHH +
partners)
Based on the findings of the first work package, it will be explored whether areas
of homogeneous Z-R modes can be detected from the polarimetric information of
the upcoming C-band weather radars of German Weather Service. This detection
can be used to extrapolate spatially the estimated Z-R relation of a MRR at a
single point. This will enhance the quantitative accuracy of surface rain rates
estimates from weather radar systems.
 Comprehensive assimilation of observations in a spectral microphysical model (
UHH + partners)
Even the envisioned comprehensive set of instruments can not provide the full
information about the distribution of various hydrometeors depening on time,
height and their size, which is ideally required for process studies and hypothesis
testing. Data assimilation provides an elegant framework to narrow this gap
between these demands and the limited observational information: A spectral
microphysical model (e.g. Khain and Sednev, 1996) is constrained by the
observations by requiring a close match between simulation and measurements.
Such efforts (e.g. Benedetti et al 2003, Delanoe and Hogan, 2008) are very
challenging and can only be realized as joined initiative of the Forschergruppe: at
least, contributions in the fields of data assimilation (probably UHH), forward
operators and microphysical modeling are needed.
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