An integrative approach towards understanding the life cycle of
deep convective weather systems
Group contribution:
Russchenberg)
ATMOS-RSE
group
(Y.
Dufournet,
H.W.R.
Observation challenges
From the process of nucleation to frontal system development, the life cycle of deep
convective systems spans several spatial and time scales, ranging from a few microns to
hundreds kilometers and from a few seconds to days respectively. A complete observation
of such a phenomenon is often based on strategies where synergies of several observations
techniques are employed.
One of the main challenges comes from the way microphysical properties of microscopic
sized cloud particles can be monitored at several kilometers above the ground within
convective clouds. Radar sensors are commonly used to observe and retrieve microphysical
properties of cloud hydrometeors as the radiowave signal, transmitted from the radar
through the atmosphere, is reflected without being fully attenuated by hydrometeors in a
way that depends on their phases, habits, densities, motions as well as their respective size
and orientation distributions. Based on a synergetic use of different cloud observation
sensor(s) and microphysical algorithm(s), Radar-based retrievals are later employed in order
to extract information from both sensors and models and convert them into microphysical
cloud properties. However, the amount of available information is often limited, which
forces current microphysical retrievals to base their algorithms on several microphysical
assumptions which affect the retrieval accuracy.
TARA observations
Central to our activity will be the TARA radar, owned by the Delft University of Technology.
The TARA radar is a ground-based sensor which aims to provide the microphysical and
dynamical properties of convective and stratiform cloud systems at high spatial and
temporal resolution in a single column of atmosphere around the radar location (profiling
radar). Because of its working frequency, a main focus is given to ice/mixed-phase clouds as
well as different precipitation regimes. The achieved time and range resolutions (up to 3m
range resolution and 3s time resolution) give the possibility of obtaining good
characterizations of small meso-scale atmospheric processes.
This radar is able to provide the standard radar measurements: reflectivity, radial mean
Doppler velocity, and Doppler width at each range gate of a vertical column of atmosphere.
Simultaneous Doppler and polarimetric measurements available on TARA also can be
performed at flexible time and spatial resolutions (Unal et al, 2004). Therefore, beside the
standard radar parameters, spectral polarimetric parameters are obtained, i.e., sZdr, sLdr and
sρco. Based on these parameters, unique cloud microphysical information can be obtained,
reducing the number of assumptions employed in current cloud retrievals.
Technical investigations
New radar-based retrievals will be created in order to improve convective cloud
microphysical information by investigating the potential use of radar spectral polarimetric
parameters:


Mid-level ice cloud dynamic: vertical and horizontal cloud particle motion (Unal et al,
2009).
Mid-level ice cloud microphysical properties: ice crystal type (cloud categorization),
particle orientation, IWC and particle size distribution,
Mid-level cloud retrievals and categorizations should also be created based on lidar-radar
combination techniques. For cloud observation, the difference in working frequency makes
radars and lidars sensitive to distinct ranges of cloud particle sizes and different scattering
regimes (Donovan and Lammeren, 2001). Synergy between radars and lidars are, therefore,
potentially very useful for cloud characterization. The Raman Lidar from KNMI (CAELI – Ar
noud Apituley) could be used to achieve this activity.

Precipitation: Drop size distribution and rain rate
Finally, several comparison / validation techniques will be considered depending on the
instrumental availability deployed in each observation sites (see next section). For example,
rain rates provided by both, the TARA radar and the network of disdrometers (TU DELFT,
Susan Steele-Dunne) could be considered.
Scientific investigations
Several scientific investigations related to the understanding of deep convection can gain
from the technical achievements mentioned in the last section:

Effect of dynamic on supercooled water droplet activation within mixed-phase
clouds: it is proposed to investigate how the magnitude of vertical updrafts acts on

mixed-phase cloud formation by changing the amount of water droplets activated
within already formed ice clouds.
Effect of dynamic on cloud glaciation time (effect on Bergeron Feidesen processes).
It is proposed to investigate how vertical updrafts can maintain a mixed-phase state
within the cloud as the magnitude of the updraft acts on the cloud glaciation time
(glaciation time increase as uniform ascent increase).
Both first items should be performed in collaboration with the Institute for
Troposhperic research (IFT, Leipzig) which already study these phenomena using the
large amount of microphysical information (particle phase in optically thin cloud
regions) and dynamical information (updraft in optically thin cloud regions) obtained
from Raman Lidar measurements (Buehl). For these activities, the Caeli lidar
measurements would be complemented with TARA measurements in order to
complement some missing microphysical (particle shape, orientation in optically thick
cloud regions) and dynamical key information (updraft in optically thick cloud regions)
which could not be obtained from Raman lidar measurements. With such a
complementary setup, vertical updraft as well as better particle phase categorization
(required to determine water droplet formation and glaciation time) can be obtained
from newly developed retrieval described in the ‘technical investigation’ section.


Effect of glaciation time on cloud dynamic (Stephan de Roode point): as a counter
effect, the glaciation processes release latent heat that is associated with phase
changes which will enhance the updraft velocity (Carey and Rutledge 2000).
Glaciation time and cloud dynamic can be obtained from lidar-radar combination
techniques as previously stated.
Effect of evaporation rate of precipitation on cold pool formation (and downdraft
velocities): as stated by Stephan de Roode ‘The evaporation of precipitating water
falling out of convective clouds leads to a local cooling and the subsequent formation
of downdrafts which transport this air towards the ground leaving clear signatures of
so-called cold pool structures. The amount of precipitating water that evaporates is
key to the further evolution of the cloud system’. Retrieving Droplet Size Distribution
within convective rain from new TARA algorithms would help to monitor evaporation
rate required to understand cold pool formation.
This activity will be performed in addition to the work of Stephan de Roode.
Resources
-
-
‘Test beds’ : two locations are proposed : The south western
Mediterranean area which will be instrumented during the HyMeX
campaign (www.hymex.org), Cabauw.
Collaboration: IFT(for the radar-lidar technique), HyMex participants
(tto1.f mainly), CESAR members (Arnoud), TU Delft (Stephan, Susan).
Bibliography
Donovan, D. and A. Lammeren, 2001: Cloud effective particle size and water content profile
retrievals using combined lidar and radar observations 1. theory and examples. J. Geophys.
Res., 106, 27,425 – 27,448.
Unal, C. and D. Moisseev, 2004: Combined Doppler and polarimetric radar measurements:
Correction for spectrum aliasing and nonsimultaneous polarimetric measurements. J. Atmos.
Oceanic Technol., 21, 443–455.