The U.S. Department
of Energy Proteus
research aircraft.
Peter May and Christian Jakob
Bureau of Meteorology
Jim Mather
Pacific Northwest National Laboratory
Geraint Vaughan
University of Manchester
Characterizing oceanic
convective cloud systems
Vaisala played an important
role in collecting one of the
most complete datasets ever
describing tropical convection
during the Tropical Warm Pool
International Cloud Experiment (TWP-ICE) in the area
around Darwin, Northern
Australia in January-February
2006.
The experiment examined convective
cloud systems from their initial stages
through to the decay of the cirrus generated, and measured their impact on the
environment, focusing on data critical
for the improvement of understanding
deep convective clouds in weather forecasting and climate models. This was
the first field program in the tropics
that describes the evolution of tropical
convection, including the large scale heat,
moisture, and momentum budgets, while
also obtaining detailed observations of
cloud properties and the impact of the
clouds on the environment.
The experiment included an unprecedented network of ground-based observations (soundings, active and passive
remote sensors), combined with a large
range of low, mid and high altitude
aircraft for in-situ and remote sensing
measurements.
16
|
Vaisala News 171 / 2006
A crucial product resulting from
the experiment is a dataset suitable to
provide the forcing and evaluation data
required by cloud resolving and single
column models as well as global climate
models (GCMs), and to aid in parameterization development. This dataset
will provide the necessary link between
the observed cloud properties and the
models that are attempting to simulate
them.
Large multi-agency
experiment
TWP-ICE is a large multi-agency experiment, including substantial contributions from the United States Department
of Energy (DOE) Atmospheric Radiation
Measurement (ARM) program, the ARM
Unmanned Aerial Vehicle (UAV) program,
the National Aeronautics and Space
Administration (NASA), the Australian Bureau of Meteorology, Australia’s
Commonwealth Scientific and Industrial
Research Organization (CSIRO), the UK
Natural Environment Research Council
(NERC), and many universities.
Tropical convection and cirrus
The timing of the experiment enabled the
sampling of both oceanic and coastal/
continental convection. Figure 1 shows
a time series of the average rainfall accumulation across the sounding domain.
Active monsoon conditions were
experienced early in
the exper-
iment. There were widespread storms
with cloud tops often extending to 16 km.
The entire region was covered by cirrus
almost continuously.
Following the passage of a large mesoscale convective system (MCS), a low
spun up to the south of the domain. The
storms in the area were more sporadic
and had tops only to about 10 km as dry
air was advected in from the south. An
active break period developed in the
latter part of the experiment with intense
afternoon storms and squall lines with
cloud tops above 18 km.
The experiment goal was to describe
the evolution of tropical convection,
including the large-scale heat, moisture,
and momentum budgets, while at the
same time obtaining detailed observations of cloud properties and the impact
of the clouds on the environment. A
major focus of the cloud component
was on cirrus. Cirrus clouds are ubiquitous in the tropics and have a large
impact on their environment. However,
the properties of these clouds are poorly
understood.
Another important component was
the measurement of a variety of active
chemical species generated within the
convection that are then transported
into the anvil clouds and detrained
into the upper troposphere and lower
stratosphere.
The Southern Surveyor
research vessel
As Darwin is a coastal site, it was important to characterize the oceanic region
off the Australia coast. For this
purpose, the CSIRO’s Marine
National Facility Research
Vessel, the Southern
Balmy tropical seas. Photo courtesy of Ron Plaschke, CSIRO.
Primary role/instruments
DOE Proteus
Active/passive remote sensing of
clouds and in-situ microphysical
measurements of cirrus
ARA Egrett
In-situ measurements of cloud
microphysics, aerosols and chemistry
in cirrus
NERC Dornier In-situ measurement of aerosols,
and chemistry in the boundary layer
and lower troposphere
Twin Otter
International
Cloud Radar and lidar observations
of clouds
ARA Dimona
In-situ state measurements, and
fluxes in the boundary layer
Table 1. TWP-ICE aircraft and
their principal roles.
400
Area averaged accumulation (Z=305R1.36)
“Active monsoon” “Dry monsoon”
350
Break
~7 mm/day
300
~5 mm/day
250
mm
Aircraft
150
Specific goals of the Tropical Warm
Pool International Cloud Experiment (TWP-ICE) were:
Jan. 23 MCS
45 mm in a day
200
~15 mm/day
100
50
0
IOP Start
15
20
25
30
35
Julian Day 2006
IOP End
40
45
Figure 1. Rainfall accumulation thru’
experiment period with IOP extending
from Jan 21 to Feb 13 (dashed lines).
A key component of the experiment was the
circular network of six stations launching Vaisala
Radiosondes RS92 every three hours
Surveyor, was stationed in the Timor Sea
to the northwest of Darwin.
The ship served as a launch site for
sondes, to complete the ring around
Darwin, and carried surface flux instruments. In addition to flux observations,
the ship carried radiation measurements,
a cloud radar and a lidar.
With the very active monsoon
followed by the development of the
deep low to the south, the ship experienced winds of more than 30 knots and
3 m seas for much of the first half of the
experiment. This was very challenging
for the sounding team releasing sondes
from a platform on the stern of the ship.
This unprecedented data set at a tropical
location will allow us to address fundamental questions such as what controls
the diurnal cycle of convection over the
open ocean.
Researching
tropical clouds
Energy System (CERES) observations
will provide top of atmosphere radiative fluxes and cloud properties over the
entire domain while more specialized
satellite products will be collected both
as an additional source of information
for understanding cloud processes and
to test satellite remote sensing retrievals
in much the same way that surface-based
retrievals will be tested.
The ground-based observational
network included two sites (one being
the research vessel) equipped with cloud
radar, lidar, and radiometry, profiler sites,
several surface flux sites, three hourly
radiosondes launched from six sites
within a 250 km diameter circle centered
on Darwin, as well as a Doppler weather
radar and a 5 cm wavelength polarimetric weather radar.
A key component of the experiment
was the circular network of six stations
Aircraft missions
launching Vaisala Radiosondes RS92
To fully describe cirrus cloud properties, every three hours for the intensive period,
both in-situ and remote sensing airborne
and an observing station in Darwin near
observations are required. The aircraft the centre launching sondes every six
involved and their principal payloads
hours. These sites were mostly remote
are listed in Table 1. These included an - an aircraft was required to visit two of
extensive set of in-situ cloud microphys- them, and the ship was located approxiical observations, detailed radiometry, mately 100 km off shore. Vaisala supplied
aerosol and chemistry measurements, the ground stations for the experiment.
as well as remote sensing with airborne
The statistics of the data gathering
radar, lidar, and an IR interferometer. are impressive – over 1000 radiosondes
More than 20 different aircraft missions were launched, 7000 weather radar
were flown.
volumes and more than 600,000 cloud
radar/lidar profiles collected as well as
Extensive data gathered
over 20 aircraft missions flown during
Satellites will be important for character- the experiment.
izing the upper boundary of the experiment region. Geostationary satellite
and the Clouds and the Earth’s Radiant
1) Collect detailed measurements
of the cirrus microphysics
and relate them to storm
intensity and proximity (spatial
and temporal) to the parent
convection.
2) Verification of retrievals of
microphysical properties from
satellites and the ground.
3) Provide datasets for forcing
cloud resolving and single
column models that will
attempt to simulate the
observed characteristics and
impacts.
4) Document the evolution of
oceanic convective clouds from
the early convection phase
through to the remnant cirrus
with particular emphasis on
their microphysics.
5) Measure the dynamical and
radiative impacts of the cloud
systems.
6) Measure chemical species
generated within the convection
that are important for both the
radiation budget and ozone
chemistry.
7) Characterize the environment
in which the cloud systems
occur.
8) Document the evolution of
the convective boundary layer
throughout the diurnal cycle
and through the lifecycle of
convective systems.
9) Observe the characteristics of
convectively generated gravity
waves.
Further information:
www.bom.gov.au/bmrc/wefor/
research/twpice.htm
www.science.arm.gov/~mather/
darwiniop/
www.ozone-sec.ch.cam.ac.uk/scout_o3/
171 / 2006 Vaisala News
|
17