Tropical Warm Pool
International Cloud Experiment
Cloud and rain characteristics in the Australian Monsoon
Peter T May, Christian Jakob, and James H. Mather
With contributions from
The impact of oceanic convection on its environment and the relationship between the characteristics of the convection and the resulting cirrus characteristics is still not understood. An intense airborne measurement campaign combined with an extensive network of ground-based observations is being planned for the region near Darwin, Northern Australia during January-February, 2006 to address these questions. The Tropical Warm Pool – International Cloud Experiment (TWP-ICE) will be the first field program in the tropics that attempts 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. The emphasis will be on cirrus for the cloud properties component of the experiment. Cirrus are ubiquitous in the tropics and have a large impact on their environment but the properties of these clouds are poorly understood. A crucial product from this experiment will be a data set suitable to provide the forcing and testing required by cloud resolving models and parameterizations in Global Climate Models (GCMs). This data set will provide the necessary link between cloud properties and the models that are attempting to simulate them.
The experiment is a collaboration between the US DOE ARM project, the Bureau of Meteorology, NASA, the European Commission DG RTD-1.2 and several United States, Australian, Canadian and European Universities. This experiment will be undertaken over a four week period in early 2006. January/February corresponds to the wet phase of the Australia monsoon. This season has been selected because, despite Darwin’s coastal location, the convection that occurs over and near Darwin at this time is largely of maritime origin with a large fetch over water. Based on previous experiments, the convection appears typical of maritime convection with widespread convection that has complex organization, but is not as deep or as intense as continental or coastal convection. Therefore, it is expected that the convection and cloud characteristics will be representative of conditions typical for wide areas of the tropics.
TWP-ICE seeks to build on a number of past experiments. TOGA-COARE was an extensive tropical campaign that focused largely on the organization of deep convection and on processes associated with the ocean/atmosphere interface. More recently, the CRYSTAL-FACE and Emerald experiments have focused on deep organized convection systems typical of coastal areas. In both of these experiments, the emphasis was on observing properties of cirrus clouds and processes of the upper troposphere. There was some effort during CRYSTAL-FACE to describe the convective environment through the use of the Eldora radar, ground based weather radars and radiosondes; however, the emphasis of the experiment was on upper troposphere characterization rather than convective processes.
The Darwin experiment is being designed to include the characterization of cirrus and the upper troposphere along with the convective environment. This dual focus is extremely important. A primary reason for ARM’s attention to tropical cirrus is to improve the representation of these clouds in climate models. While characterization of tropical cirrus will be valuable on its own, as few such observations are currently available, this information without corresponding information about the processes that led to the cirrus formation will be of limited use to cloud modelers. The dual focus if TWP-ICE will result in a data set that can be used to address a range of important scientific questions related to tropical clouds. Specific goals of the experiment are:
Make detailed measurements of the cirrus microphysics and how they relate to storm intensity and proximity (spatial and temporal) to the parent convection.
Verification of remotely sensed microphysical measurements.
Provide data sets for forcing cloud resolving and single column models that will attempt to simulate the observed characteristics and impacts.
Document the evolution of oceanic convective clouds from the early convection phase through to the remnant cirrus with particular emphasis on their microphysics.
Measure the dynamical and radiative impacts of the cloud systems.
Characterize the environment in which the cloud systems occur.
Document the evolution of the convective boundary layer throughout the diurnal cycle and through the lifecycle of convective systems.
The European participants will also focus on issues such as troposphere/stratosphere exchange, and the water budget of the lower stratosphere while there will be a large chemistry component from NASA focusing on short-lived species generated in the convection. Other groups will also be adding smaller components focused on specific areas of research, such as convectively generated gravity waves, that can use the data gathered by ARM synergistically, but are not core areas of the program.
Observations of cloud properties will be provided by an extensive set of in situ and remote sensing instruments. The Darwin ARM site will provide continuous measurements of cloud properties through remote sensing retrievals. A second set of instruments will be deployed on a ship in the Timor Sea, approximately 100 km northwest of Darwin. Added to these surface-based observations will be several research aircraft including the NASA WB-57, the DOE Proteus, the DLR Falcon and the Geophysica, as well as three aircraft provided by Airborne Research Australia (ARA): the Egrett, King Air and Dimona. These aircraft fulfill a variety of observational requirements to address the science goals.
To fully describe cirrus cloud properties, both in situ and remote sensing airborne observations are required. The WB-57 is primarily an in situ platform while the Proteus is capable of making both in situ and remote sensing measurements. The Geophysica is again a primarily in-situ platform, but includes accurate water vapor measurements and has by far the highest operating ceiling (>20 km) of the aircraft in the campaign. With a ceiling of approximately 15 km, the Proteus will be able to fly above much of the cirrus thereby providing a very useful top-down view of the cloud systems. The King Air will complement the Proteus by adding remote sensing from the middle troposphere with both lidar and 94 GHz cloud radar while the Falcon carries a lidar and sensors for in-situ measurements of cloud microphysics. In some cases, cloud attenuation will preclude the Proteus instruments from observing cloud properties through the column, a bottom-up view from the King Air will help fill this gap. The Egrett is primarily an in situ aircraft with a somewhat lower ceiling than the WB-57 or Proteus. The addition of the Egrett adds considerable flexibility for spatial sampling. Finally, the Dimona is a small boundary layer aircraft. The purpose of this aircraft is to characterize surface turbulent and radiative fluxes throughout the experiment region. This aircraft is one part of the set of observations needed to describe the convection environment.
The observation strategy for describing the convective environment includes several elements: a network of radiosonde stations, a pair of precipitation radars, wind profilers, surface flux stations, and the boundary layer aircraft. A ring of five radiosonde stations will encircle the station at Darwin. The Darwin sonde station is co-located with the BOM facility and the ARM site. Sondes will be launched with very high temporal resolution (3-hourly launches) from all six sites to provide a detailed description of the experiment region’s lateral boundary and core. This will be an unprecedented data set for a tropical location. The radius of this sonde network will be 100-150 km. This dimension was selected to match the range of the dual Doppler radars stationed near Darwin. These radars will provide a 3-dimensional view of convection intensity throughout the Darwin region. Turbulent and radiative fluxes at the lower boundary will be characterized with instruments at selected sites including a ship and with observations from the boundary layer aircraft. The instrumented sites will be positioned to characterize diverse surface types while the aircraft will provide the means to apply these point measurements to the larger region.
Darwin is a coastal site and it will be important to characterize the oceanic region off the Australia coast. For this purpose, the CSIRO research vessel Southern Surveyor will be stationed in the Timor Sea to the northwest of Darwin. This ship will serve as a launch site for sondes, to complete the ring around Darwin and will also carry surface flux instruments. In addition to flux observations, remote sensing instruments are also being considered for the ship. The addition of these instruments will provide a complete second surface site. This capability will increase the likelihood of obtaining good coordinated surface/aircraft observations.
Finally, satellites will be important for characterizing the upper boundary of the experiment region. Geostationary observations will provide top of atmosphere fluxes 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. Of particular interest will be comparisons of in situ aircraft observations of cirrus properties with retrievals from CloudSat and Calypso which are both due to be launched in early 2005.
The occurrence of tropical clouds in the form of shallow non-precipitating cumulus to deep precipitating cumulonimbus with extensive anvils have a significant and fundamental impact on the atmospheric energy balance. Understanding the properties of these clouds and parameterizing their impact within climate prediction models is one of the major challenges facing the international community. It is well recognized that the representation of these process is a major uncertainty in our ability to represent and predict climate. In that context modeling activities, including the application of Cloud Resolving and Single Column Models as well as a suite of Numerical Weather Prediction models, will form a major component of the experiment activities. Those activities include pre-experiment model evaluation, NWP support during experiment and a large post-experiment program that will make use of the collected data in evaluating and improving the representation of cloud, convective and radiative processes in the models.
The ARM project has been taking long term observations to provide data to address the cloud characteristics and their radiative effects. TWP-ICE will provide important in situ data to both verify the remote retrievals and to provide high resolution in situ data that cannot be remotely sensed. The experiment will also provide an invaluable description of the convective environment. Together, the observations of cloud properties and the atmospheric state will make this a data set unique for improving cloud parameterizations in the tropics.