TWP-ICE

Tropical Warm Pool - International Cloud Experiment (TWP-ICE)

21 January 2006 - 13 February 2006

Lead Scientist: Peter May

Observatory: TWP

The Tropical Warm Pool – International Cloud Experiment (TWP-ICE) was an intense airborne measurement campaign conducted in the region near Darwin, Northern Australia in early 2006. The experiment was a collaborative effort 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. TWP-ICE aimed to describe the properties of tropical cirrus and the convection that leads to their formation. 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 was 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 provided the necessary link between cloud properties and the models attempting to simulate them. This experiment was undertaken over a four week period in early 2006. January/February corresponding to the wet phase of the Australia monsoon. This season was 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 was expected that the convection and cloud characteristics would be representative of conditions typical for wide areas of the tropics. Observations of cloud properties were provided by an extensive set of in situ and remote sensing instruments. The Darwin ARM site provided continuous measurements of cloud properties through remote sensing retrievals. A second set of instruments was deployed on a ship in the Timor Sea, approximately 100 km northwest of Darwin. Added to these surface-based observations were 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 fulfilled a variety of observational requirements to address the science goals. The strategy for utilizing these aircraft included obtaining in situ observations of cirrus properties combined with remote sensing observations obtained from above and below the clouds. In addition, a boundary layer aircraft was used to characterize radiative and turbulent fluxes in the boundary layer. The boundary layer observations were part of the effort to characterize the convective environment in which the cirrus are formed. Other observational components geared toward describing the environment included a network of radiosonde stations encircling Darwin, a pair of precipitation radars, wind profilers, and surface flux stations. Darwin is a coastal site and it was important to characterize the oceanic region off the Australia coast. For this purpose, the CSIRO research vessel Southern Surveyor was stationed in the Timor Sea to the northwest of Darwin. This ship served as a launch site for sondes, to complete the ring around Darwin and also carried surface flux instruments. In addition to flux observations, remote sensing instruments were also planned for the ship. The addition of these instruments provided a complete second surface site. This capability increased the likelihood of obtaining good coordinated surface/aircraft observations. 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, formed a major component of the experiment activities. Those activities included pre-experiment model evaluation, NWP support during experiment and a large post-experiment program that made use of the collected data in evaluating and improving the representation of cloud, convective and radiative processes in the models. The ARM project takes long term observations to provide data to address the cloud characteristics and their radiative effects. TWP-ICE provided important in situ data to both verify the remote retrievals and provide high resolution in situ data that cannot be remotely sensed. The experiment also provided an invaluable description of the convective environment. Together, the observations of cloud properties and the atmospheric state make this a data set unique for improving cloud parameterizations in the tropics.