ORCESTRA (Organized convection and EarthCare studies over the tropical atlantic)
During the summer 2024, several initiatives aiming at understanding the mesoscale organisation of tropical convection with field data and satellite observations took place over the tropical ocean. This international initiative, named ORCESTRA, included the French airborne field campaign MAESTRO (Mesoscale organisation of tropical convection), the German airborne field campaign PERCUSSION (Persistent EarthCare underflight studies of the ITCZ and organized convection), the German ship cruise BOW-TIE (Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) and the US initiative PICCOLO (Process investigation of clouds and convective organization over the tropical ocean).
The overarching objective of ORCESTRA is to better understand the physical mechanisms that organize tropical convection at the mesoscale, including the interaction of convective organization with tropical waves and air-sea interaction, and the impact of convective organization on climate and the Earth’s radiation budget and processes of tropical cyclogenesis. In addition to advancing understanding of tropical meteorology and atmospheric processes, ORCESTRA observations will to help calibrate and validate satellite remote sensing (especially EarthCARE) and a new generation of global ocean-eddy and storm-resolving climate models.
The ORCESTRA/MAESTRO field campaign
The ORCESTRA/MAESTRO campaign took place from Aug 10 to Sept 10 2024 over the Atlantic ocean near Cape Verde, with research aircraft operating out of Sal (16o4’N, 22o6’W).
The SAFIRE ATR-42 aircraft flew in the lower troposphere with a rich ensemble of in-situ and remote sensing instruments to characterize the interplay between coherent structures in the subcloud-layer, cloud properties near the cloud base level, the geometry of convective clouds and the horizontal and vertical distribution of water vapor and temperature in-between clouds. These observations were complemented by measurements from the HALO aircraft flying a large circle in the upper troposphere and dropping sondes intensively to characterize the dynamical and thermodynamical properties of the environment at the mesoscale (e.g. divergence and vertical motion), geostationary satellite measurements from which informations about clouds and vertical motions in the clear-sky atmosphere can be inferred, and EarthCARE measurements of radiative fluxes, precipitation, clouds and vertical motion within clouds.
MAESTRO/ORCESTRA built on experimental advances from the EUREC4A field campaign, and developed new experimental techniques to provide unprecedented observations of the convective atmosphere. In particular, the ATR-42 probed the atmosphere in multiple directions with a W-band pulsed Doppler cloud radar (RASTA) pointing up and down, a Doppler High Spectral Resolution Lidar (LNG) pointing up or down, a Doppler cloud radar (BASTA) pointing horizontally and, for the first time, a Raman lidar pointing horizontally (AWALI). These measurements were complemented by an ensemble of probes and sensors to measure temperature, humidity and winds at a fast rate and to characterize aerosols and cloud microphysics of liquid and ice clouds.
Most ATR flights consisted of a series of straight legs flown at different altitudes (near the surface, in the subcloud-layer, at cloud base and in the mid-troposphere around 6 km). The ATR usually five days a week. Each ATR flight is expected to lasted between 3-4 hours, including one hour during in which it flew in coordination with HALO. Some flights were devoted to the calibration and validation of the European Space Agency (ESA) EarthCARE satellite, which launched in Spring 2024. These flights took place as close as possible to the satellite orbit.
See below for additional details about the MAESTRO campaign.
MAESTRO Overview
The project aimed to collect the observations needed to:
- Test mechanisms hypothesized to control the mesoscale organisation of shallow and deep convection
- Assess the quality of the new EarthCARE satellite observations
- Understand how the organisation of convection impacts the Earth’s radiation budget through water vapour and clouds
- Assess the ability of high resolution models to predict the interplay between the mesoscale organisation of convection and climate
All types of mesoscale organization were of interest, including:
- Transitions among patterns of shallow convection
- Transitions from shallow to deep convection
- Convergence lines
- Cloud flowers (mini mesoscale convective systems)
- Early growth of mesoscale convective systems
- Isolated vs clustered convection
- Range of precipitable water regimes
- Northern edge of the ITCZ
Conceptual Framework
The guiding conceptual framework behind this study is that coherent structures in the boundary layer help dicate cloud geometry, which in turn controls the distribution of water vapor. Clouds and water impact the radiation budget, driving mesoscale circulations that act to create, reinforce, and evolve coherent structures. The goal is to measure the populations of these thermals and clouds and relate them to the mesoscale circulations they are embedded within.
Measurement Strategy
To measure these elements, the aircraft will be equipped with in-situ and remote sensing instrumentation.
A standardized flight pattern of about 1000 km over 3.5 hours was used to allow for statistical sampling. About 25 flights occurred over the entire campaign.
A typical flight began by departing from Sal, ascending to FL200 (~6000 m), and completing a 200 km transect. Then, the plane descended to cloud base to fly the transect twice (out and back). These two legs were sometimes flown with a slight offset so that the remote sensing measurements could be verified with in-situ measurements. Then, the plane descended to the sub cloud layer and flew one more transect. If conditions were favorable, the plane sometimes flew a portion of this leg near the surface (~100 m). Finally, the plane returned to Sal.
Some of these flights were supported the HALO aircraft flying a circle over the measurement area. Dropsondes were launched along this circle, allowing the mesocale vertical velocity to be computed.
Integration into ORCESTRA
There are strong connection to the other parts of the ORCESTRA campaign, especially space observations (EarthCARE CalVal, SAR, MTG, IASI), radiosondes, and GNSS data on Cape Verde.
Further Reading
Details on the scientific questions, flight strategy, and instrumentation will be available in a forthcoming MAESTRO overview paper