What role does convective aggregation play in climate – this is one of four key questions formulated by the World Climate Research Program (WCRP) initiative Grand Science Challenge on Clouds, Circulation and Climate Sensitivity. Moist convection can be organized in very different ways – from randomly scattered shallow clouds up to mesoscale cloud clusters or even large-scale systems affecting the planetary circulation. There is evidence both from observations and from numerical simulations that the degree of organization affects the overall stratification of the atmosphere: an atmosphere in which convection is more aggregated is drier, clearer and, thus, more efficient at radiating energy to space. Recent numerical studies have established the concept of self-aggregation which occurs even without any external driver. This process might intensify with increasing temperature and may, thus, be sensitive to climate change. These findings are based solely on large-eddy resolving numerical simulations. Comprehensive and highly resolved observational records are needed to constrain and evaluate such numerical simulations.

NARVAL 2 will contribute to this very timely issue by analyzing organized convection in the deep tropics in the vicinity of Barbados. This region is designated by weak external forcing. HALO will be stationed at Barbados for a four weeks period in August 2016 to perform targeted observations of organized connective systems. If possible, we will follow each system by a series of flights at subsequent days to trace the systems life cycle. The focus will be on non-rotating systems, since up to now they have been paid less attention to compared to larger systems evolving into Hurricanes. We intend to describe both the organization of the convection itself as well as the status of the ambient boundary layer. Disentangling the relation between convective aggregation and boundary layer evolution is an important key to assess the climate sensitivity of organized convection.

Stationing HALO on Barbados close to the target area minimizes the time of transfer flights and allows flying at moderate altitude to enhance the sensitivity of all remote sensing instruments. In addition we will benefit from technical and scientific experience of the South part of the first NARVAL campaign. NARVAL 2 will expand the statistics of the first campaign and will again broaden the long-term measurement program of the Barbados cloud observatory by adding a spatial component. The campaign will be led by the Max-Planck-Institute for Meteorology, which envisions investing 100 flight hours. The instrumentation will be identical to the first NARVAL campaign to reuse the certifications and the well-proven setup. However, the campaign is open to other groups and can be expanded by related science topics and contributions.

NARVAL 2 is closely related to NAWDEX, which will take part in autumn 2016 using exactly the same aircraft configuration. The missions feature also a scientific overlap: NAWDEX will study cyclones as mature convective systems over the extratropical Northern Atlantic. Expanding the science questions of NARVAL 2 to other regimes and different regions will support the WCRP Grand Science Challenge on Clouds, Circulation and Climate Sensitivity initiative effectively.