The origin of water-vapor rings in tropical cold pools

Friday, 19 December 2014
Wolfgang Langhans, Lawrence Berkeley National Laboratory, Berkeley, CA, United States and David M Romps, University of California Berkeley, Berkeley, CA, United States
An invigoration of deep convection by cold pools is supported by two conceptually different theories: a) mechanic lifting of air-parcels at the cold-pool boundary and b) thermodynamic preconditioning of boundary-layer air due to rings of enhanced water-vapor content (~ +1 g/kg). The latter have been associated with the leading edges of radially spreading cold pools in studies of precipitating convection over tropical oceans. Even recovered cold pools exhibit these moisture anomalies and the formation of such rings thus also plays a critical role in theory (a) through the moistening of ambient air that is later lifted by another cold-pool.

Despite the described relevance, the origin of these water-vapor rings is unclear. This motivates us to conduct idealized large-eddy simulations with the purpose of explaining the origin of these water-vapor rings. The simulations are coupled with a recently designed framework to track Lagrangian water particles and allows us to decompose the emerging vapor distribution according to its origin. The emerging quasi-axisymmetric flow transitions from a vortex-dominated downdraft in the early stage to a radial gravity current in the later stage. Preliminary results highlight the dominating role of moisture that resides in the boundary layer before deep convection is initiated. A delineation of the individual contributions from boundary-layer moisture, evaporated hydrometeors, and latent heat fluxes reveals that the latter two sources may not be crucial for the anomalous moisture content over the radial distances considered. The sensitivity to the initial moisture of the boundary-layer and the effects of entrainment into the cold pool will be discussed.