Interplay between Convection and Marine Atmospheric Boundary Layer Observed during DYNAMO Filed Experiment: A Case Study

Madden-Julian Oscillation (MJO) is the dominant convective disturbance in tropics. This eastward propagating large-scale atmospheric convection anomaly is the dominant intra-seasonal (with a period of 30-90 days) variability in the tropics. MJO is most active in the Indian and Western Pacific Oceans and influences all major tropical as well as extra tropical weather and climate. Strong and widespread precipitating cumulus convection is characteristic of Madden-Julian Oscillation (MJO). Cumulus clouds producing significant precipitation (>1 mm h^-1) also gives rise to cold pools. Cold pools are formed when the precipitation driven downdraft of air is cooled by the evaporation of the precipitation. At the surface level, this cold and dry air downdraft pushes the moist environment air in the mixed layer outward thereby modifying the Marine Atmospheric Boundary Layer (MABL) thermodynamic structure. This case study unravels the complex interplay between MABL and convection observed on 28 November 2011, during the Dynamics of MJO (DYNAMO) field campaign, conducted in the tropical Indian Ocean. The results presented are based on the analysis dropsonde and radar reflectivity data collected using NOAA WP-3D (P-3) Orion aircraft.

DYNAMO domain experienced intense and widespread convective activity on 28 November 2011. The convective system is comprised of very cold (IR cloud top temperature < 208 K) clouds in the interior often reaching 177 K. However, moderately cold clouds with IR cloud top temperature <235K was also present in the periphery of the system. On 28 November, P-3 made dedicated MABL sorties across the convection cells that formed in the periphery of the system. On this day, we noticed a convection band (CB) marked by enhanced radar reflectivity (> 30 dBZ), distributed roughly between 74°E and 75°E. Interestingly, MABL profiles showed that relatively cooler and drier mixed layers formed in the east of the enhanced reflectivity band in comparison with west of it. Moreover, the soundings from east of CB also showed strong easterly flow (~ 10 m s^-1) in the mixed layer. These features signaled the presence of cold pool at the eastern boundary of CB.

Further, we found that cold pools played an important role in triggering the convection observed between 74°E and 75°E, seen as enhanced reflectivity in the radar data. Our analysis of the lower level wind fields showed strong convergence and ascent at the boundary where the cold pool air meets the warm and humid air at west. CB roughly aligns with the region of strong convergence. This observation suggests the mechanical lifting at the cold pool edge as the triggering mechanism for the development of new convection cells. The cold pool observed in the present study had an average depth of 860 m.