Studies of Marine Convective Disturbances Based on MISO-BOB Field Observations: Topographic Enhancement of Precipitation and Cold-Pool Dynamics

Convective disturbances over oceans have propensity to couple with background atmospheric circulation as well as favorable air-sea processes, leading to severe weather phenomena. Some examples are the Convectively-Coupled Equatorial Waves (CCEWs) and concatenating deep-convective events associated with descending/collapsing cold air masses over ocean known as Atmospheric Cold Pools (ACP). During the Monsoon Intra-seasonal Oscillations in the Bay of Bengal (MISO-BOB) field campaigns in 2018 and 2019, opportunities arose to observe and study such disturbances in the BOB, which form the theme of this thesis. The first part of the thesis concerns the interaction of CCEWs with an island topography, based on spatially-separated, extreme wind and precipitation events observed in Sri Lanka on 18 July 2019, during the southwest Monsoons. Such events cause severe floods, landslides, and socio-economic impacts. During the event in point, Sri Lanka recorded over 200 mm of rainfall in the mountainous Nuwara Eliya District (elevation 1900m) in a 24-hr period with gale-force winds in [coastal] Colombo. This research examines how CCEW, in particular, a Convective-Coupled Kelvin Wave (CCKW) influenced this event, based on data from the MISO-BOB instrument array. Ground-based observations and a Froude number analysis indicated that orographic lifting of a northeast propagating CCKW that first enters the west coast of Sri Lanka and then arriving at countries’ hill country is responsible for the extreme winds in Colombo followed by enhancement of rainfall in Nuwara Eliya. This study also highlights the challenges of using satellite measurements such as the Global Precipitation Measurement (GPM) mission that failed to accurately detect localized extreme precipitation. The second part of the thesis examines ACPs in the Indian Ocean, an important (but understudied) ingredient for deep convection. ACPs are formed by the evaporation of rain beneath deep-convective clouds by cold, dense air pockets descending to the surface and spreading out as low-level outflows that are similar to gravity currents (GCs). As ACPs interact with surrounding warmer and moist air, they can trigger secondary convection by lifting the latter over the level of free convection. During MISO-BOB campaigns, the propagation and structure of an ACP were captured over the equatorial Indian Ocean by an WC-130J research aircraft. It was hypothesized that moist, warm, background air flowing (and lifted) above the flanks of ACP may trigger secondary convection, if ACP can maintain its integrity irrespective of shear introduced between ACP and background flow. To this end, laboratory experiments were performed to investigate the interaction between a GC induced by a descending buoyant plume and an opposing mean flow. Particle Image Velocimetry (PIV) and flow visualization were used for flow diagnostics. The governing parameters for the problem were identified as the Richardson Ri= ?bh/?U2 and Reynolds numbers Re= ?Uh/? based on the relative velocity (?U) and buoyancy jump (?b) between the GC and ambient air, the GC height (h) and kinematic viscosity (?). Experiments revealed that to generate updrafts it is necessary to have a Richardson number greater than a critical value Ric ˜ 6. The laboratory findings were compared with WC-130J dropsonde observations and a fair agreement was noted, indicating the usefulness of the criterion derived above in designing future ACP studies.