Aircraft observations in a tropical supercluster over the equatorial Indian Ocean during MISO-BOB field campaign

Abstract

Abstract The Monsoon Intra-Seasonal Oscillations in the Bay of Bengal (MISO-BOB) field campaign was conducted in the Indian Ocean during the 2018 and 2019 summer monsoon seasons. WC-130J aircraft of the 53rd Weather Reconnaissance Squadron of the US Air Force participated in the campaign in June 2018. The dropsonde observations across a tropical supercluster showed zonal wind variations in association with the structure of the convectively coupled Kelvin wave (CCKW). Within the supercluster, easterlies (westerlies) were observed in the upper (lower) troposphere; this transformation occurred just below the 0 $$^{\circ }$$ ∘ C level. The cold pool had an easterly component throughout, and it was coldest (by 2.5 $$^{\circ }$$ ∘ C) at the center of the supercluster, deepest ( $$\sim$$ ∼ $$1000\,\hbox {m}$$ 1000 m ) at its rear/western end, and shallowest ( $$\sim$$ ∼ 300 m) at the front/eastern end. The level of free convection (LFC) at the front end was at $$897\,\hbox {m}$$ 897 m altitude. At the eastern flank of the supercluster, zonal convergence in the lower troposphere occurred between 500-1500 m levels above the surface between the westerlies within the supercluster and opposing ambient easterlies. Thus, the uplifting of conditionally unstable air parcels above LFC to the east of the supercluster was likely to occur due to this convergence rather than the cold pool influence. Conversely, the western flank of the supercluster had low-level zonal divergence. These observations support the notion of ‘self-similarity’ among the mesoscale convective systems and large-scale waves.