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Master of Science (MS)
Meteorology and Physical Oceanography (Marine)
Date of Defense
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One prominent feature while observing marine boundary layer (MBL) cloud morphology in the tropical and sub-tropical eastern ocean basins is the transition of shallow stratocumulus (Sc) to fair-weather marine cumulus (Cu) in the trade wind region. As the sea surface temperature (SST) increases equatorward, the shallow Sc, generally in a well-mixed boundary layer topped with a strong temperature inversion evolves, into scattered and broken Cu with diffuse inversion heights. This stratocumulus-to-cumulus transition (SCT), has a strong effect on the planetary albedo. The SCT was studied during the Cloud System Evolution in the Trades (CSET) experiment, using the NSF/NCAR Gulfstream V aircraft to collect in-situ, dropsonde and remote sensing datasets between California and Hawaii from 1 July to 15 August 2015. A unique aspect of the experimental design was a Lagrangian sampling strategy, whereby the second flight of a pair sampled air corresponding to a HYSPLIT-calculated forward-trajectory from the earlier flight two days prior. Observations from one such flight pair (RF06- 17 July CA to HI, and RF07- 19 July HI to CA), are analyzed. Guiding questions are how well the observations conform to the ‘deepening-warming-decoupling’ paradigm of Bretherton and Wyant (1997), using estimates based on: (a) vertical thermodynamic profiles, (b) the correspondence between cloud base and lifting condensation level and c) turbulence profiles. The surface latent heat fluxes (LHF), drizzle evaporation in the sub-cloud layer, and net longwave (LW) cooling at cloud top are analyzed for their contribution towards boundary layer (BL) decoupling. Unlike in Bretherton and Wyant (1997), where surface LHF was the dominant factor contributing to BL decoupling, during this case study, surface LHF decreased with increasing SST, due to the weakening of near-surface horizontal wind towards the equator, hypothesized to reflect mesoscale subsidence from a neighboring tropical storm. Sub-cloud evaporative cooling fluxes increase for the deeper, more convective, clouds sampled closer to Hawaii, hinting at an increasing stabilization between the cloud and sub-cloud layers. The net LW cooling at cloud top is also higher for the clouds closer to Hawaii because of their increased depth, which would support more entrainment-led decoupling. Finally, the large-scale influences are studied using re-analysis and satellite datasets. While the SST increases towards the equator, the lower tropospheric stability (LTS) decreases, implying a weaker inversion along warmer water. Free tropospheric (FT) specific humidity is decreasing and FT potential temperature is increasing which implies a warmer and drier FT towards the equator. The southerly decrease in surface LHF along with weakening near-surface horizontal wind is confirmed by OAFlux LHF and AMSR-2 10-m wind datasets.
Cloud System Evolution in the Trades; Stratocumulus to Cumulus transition; Decoupling; Marine boundary layer; drizzle; parcel lagrangian evolution
Sarkar, Mampi, "Stratocumulus-to-Cumulus Transition in the North Pacific Ocean as Understood from an Observational Case Study" (2017). Open Access Theses. 671.