Relationship between Low Stratiform Cloud Amount and Estimated Inversion Strength in the Lower Troposphere over the Global Ocean in Terms of Cloud Types

抄録

Low stratiform clouds (LSCs) are of three different types: stratocumulus (Sc), stratus (St), and sky-obscuring fog (FOG). Ship-based cloud observations (September 1957-August 2002) and air-temperature and sea-level pressure data obtained from the ERA-40 reanalysis are used to investigate the seasonal relationships between the amounts of these LSC types and the estimated inversion strength (EIS) over the global ocean. Although it is known that a single linear relationship applies to the variations in the LSC amount as the sum of those of the LSC types and EIS, two relationships with different sensitivities are found between each LSC-type amount and EIS. The boundary lies at a sea surface temperature (SST) of approximately 16°C. The Sc amount is strongly correlated with EIS in the warm SST regime, whereas no correlation can be observed between them in the cold SST regime. In contrast, although FOG rarely occurs in the warm SST regime, its amount increases with EIS in the cold SST regime. The St amount increases with EIS in both regimes, with higher sensitivity in the cold SST regime. Examination of vertical layers contributing to EIS reveals that an increase in the inferred inversion strength between 850- and 925-hPa levels corresponds to that in the Sc amount in the warm SST regime. In the cold SST regime, as EIS increases, relatively high values of inferred inversion strength between 700- and 850-hPa levels change to a rapid increase in that between 925-hPa level and the surface, which coincides with the transition from Sc to FOG. Temperature advection implied by the air-sea temperature difference provides favorable conditions to the different variations in the two regimes: general occurrence of cold advection in the warm SST regime and cold-to-warm transition of advection in the cold SST regime.