85th AMS Annual Meeting

Thursday, 13 January 2005
Summertime North Pacific cloud feedbacks inferred from synoptic-scale dynamic and thermodynamic relationships
Joel R. Norris, SIO/Univ. Of California, La Jolla, CA; and S. F. Iacobellis
Poster PDF (517.9 kB)
Daily satellite cloud observations and reanalysis dynamical parameters are analyzed to determine the how midtropospheric vertical velocity and meridional advection over the sea surface temperature gradient control cloud properties over the summertime midlatitude North Pacific. Optically thick clouds with high tops are generated by synoptic ascent, but two different cloud regimes occur under synoptic descent. When vertical motion is downward, extensive stratocumulus cloudiness is associated with near-surface northerly wind, while frequent cloudless pixels occur with southerly wind. Examination of ship-reported cloud types indicates that midlatitude stratocumulus breaks up as the boundary layer decouples when it is advected equatorward over warmer water. Poleward advection of subtropical air over colder water causes stratification of the near-surface layer that inhibits upward mixing of moisture and suppresses cloudiness until a fog eventually forms. Averaging of cloud and radiation data into intervals of 500-hPa vertical velocity and 1000-hPa meridional wind enables the cloud response to changes in temperature and the stratification of the lower troposphere to be investigated independently from the dynamics. Vertically uniform warming results in decreased cloud amount and optical thickness over a large range of dynamical conditions. Increased stratification of the lower troposphere is associated with more low-level cloudiness and less mid- and high-level cloudiness. Further calculations indicate that a decrease in the variance of vertical velocity would lead to a small decrease in mean cloud optical thickness and top height. These results suggest reflection of solar radiation back to space by midlatitude oceanic clouds will decrease as a direct response to global warming with an additional decrease were the storm track to also weaken, thus producing an overall positive feedback on the climate system.

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