A new scaling for low cloud cover

  Observations show that on daily to interannual timescales, stratiform low cloud fraction CF is strongly correlated with the lower tropospheric stability LTS, defined as the difference between the potential temperature q of the free troposphere (700 hPa) and the surface, LTS = q700-q0 (Klein and Hartmann 1993).

Relationships between LTS and CF from observations in the tropics (Slingo 1980) and subtropics (Klein and Hartmann 1993) have been used in the parameterization of low cloud cover in general circulation models (e.g. Slingo 1987; Rasch and Kristjansson 1998) used to predict climate changes. They are also a key assumption in the thermostat hypothesis of Miller (1997) and in the climate sensitivity study of Larson et al. (1999). Both of these studies result in a strong negative low cloud feedback on climate changes due to a marked increase in low cloud cover as the sea surface temperature SST increases. However, it has yet to be demonstrated whether the observatlts_eisionally-derived LTS-CF relationships will hold in a changed climate.

We have derived a new formulation, called the estimated inversion strength (EIS) to estimate the strength of the PBL inversion given the temperatures at 700 hPa and at the surface (Wood and Bretherton 2006). The EIS, which like LTS depends only upon the 700 hPa and surface temperatures, accounts for the general observation that the free-tropospheric temperature profile is often close to a moist adiabat and its lapse rate is strongly temperature dependent. Therefore, for a given LTS, the EIS is greater at colder temperatures. We demonstrate that while the seasonal cycles of LTS and low cloud cover CF are strongly correlated in many regions, no single relationship between LTS and CF can be found that encompasses the wide range of temperatures occurring in the tropics, subtropics, and midltatitudes. However, a single linear relationship between CF and EIS explains 83% of the regional/seasonal variance in stratus cloud amount (see Figure below), suggesting that EIS is a more regime-independent predictor of stratus cloud amount than is LTS under a wide range of climatological conditions.

The result has some potentially important implications for how low clouds might behave in a changed climate. In contrast to Miller's (1997) thermostat hypothesis that a reduction in the lapse rate (Clausius-Clapeyron) will lead to increased LTS and increased tropical low cloud cover in a warmer climate, our result suggests that low clouds may be much less sensitive to changes in the temperature profile if the vertical profile of tropospheric warming follows a moist adiabat. There is some evidence that recent syntheses of state-of-the-art climate models are demonstrating a weaker cloud feedback than previously thought (Soden et al. 2006). Our results give some physical basis for why this might be expected. They also provide strong constraints for evaluating these models.


LEFT: Low cloud cover CF, vs LTS (top) and vs EIS (bottom). Solid circles show long term seasonal means from the tropics and subtropics, while open circles are for the colder midlatitude regions. Notice that EIS is a much more appropriate measure across a broader range of temperatures, which suggests it may have skill in predicting how low clouds may change in a future climate.






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