Klein and Hartmann (1993) showed an empirical correlation between mean boundary layer
cloud cover and lower-tropospheric stability (defined in their study as the difference
of 700-hPa and near-surface potential temperature). When imposed in simple two-box
models of the tropical climate (Miller 1997; Clement and Seager 1999; Larson et al.
1999) or into some GCMs’ parameterizations of boundary layer cloud amount [e.g.,
in the National Center for Atmospheric Research (NCAR) Community Climate System Model
verion 3 (CCSM3)], this empirical correlation leads to a substantial increase in
low cloud cover in a warmer climate driven by the larger stratification of warmer
moist adiabats across the Tropics, and produces a strong negative feedback.
The observed relationship between low-level cloud amount and a particular measure
of lower tropospheric stability (Klein and Hartmann, 1993), which has been used in
some simple climate models and in some GCMs’ parametrizations of boundary layer cloud
amount (e.g., CCSM3, FGOALS), led to the suggestion that a global climate warming
might be associated with an increased low-level cloud cover, which would produce
a negative cloud feedback (e.g., Miller, 1997; Zhang, 2004).
The next sentence was identical other than trivial wordsmithing. Bony et al 2006
had stated that the "empirical" Klein and Hartmann (1993) correlation "leads" to
a substantial increase in low cloud cover, which resulted in a "strong negative"
cloud feedback. Again IPCC watered this down: "leads to" became a "suggestion" that
it "might be" associated with a "negative cloud feedback" - the term "strong" being
dropped by IPCC.
Bony et al said that boundary layer clouds had "strongly negative CRF" (Cloud Radiative
Forcing), which IPCC watered down to "strong impact". I guess that the idea of "strongly
negative" feedback was too salacious for the IPCC audience.
Boundary layer clouds have a strongly negative CRF (Harrison et al. 1990; Hartmann
et al. 1992) and cover a very large fraction of the area of the Tropics (e.g., Norris
Boundary-layer clouds have a strong impact on the net radiation budget (e.g., Harrison
et al., 1990; Hartmann et al., 1992) and cover a large fraction of the global ocean
(e.g., Norris, 1998a,b).