Variability in clear‐sky longwave radiative cooling of the atmosphere

Abstract

[1] The longwave radiative cooling of the clear-sky atmosphere (Q LWc ) is a crucial component of the global hydrological cycle and is composed of the clear-sky outgoing longwave radiation to space (OLRc) and the net downward minus upward clear-sky longwave radiation to the surface (SNLc). ), explained by the least negative SNLc. On the basis of comparisons with data derived from satellite measurements, ERA40 provides the most realistic Q LWc climatology over the tropical oceans but exhibits a spurious interannual variability for column integrated water vapor (CWV) and SNLc. Interannual monthly anomalies of Q LWc are broadly consistent between data sets with large increases during the warm El Niño events. Since relative humidity (RH) errors applying throughout the troposphere result in compensating effects on the cooling to space and to the surface, they exert only a marginal effect on Q LWc . An observed increase in CWV with surface temperature of 3 kg m À2 K À1 over the tropical oceans is important in explaining a positive relationship between Q LWc and surface temperature, in particular over ascending regimes; over tropical ocean descending regions this relationship ranges from 3.6 to 4.6 ± 0.4 W m À2 K À1 for the data sets considered, consistent with idealized sensitivity tests in which tropospheric warming is applied and RH is held constant and implying an increase in precipitation with warming.

The sensitivity of the modeled energy budget and hydrological cycle to CO<sub>2</sub> and solar forcing

“…2d). In those scenarios, changes in LW back radiation are caused by increases in CO 2 (although Allan (2006) showed that this effect is small in the tropics), water vapor and low clouds, while in the solar scenarios, only the increasing greenhouse effect of water vapor and low clouds is seen. In addition, changes in surface temperature are larger in CO 2 scenarios, which in itself causes a larger increase in water vapor and consequently larger back radiation.…”

Physically Consistent Responses of the Global Atmospheric Hydrological Cycle in Models and Observations

“…3), it has been known for some time that the total amount of precipitation (P) increases with warming at a slower rate than water vapor (*2-3 %/K), responding instead to a changing heat balance of the atmosphere (Manabe and Wetherald 1975;Mitchell et al 1987;Allen and Ingram 2002). The primary physical basis for this is that a warming atmosphere radiates energy away more effectively, particularly to the surface (e.g., Allan 2006;Stephens and Ellis 2008;Prata 2008).…”

“…In (2), L = 2.5 9 10 6 J kg -1 is the latent heat of vaporization and k * 2 W m -2 K -1 is the response of atmospheric radiative cooling to surface temperature, qQ atm /qT (e.g., Allan 2006;Lambert and Webb 2008;, set by the atmospheric temperature and humidity lapse rates (e.g., moist adiabatic lapse rate with near-constant mean relative humidity is a reasonable approximation).…”

An assessment of climate feedback processes using satellite observations of clear‐sky OLR

“…Current generation climate models also exhibit significant biases in tropospheric temperature and humidity compared to satellite observations [John and Soden, 2007]. Since the clear-sky OLR is highly sensitive to temperature and atmospheric water vapor (and also anthropogenic greenhouse gases and aerosols), it is useful to evaluate the sensitivity of clear-sky OLR to changes in temperature and water vapor arising from different sources of observable climate variability [e.g., Allan, 2006].…”

“…requires water vapor variations to be positively correlated with changes in temperature, such that when taken at a global-scale, the slope is similar to that expected from a constant relative humidity moistening [e.g., Allan, 2006Allan, , 2009.…”