ZHANG PAPER

Cloud Effects on Radiative Flux Profiles

The feedback by clouds on the atmospheric motions that produce them is determined by the vertical and horizontal gradients of diabatic heating by both the precipitation formed in clouds and the radiative flux perturbations produced by the clouds. So to understand cloud-dynamical feedback the relations of radiative (and latent) heating rate profiles, induced by cloud vertical structure, to the atmospheric circulation must be determined. As a first step towards this goal, studies of cloud effects on top-of-atmosphere and surface radiative fluxes (Rossow and Lacis 1990, Zhang et al. 1995, Rossow and Zhang 1995) have been extended to the determination of cloud effects on radiative heating rate profiles in the atmosphere (Zhang et al. 2004) using newly developed information about cloud vertical structure (Rossow et al. 2005).

Figure 1 summarizes the annual mean, net full sky, latitude-pressure distribution of (a) shortwave (NS), (b) longwave (NL) and (c) net (N) radiative flux profiles in the atmosphere (the values indicate the amount of energy gained or lost below a particular pressure level, not the heating or cooling rate at a particular level). Figure 2 shows that the basic structure of the radiative flux profiles is established primarily by the clear atmosphere and surface properties - the magnitude of the cloud effects is relatively small. However, the cloud effects (Figure 3) are still significant because they feedback on the atmospheric general circulation. More details can be examined in the maps that can be viewed below.

Figure 1: Mean pressure-latitude cross sections of full-sky net fluxes in Wm^-2 (linearly interpolated from the original 5-level profiles) averaged over 1985-1989: (a) NS (net SW), (b) NL (net LW) and (c) N (total net = SW + LW). The sign convention indicates the energy gained (positive values, solid curves) or lost (negative values, dashed curves) below a given pressure level, e.g., the net SW at the top is the total amount absorbed at that latitude by the atmosphere and surface, whereas the net SW at the bottom is the total amount absorbed at that latitude by the surface.)

Figure 2: Mean pressure-latitude cross sections of clear-sky net fluxes in Wm^-2 (linearly interpolated from the original 5-level profiles) averaged over 1985-1989: (a) CLR-NS, (b) CLR-NL and (c) CLR-N. The sign convention indicates the energy gained (positive values, solid curves) or lost (negative values, dashed curves) below a given pressure level. Thus the net SW at the top is the total amount absorbed at that latitude by the atmosphere and surface, whereas the net SW at the bottom is the total amount absorbed at that latitude by the surface.

Figure 3: Mean pressure-latitude cross sections of the cloud effects on net fluxes in Wm^-2 (linearly interpolated from the original 5-level profiles) averaged over 1985-1989: (a) CFC-NS, (b) CFC-NL and (c) CFC-N. Adding these values to the clear-sky net fluxes, shown in Figure 1, gives the full-sky net fluxes.

The dominant equator-to-pole decrease of NS (Fig. 1a and 2a) because of the spherical shape of the earth is obvious; the nearly vertical contours indicate the relative transparency of the atmosphere with the strongest absorption by water vapor in the lower tropical troposphere causing the largest deviations of the contours from vertical. On the other hand, the nearly horizontal contours of NL (Fig. 1b and 2b) indicate the relatively opaque atmosphere and the effects of poleward heat transports by the ocean and atmosphere that produce a more uniform temperature and longwave emission. Figures 1c and 2c show that N varies from a maximum at the equatorial surface to minima at the polar tropopause, a pattern that is balanced by the atmospheric and oceanic heat transports.

Clouds generally reduce NS (all negative values in Figure 3a), most notably in the midlatitude and tropical storm zones, acting to reduce the strength of the oceanic circulation. The generally vertical contours indicate that clouds do not change the SW absorbed by the atmosphere much: small maxima in the midlatitude and tropical storm zones indicate a small upward shift of the SW heating.

The more important cloud effects are the larger ones on NL (Fig. 3b), where the positive values indicate decreased cooling (an effective heating). The fact that the contours are not vertical indicates that the clouds alter the vertical gradients in longwave cooling, producing a high-level heat source in the tropics that suppresses convection. The cloud effect on the horizontal longwave cooling gradient enhances the mean Hadley circulation, but the feedback between these radiative effects on convection and the large scale circulation can produce more complicated responses. The cloud effects at higher latitudes are more complicated because they act to reduce the horizontal temperature gradient by heating more at higher latitudes and shifting the heating into the middle atmosphere; both of these changes may weaken the midlatitude storms. A more detailed analysis of these cloud-radiative effects is needed to determine how each meteorological situation is affected.

References

Rossow, W.B., and A.A. Lacis, 1990: Global, seasonal cloud variations from satellite radiance measurements. Part II: Cloud properties and radiative effects. J. Climate, 3, 1204-1253. (Read abstract.)

Rossow, W.B., and Y-C. Zhang, 1995: Calculation of surface and top-of-atmosphere radiative fluxes from physical quantities based on ISCCP datasets, Part II: Validation and first results. J. Geophys. Res., 100, 1167-1197. (Read abstract.)

Rossow, W.B., Y-C. Zhang and J-H. Wang, 2005: A statistical model of cloud vertical structure based on reconciling cloud layer amounts inferred from satellites and radiosonde humidity profiles. J. Climate, 18, 3587-3605. (Read abstract.)

Zhang, Y-C., W.B. Rossow and A.A. Lacis, 1995: Calculation of surface and top-of-atmosphere radiative fluxes from physical quantities based on ISCCP datasets, Part I: Method and sensitivity to input data uncertainties. J. Geophys. Res., 100, 1149-1165. (Read abstract.)

Zhang, Y-C., W.B. Rossow, A.A. Lacis, M.I. Mishchenko and V. Oinas, 2004: Calculation of radiative fluxes from the surface to top-of-atmosphere based on ISCCP and other global datasets: Refinements of the radiative transfer model and the input data. J. Geophys. Res., 109, doi 10.1029/2003JD004457 (1-27 + 1-25). (Read abstract.)