What controls the variability of net incoming solar radiation? Alex Hall, Xin Qu, UCLA Dep’t of Atmospheric and Oceanic Sciences standard deviation of planetary albedo (%) in the ISCCP D2 data set broken down by season

For both clear and all-sky cases, the ISCCP data set (D2) contains We assess the controls on planetary albedo variability by examining the ISCCP D2 data set (1983-2000). For both clear and all-sky cases, the ISCCP data set (D2) contains (1) surface radiation fluxes (2) TOA radiation fluxes These were generated based on observations at 3 different channels (visible, near IR, and IR) and a radiative transfer model. Rossow and Gardner (1993a and b) J Clim. Rossow and Schiffer (1999) BAMS.

SURFACE CLOUD RESIDUAL COVARIANCE Planetary albedo variability can be divided into contributions from four components: SURFACE: the portion unambiguously related in linear fashion to surface albedo variability CLOUD: The portion unambiguously related in linear fashion to cloud cover and optical depth variability RESIDUAL: The portion that cannot be linearly related to either surface or cloud variability COVARIANCE: The portion linearly related to surface and cloud variability but not unambiguously attributable to either. (1) (2) (3) (4) SURFACE CLOUD RESIDUAL COVARIANCE

SURFACE CLOUD RESIDUAL COVARIANCE We defined six regions, guided by known differences in the behavior of surface albedo variability: northern hemisphere snow-covered lands northern hemisphere sea ice zone southern hemisphere sea ice zone snow-free lands ice-free ocean Antarctica We averaged the contributions of the four components over each region for each season and normalized by the total planetary albedo variability. Note that the definition of the regions varies seasonally. SURFACE CLOUD RESIDUAL COVARIANCE

SURFACE CLOUD RESIDUAL COVARIANCE The surface contribution to planetary albedo variability in ISCCP is significant everywhere except for the ice-free oceans. It is dominant in the SH sea ice zone year around, and in the other cryosphere regions for most of the year. SURFACE CLOUD RESIDUAL COVARIANCE

Comparison to CCSM3 To allow for as direct a comparison with the ISCCP data as possible, we used a simulated time series with approximately the same mix of internal variability and externally-forced climate change: --a CCSM3 scenario run was used (b30.030c and b30.040c time series). --Data was taken from the same time period as ISCCP (1983-2000).

SURFACE CLOUD RESIDUAL COVARIANCE Controls on planetary albedo variability in CCSM3 SURFACE CLOUD RESIDUAL COVARIANCE

SURFACE CLOUD RESIDUAL COVARIANCE CCSM3 ISCCP A side-by-side comparison of CCSM3 and ISCCP reveals much more contribution from the surface in ISCCP to interannual planetary albedo variability in all regions except for ice-free oceans. SURFACE CLOUD RESIDUAL COVARIANCE

SURFACE CLOUD RESIDUAL COVARIANCE CCSM3 ISCCP A prominent example of the difference between CCSM3 and ISCCP is in the NH snow-covered land areas during all seasons. SURFACE CLOUD RESIDUAL COVARIANCE

Why is the contribution of the surface so much smaller in CCSM3? --Is it that clouds are more variable, increasing the relative contribution of clouds? --Or is surface albedo itself less variable in CCSM3? --Or is it that the CCSM3 atmosphere is more opaque to solar radiation, attenuating the effect of surface albedo anomalies?

ISCCP Clear-sky surface albedo standard deviation (%) in ISCCP and CCSM3, broken down by season. ISCCP has consistently more surface albedo variability. CCSM3

ISCCP Clear-sky surface albedo standard deviation (%) in ISCCP and CCSM3, broken down by season. The larger surface albedo variability in ISCCP is particularly apparent in the interior of the northern hemisphere snowpack. (e.g. DJF, MAM) CCSM3

CONCLUSIONS --The surface contribution to planetary albedo variability in ISCCP is significant everywhere except for the ice-free oceans. It is dominant in the SH sea ice zone year around, and in the other cryosphere regions for most of the year. --CCSM3 has substantially less surface albedo variability than ISCCP, resulting in a significantly smaller contribution of the surface to planetary albedo variability.