What Controls Planetary Albedo and its Interannual Variability over the Cryosphere? Xin Qu and Alex Hall Department of Atmospheric and Oceanic Sciences, UCLA 85th AMS Annual Meeting San Diego, CA 9-13 January 2005 January 13, 2005

Q1: Is the surface contribution to climatological planetary albedo over the cryosphere regions larger than the atmospheric contribution?  On a global-mean basis, a large portion of upwelling solar photons at the top of atmosphere are reflected by the atmosphere rather than the surface. However, it is unclear whether this is true over the cryosphere regions. Large surface albedo there may increase the surface contribution considerably.  Surface albedo in the cryosphere regions changes from year to year due to fluctuations in sea ice and snow. On the other hand, fluctuations in atmospheric constituents, such as clouds, also result in variability in atmospheric albedo. It is unclear which component contributes more to planetary albedo variability over the cryosphere regions. Q2: Is the surface contribution to planetary albedo variability over the cryosphere regions dominant over the atmospheric contribution? Motivations The answer to this question can be used to examine the effectiveness of surface albedo feedback in the climate. For example, if the surface’s contribution to planetary albedo variability is small, then the contribution of surface albedo feedback in the climate would be negligible.

Climatological case (1) (2) (3) (1)Atmospheric albedo (2)Surface albedo (3)Atmospheric effective transmissivity The atmosphere attenuates the surface’s contribution to planetary albedo in two ways:  The atmosphere absorbs and scatters incoming solar radiation, reducing the number of photons ultimately reaching the surface.  The atmosphere absorbs and scatters solar radiation reflected by the surface, preventing these photons from reaching the top of atmosphere.  p =  a +  s * T e A simple equation for planetary albedo:

Surface Vs atmosphere The surface contribution is still considerably smaller compared to atmospheric contribution in most of the cryosphere regions. However, in Antarctica, the surface accounts for more of planetary albedo than the atmosphere. The cryosphere is segregated into 4 regions: NH snow- covered lands, NH sea ice zone, SH sea ice zone and Antarctica. We also include non-cryosphere regions in our analysis for sake of comparison. ISCCP D-series flux data set, containing: surface and TOA radiation fluxes ( ). These were generated based on ISCCP D-series cloud data set ( ) and a radiative transfer model. Climatological case

Springtime snow-free lands, NH snow-covered lands and Antarctica Surface albedo Atmospheric albedo Atmospheric effective transmissivity Snow-free lands NH snow- covered lands Antarctica Climatological case

(1) Surface: the portion unambiguously related in linear fashion to surface albedo variability (2)Cloud: The portion unambiguously related in linear fashion to cloud cover and optical depth variability (3)Covariance: The portion linearly related to surface and cloud variability but not unambiguously attributable to either. (4)Residual: The portion that cannot be linearly related to either surface or cloud variability Planetary albedo variability can be divided into surface and cloud contributions: (1) (2) (3) (4) Variability case (  p ’ ) 2 =  2 * (  s ’ ) 2 + (  pc ’ ) *  * (  s ’,  pc ’ ) + (  r ’ ) 2

Four components to planetary albedo variability In nearly all cryosphere regions, surface contribution is dominant over cloud contribution, accounting for more than 50% of total variability. Variability case

Springtime NH snow-covered lands, NH sea ice and SH sea ice zones (s’)2(s’)2 (  pc ’ ) 2 22 NH Snow- covered lands 2.6* * NH Sea ice zone 3.0* * SH Sea ice zone 6.3* * Variability case

 Atmospheric albedo accounts for much more of climatological planetary albedo than surface albedo in most of the cryosphere regions. This is because the attenuation effect of atmosphere on the surface’s contribution to climatological planetary albedo is significantly large.  In all cryosphere regions, surface contribution to interannual variability in planetary albedo is dominant over cloud contribution at nearly any time of year. This is because the surface albedo variability associated with snow and ice fluctuations in the cryosphere regions is much larger than atmospheric albedo variability due to clouds. Even damped by the atmosphere, the surface contribution is still dominant over the cloud contribution. Qu X. and Hall A. (2005): What controls planetary albedo and its interannual variability? Accepted by J. Climate. Conclusions