Space observations of cold-cloud phase change

Abstract

This study examines the vertically resolved cloud measurements from the cloud-aerosol lidar with orthogonal polarization instrument on Aqua satellite from June 2006 through May 2007 to estimate the extent to which the mixed cloud-phase composition can vary according to the ambient temperature, an important concern for the uncertainty in calculating cloud radiative effects. At -20 degrees C, the global average fraction of supercooled clouds in the total cloud population is found to be about 50% in the data period. Between -10 and -40 degrees C, the fraction is smaller at lower temperatures. However, there are appreciable regional and temporal deviations from the global mean (> +/- 20%) at the isotherm. In the analysis with coincident dust aerosol data from the same instrument, it appears that the variation in the supercooled cloud fraction is negatively correlated with the frequencies of dust aerosols at the -20 degrees C isotherm. This result suggests a possibility that dust particles lifted to the cold cloud layer effectively glaciate supercooled clouds. Observations of radiative flux from the clouds and earth's radiant energy system instrument aboard Terra satellite, as well as radiative transfer model simulations, show that the 20% variation in the supercooled cloud fraction is quantitatively important in cloud radiative effects, especially in shortwave, which are 10-20 W m(-2) for regions of mixed-phase clouds affected by dust. In particular, our results demonstrate that dust, by glaciating supercooled water, can decrease albedo, thus compensating for the increase in albedo due to the dust aerosols themselves. This has important implications for the determination of climate sensitivity.

Fig. 1.

(A) Annual mean (June 2006–May 2007) SCF at -20 °C isotherm. The fraction is lowest over the Asian continents and South America, but highest over the Southern Hemisphere high latitudes. (B) Annual mean supercooled cloud fraction with respect to temperature over the selected regions in (A): Asia, South America, North America, Europe, Africa, and the Antarctic. The error bar corresponds to a standard error of ± 3.

Fig. 2.

Seasonal variation in SCF and relative dust frequency (to the highest dust frequency), both at the -20 °C isotherm. Pattern correlation coefficients between SCF and dust frequency for the Northern Hemisphere are negative for all seasons: -0.1, -0.5, -0.5, and -0.6 for JJA, September-October-November (SON), December-January-February (DJF), and MAM, respectively.

Fig. 3.

Dependence of SCF on relative dust frequency (to the maximum dust frequency) at the -20 °C isotherm. Plots are monthly means taken from the regions where the correlation is significant at the 95% confidence level.

Fig. 4.

Change in CRE of mixed-phase clouds at TOA when liquid (ice) water content decreases (increases) by 20%. Warming is positive, and cooling is negative. The dashed line indicates the effective radii of cloud particles over land, as diagnosed by temperature in most global climate models.

Fig. 5.

The same as Fig. 4, but for the earth’s surface.

Fig. 6.

Observed and simulated CRE differences at TOA, the atmosphere, and the earth’s surface, in response to a 20% reduction in SCF with respect to total cloud water path. The observations are from CERES SFC data obtained during 2004, with values statistically significant at a 95% confidence level. Possible ranges of the simulated values are shaded, and the means are indicated by red lines.