A-Train Observations of Maritime Mid-latitude Storm Track Cloud Systems: Comparing the Southern Ocean against the North Atlantic and the North Pacific

Mace (2010) studied the mid-latitude storm track clouds over the Southern Ocean (SO) and the North Atlantic (NA) with a synthesis of A-Train observations, concluding that there is a ‘high degree of similarity in cloud occurrence statistics, in cloud properties, and in the radiative effects of the clouds.' This conclusion is perhaps remarkable when reflecting upon the difference between the NA and the SO in terms of the seasonality of thermodynamic structures and the inherent differences with respect to aerosol characteristics. Aerosol conditions over the NA contain anthropogenic and terrestrial emissions, whereas clean marine conditions prevail over the SO (Kanitz et al. 2011; Minikin et al. 2003).

This study revisits the conclusion in Mace (2010) by a direct examination on the observable parameters (i.e. radar reflectivity and lidar attenuated backscatter) from the cloud profiling radar (CPR) on CloudSat and the CALIOP lidar on CALIPSO, in conjunction with an A-Train merged product DARDAR-MASK. The Northwest Pacific (NWP) and Northeast Pacific (NEP) regions are also examined to extend our understanding of the cloud nature over the two hemispheres.

In contrast to Mace (2010), our analysis suggests that while the SO and NA cloud systems display many similarities, i.e. the prevalence of boundary layer clouds and deeper frontal systems, there are observable differences: the NA clouds experiences heavier glaciation at warmer temperatures (-5 to -25ºC) during winter and a greater increase of precipitation during summer; the presence of supercooled liquid water is more pronounced over the SO, especially in summertime; the boundary layer (BL) cloud-top height is ~0.5-1.0 km lower over the SO in winter - predominantly below 1 km (~35% with respect to the overall cloud cover according to CALIOP).

The NWP cloud system displays the strongest seasonal variability, with significant glaciation (a substantial increase of rainfall) during winter (summer). Heterogeneous ice formation appears to be the most efficient over the NWP (0 to -15ºC), which is likely to be associated with the aerosol emissions from Southeast Asia. The NEP cloud system exhibits some similarities in comparison to that of the NA, except that the wintertime glaciation is relatively weaker. The differences between the west and the east of the Pacific storm track clouds are also pronounced.

The heavy presence of the BL clouds remains a major challenge in advancing our current understanding of the storm track clouds properties, especially over the SO where the majority of the BL clouds reside in the lowest 1 km. Detection of these clouds is only possible for CALIOP but not the CPR (due to the ground clutter contamination), or other passive remote sensors when multilayer clouds are present. Thus, the retrievals of microphysical properties, radiative effects, and hydrometeor-induced heating profiles of these clouds that are dependent heavily on the CPR and passive remote-sensing observations remain highly uncertain. In addition, the great contrast in the seasonality of the cloud systems over the two hemispheres may have further implications on cloud feedback and the earth energy budget.