Cloud Properties and Radiative Effects within Atmospheric Blocks

Atmospheric blocks are high pressure systems that can influence extreme weather due to their size, persistence, and circulation. For instance, in cases in which heatwaves over land are co-located with blocks, there are cloud-free conditions because of the large-scale mid-tropospheric descent created by the anticyclonic flow of the block. However, recent work focused over midlatitude oceans has demonstrated that low-level clouds are often present underneath regions where the vertical motion in the mid-troposphere is downwards. This result, in addition to recent results showing that diabatic heating can have an important role in a block’s lifecycle, is the motivation for our analysis of cloud properties in atmospheric blocks.

After identifying blocks in reanalysis using an automated detection scheme, we conditionally sort CERES-MODIS level 3 data in a block-centered compositing approach. The MODIS observations indicate that there are clouds in blocks over the oceans. The presence of these low clouds is consistent with previous work that demonstrated the ubiquity of these clouds over the global ocean. In the block-centered composites, there is a distinct spatial pattern for all of the cloud properties. The region in the western half of the poleward side of the block centroids has anomalously large cloud fraction with high cloud types. This likely represents the region where extratropical cyclones are passing poleward of the block – a typical condition during blocking events. The cloud fractions and cloud top heights are much lower in the eastern half of the equatorward side of the blocks. This is true for blocks in both the northern and southern hemispheres. The cloud fractions, even in the least cloudy region, is nearly 70% on average. However, compared to climatology, this region of the block has anomalously low cloud fractions and shallow cloud top heights.

The cloud radiative effect of blocks is also different in the western and eastern quadrants – relative to the block centroids. While the western sector sees an excess in shortwave radiation reflection of up to 10 Wm-2 compared to climatology, the eastern sector reflects less than climatology by up to -5 Wm-2. In the longwave, the western sector displays a loss of down to -10 Wm-2 but the eastern sector an excess of over 15 Wm-2. Overall, the cloud radiative effect in the western sector of the order of -20 Wm-2 is balanced by a net warming impact of 20 Wm-2 in the eastern quadrant.