The Origins of Ice in Tropical Convective Clouds over the Atlantic Ocean during ICE-D

In this paper for the first time we are able to show that the origin of the ice phase in tropical cumulus clouds over the sea can be via primary ice nucleation of small crystals at temperatures just below 0C. Observations were made during the ICE-D experiment carried out from Cape Verde in July-August 2015, using the UK FAAM BAe 146 atmospheric research aircraft (ARA). The observations were made possible through deployment of a suite of state of the art instrumentation aboard the FAAM aircraft which included a new holographic instrument which can image particles with very high spatial resolution down to small sizes (~6 um). The environment in which the observations were conducted was notable for the presence of desert dust advected over the ocean from the Sahara desert. However, there is no laboratory evidence to suggest that these dust particles can act as efficient Ice nuclei at temperatures warmer than -10C where the first ice is observed in these clouds. These small ice particles are observed to grow rapidly by vapour diffusion and then riming, or may be captured by supercooled raindrops causing the raindrops to freeze rapidly. This in turn leads to the production of secondary ice in the cloud. Hence, although the numbers of primary ice particles are small they are very effective in initiating the rapid glaciation of the cloud, altering the dynamics and precipitation production processes. Figure 1 below shows the forward facing camera image as the aircraft approaches the ascending cloud turret on each pass; the temperatures are out of cloud temperatures which are higher than the in cloud temperatures. Figure 2 shows holographic images of ice particles detected in updraught regions within the cloud for each pass.