Clouds in the Earth's atmosphere have a significant impact on its energy budget and also on the hydrological cycle. Therefore for accurate predictions of both weather and climate, clouds must be well represented within regional and global numerical models.

The MRF C-130 aircraft is equipped with state-of-the-art instrumentation for measuring the number, size and chemical composition of aerosol particles, cloud droplets and ice crystals. Our aircraft observations are analysed in conjunction with measurements of the associated dynamical and radiative properties of the clouds to improve our understanding of the microphysical processes responsible for their formation and structure. These processes can then be better represented in the numerical models.

As an example of a current area of study, Figure 1 shows satellite imagery of a low-level sheet of stratocumulus cloud off the west coast (in green) of North America (except for high cirrus clouds shown in mauve). The picture was taken in the near infra-red (at 3.7 µm wavelength) where the reflectivity of the cloud is particularly sensitive to the size of the cloud droplets. Within this sheet of cloud, numerous lines can be seen that appear initially to resemble plumes of smoke. They are called ship tracks and can reach hundreds of kilometres in length and can persist for a number of days. While they are actually caused by pollution in the exhausts of ships, what is visible is not the pollution itself but a change in the cloud reflectivity caused by the aerosol particles in the plumes. The increased number of aerosol particles in the plume relative to the background airmass increases the number of cloud droplets formed, reducing the average droplet size and thereby locally increasing the reflectivity.

Figure 2 shows some cloud droplet measurements that are typical of a transection through a ship track such as that indicated by the red line in Figure 1. The aircraft observations show the increase in cloud droplet concentration (solid line using left y-axis) and decrease in droplet size (dotted line using right y-axis) within the ship track at an altitude of about 300m which is near the top of the cloud layer. Such data can help us understand the effect of both natural and man-made aerosols on the physical and radiative properties of clouds.

Figure 1. Advanced Very High Resolution composite image taken at 2245 GMT on 27 June 1987. Image courtesy of Kurt Nielsen (NPS, Monterey, California)

Figure 2. Measurements made during the Monterey Area Ship Track (MAST) experiment off the coast of California on 8 June 1994. [Click on graph for full size image]