James G. Hudson — Desert Research Institute
Stephen R Noble — Desert Research Institute
Category
Boundary layer structure, including land-atmosphere interactions and turbulence
Description
Fig. 1. Correlation coefficients with aerosol time lagged compared to cloud measurements.
Surface aerosol size spectra at SGP were compared with remote measurements of cloud fractions (cf) and cloud base altitudes (cba). Three instruments measured cf: a ceilometer (CEIL), total sky imager (TSI) and pyranometer gridded network (PGN). CEIL is an active beam pointing straight up. TSI is a hemispheric dome. PGN covers an area nearly the size of Oklahoma. Thus, the PGN has the greatest sky coverage and CEIL has the least sky coverage. But only CEIL operates at all hours whereas TSI and PGN operate only during daylight. Since it takes time for the aerosol to move from cloud levels to the surface it was necessary to time-lag the aerosol with respect to the cloud measurements. When this is done Fig. 1 shows that during daylight hours the relationship between cf or cba with mean diameter of the aerosol progresses from low values at zero hour (simultaneous cloud and aerosol measurements) to maximum R at 4 to 6 hours after the cloud measurements before reducing to lower values. Chemical reactions within cloud droplets and Brownian capture of cloud interstitial material as well as coalescence among cloud droplets increase the mass of particles that have nucleated cloud droplets. Thus when cloud droplets evaporate, as they usually do, larger particles emerge. Particles of larger sizes are thus characteristic of cloud processing. The positive R between cf and particle mean diameter is thus indicative of the effect of cloud processing. The R values are here negated for cba because lower cba should cause greater cloud processing. Therefore, the apparent R similarity for cf and cba of CEIL daytime (black and red). Higher R for TSI and PGN is due to their greater sky coverage that better represents cloudiness.
The dark red and cyan traces for nighttime hours seem to indicate absence of cloud processing at night. However, the low and negative R at night is more likely due to significantly lighter nighttime wind speeds that do not bring cloud aerosol to the surface. Furthermore, there are fewer clouds during the night.
Because of around-the-clock year-round coverage surface measurements provide more data more economically than aircraft campaigns. Thus, aerosol and remote sensing measurements at ARM sites can be used to advance understanding of cloud processing.
