PNNL

Abstract

Continental fair-weather cumuli exhibit significant diurnal, day-to-day, and year-to-year variability. This study describes the climatology of cloud macroscale properties, including the cloud-base height, cloud-top height, cloud thickness, and cloud chord length over the U.S. Department of Energy¹s Atmospheric Radiation Measurement (ARM) Climate Research Facility (ARCF) Southern Great Plains (SGP) site. The diurnal cycle of cloud fraction, cloud-base height, and cloud-thickness were well defined. The cloud fraction reached its maximum value near 14:00 CST. The average cloud-base height increased throughout the day, while the average cloud thickness decreased with time. In contrast to the other cloud properties, the average cloud-chord length remained nearly constant throughout the day. The sensitivity of the cloud properties to the year-to-year variability and to changes in low-level moisture were compared. The cloud-base height was found to be sensitive to both the year and the low-level moisture, the cloud thickness was much more sensitive to the year then to the low-level moisture, and the cloud fraction and cloud chord length were more sensitive to the low-level moisture than to the year. Distributions of the cloud-chord length over the ARCF SGP site were computed and were well fit by an exponential distribution. The contribution of clouds of each cloud-chord length to the total cloud fraction was computed, and it was found the clouds with a chord length of about 1 km contributed the most to the observed cloud fraction.