Cloud Observations at Niamey During the AMF Deployment
Kollias, P., Stony Brook University
Kollias, P. and M. A. Miller, 2007: Cloud and Precipitation Observations at Niamey During the 2006 ARM Mobile Facility Deployment. Submitted to Geophysical Research Letters.
Daily observed cloud fraction in Niamey during the AMF deployment. The cloud fraction is derived using measurements from the 94-GHz radar, the MPL, and the ceilometer. The vertical resolution is 260 m, and a 5-day temporal filter is applied to the daily cloud fraction profiles.
(a) Monthly-averaged cloud and precipitation fraction. The monthly mean and standard deviation of cirrus cloud top (white line), middle cloud tops (white line), cirrus cloud thickness (yellow line) and middle cloud thickness (red) are also shown. (b) The deployment-mean cloud and precipitation fraction during the AMF deployment at Niamey, Niger.
The first international deployment of the AMF in Niamey, Niger,—one of the hottest countries in the world—was a great success. Prior to the deployment, the extreme heat and dust was of concern to everyone participating in the campaign, especially with respect to the delicate instruments operating in Africa's extreme conditions. Despite these conditions, the instruments, particularly the newly-fabricated W-Band (95-GHz) ARM Cloud Radar (WACR), exhibited great stability and performance. Combining the WACR, MPL, and ceilometer observations, we developed a cloud and precipitation classification algorithm and used it to analyze the data collected during the year-long Niamey, Niger, deployment.
The deployment duration was sufficient to enable collection of data during both the dry and wet (monsoon) seasons. During the dry season (October to April), extreme dry conditions and frequent dust outbreaks were observed. Boundary-layer and middle-layer clouds were rarely observed.
Cirrus clouds were the only cloud type systematically observed with a seasonal average of fractional coverage near 20%. During the wet season (May to September), precipitation was intermittent, with days that had several hours of precipitation and days with no precipitation. Intense rainfall events accounted for most of the accumulated surface precipitation, although the most frequently observed precipitation events produced rainfall that evaporated before it reached the surface. This precipitation originated from the melting of ice particles from mid-level and cirrus clouds.
These observations suggest that stabilizing the middle and lower atmosphere through the evaporation of falling precipitation may condition the environment to facilitate a steadily increasing convective available potential energy (CAPE) near the surface as a function of time. This CAPE may fuel the exceptionally strong mesoscale convective complexes that often move through the region from the east toward the Atlantic Ocean.