Abstract
Abstract. During the July 2011 Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field experiment in Maryland, significant enhancements in Aerosol Robotic Network (AERONET) sun–sky radiometer measured aerosol optical depth (AOD) were observed in the immediate vicinity of non-precipitating cumulus clouds on some days. Both measured Ångström exponents and aerosol size distribution retrievals made before, during and after cumulus development often suggest little change in fine mode particle size; therefore, implying possible new particle formation in addition to cloud processing and humidification of existing particles. In addition to sun–sky radiometer measurements of large enhancements of fine mode AOD, lidar measurements made from both ground-based and aircraft-based instruments during the experiment also measured large increases in aerosol signal at altitudes associated with the presence of fair weather cumulus clouds. These data show modifications of the aerosol vertical profile as a result of the aerosol enhancements at and below cloud altitudes. The airborne lidar data were utilized to estimate the spatial extent of these aerosol enhancements, finding increased AOD, backscatter and extinction out to 2.5 km distance from the cloud edge. Furthermore, in situ measurements made from aircraft vertical profiles over an AERONET site during the experiment also showed large increases in aerosol scattering and aerosol volume after cloud formation as compared to before. The 15-year AERONET database of AOD measurements at the Goddard Space Flight Center (GSFC), Maryland site, was investigated in order to obtain a climatological perspective of this phenomenon of AOD enhancement. Analysis of the diurnal cycle of AOD in summer showed significant increases in AOD from morning to late afternoon, corresponding to the diurnal cycle of cumulus development.
Highlights
One of the more difficult issues concerning the analysis and remote sensing of the forcing agents of climate change is the monitoring of interactions between clouds and aerosol particles
We focus on two days during the month of July 2011 where large diurnal dynamics in measured aerosol optical depth (AOD) were noted at some Aerosol Robotic Network (AERONET) sites within the Distributed Regional Aerosol Gridded Observation Network (DRAGON) network
This is the Level 1.0 AERONET data with no cloud screening applied, in order to include data that has high temporal variance, high triplet variance. During this time period when AOD increased by approximately 75 % in only minutes, the Ångström exponent remained relatively constant with no significant temporal trend throughout the day
Summary
One of the more difficult issues concerning the analysis and remote sensing of the forcing agents of climate change is the monitoring of interactions between clouds and aerosol particles. A significant challenge to the climate science community is to link remote-sensing measurements of potential changes in aerosol optical depth (AOD) to Cloud Condensation Nuclei Concentration (e.g., Kaufman and Fraser, 1997) or to secondary particle production, free of retrieval artifacts. Non-lidar remote-sensing data sets from satellites are typically biased towards measurements made away from the immediate vicinity or edges of clouds, in addition to typically having little aerosol information when cloud fraction is very high. This bias towards low cloud fraction meteorological conditions applies to ground-based remote-sensing observations such as from sun–sky radiometers, to a lesser extent than from satellite. A fundamental question is the following: how accurately can the relative signal of aerosol production and growth/humidification around clouds be observed?
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