Sub-visual Cirrus Effect on the Brightening of Surface Clear-sky Downwelling Shortwave Irradiance

Martial Haeffelin Institut Pierre Simon Laplace
Jean-Charles Dupont Laboratoire de Météorologie Dynamique (LMD), IPSL
Chuck Long NOAA Global Monitoring Division/CIRES
Connor Flynn Pacific Northwest National Laboratory
Jennifer Comstock Pacific Northwest National Laboratory
Sally McFarlane U.S. Department of Energy
Chitra Sivaraman Pacific Northwest National Laboratory
Y. Morille Institut Pierre Simon Laplace

Category: Cloud Properties

Working Group: Cloud Life Cycle

The recent decadal clear-sky brightening of downwelling shortwave irradiance at the ARM SGP site is about +2.9 Wm-2/decade (Long et al. 2008). This trend is composed of -0.3 Wm-2/decade and +3.2 Wm-2/decade for direct and diffuse irradiance, respectively. This tendency is also observed at SURFRAD sites over the continental United States. The Long et al. results show that the clear-sky changes are correlated with increased commercial aircraft flight hours over the continental U.S. In the traditional classification of “clear sky” an inherent limit must be included that allows some amount of condensed water in the column to be included in the “clear sky” category. Dupont et al. (2008) show that traditional clear-sky classification allows up to 0.15–0.2 optical depth of primarily cirrus-level ice crystals. Here we attempt to quantify the effect of this “clear-sky” condensate on the clear-sky brightening of downwelling shortwave irradiance. In this study, we use a 10-year data set of lidar remote sensing (Micropulse and Raman lidar) to derive the high-level ice (here named “residual condensates”) properties, such as altitude, optical thickness, and occurrence, using the STRAT algorithm (Morille et al. 2006). We combine this information about residual condensates with radiative flux measurements to quantify the tendency of residual condensates not traditionally classified as cloud, using the radiative flux analysis methodology (Long and Ackerman 2000). Our results show that overall 50% of the residual condensates have an optical thickness stronger than 0.1, and 70% are higher than 5 km. In 1999, only 10% of the clear-sky periods include residual condensates compared with an almost 50% occurrence in 2006. The residual condensates layers thinner than 0.5 km represent 40% and 65% of the cases in 1999 and 2006 respectively. This study classifies three types of situations: all cases, pristine, and turbid period. The first correspond to all the situations detected as clear by the radiative flux algorithm, the second to the pristine clear-sky periods (lidar signals and Long et al. algorithm indicate 0%), and the third to condensate situations detected by lidar but classified as clear-sky by the radiative flux analysis. The radiative effect of residual condensates is also quantified, and the increase reaches 10 W m-2 for the shortwave diffuse irradiance at 45° solar zenith angle.

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