Effect of aerosols on shallow cumuli sampled during RACORO

Poster PDF

Description

The classical second aerosol indirect effect occurs when increases in aerosol concentration lead to reductions in cloud droplet sizes, affecting the precipitation efficiency, and consequently leading to increases in liquid water content (LWC) and cloud lifetime. Even though numerous studies have provided evidence of this effect in stratus and stratocumulus, an opposite effect may occur in shallow cumulus where decreases in LWC have been observed with increases in aerosol. In this study, data collected in warm shallow continental cumuli during the 2009 Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field project are used to investigate the second aerosol indirect effect. RACORO was held in the vicinity of the ARM Southern Great Plains site at Lamont, Oklahoma from January to June 2009. The CIRPAS Twin Otter aircraft measured aerosol, cloud microphysical, and radiative properties in both clear and cloudy conditions for 260 hours, 85 of which were classified as shallow cumuli conditions. Daily averaged sub-cloud aerosol concentrations in the accumulation mode (0.1 µm < D < 2.2 µm) measured by Passive Cavity Aerosol Spectrometer Probe (PCASP), NPCASP, varied from 70 to 1300 cm-3 for the 19 shallow cumuli flights, with a median value of 740 cm-3. Sub-cloud aerosol concentrations in the Aitken mode (D > 10 nm) measured by a Condensation Particle Counter varied from 800 to 9500 cm-3 with a median value of 2030 cm-3. For cloud measurements, a number of cloud probes, including the Forward-Scattering Spectrometer Probe (FSSP) and the Cloud Aerosol Spectrometer for D < 50 µm, the 2D-Stereo probe for 50 µm < D < 100 µm, and the 1d-Cloud Imaging Probe (CIP) and 2d-CIP for D > 100 um, were used to generate an integrated cloud product of the size distributions and bulk microphysical properties. Using 1-second averaged cloud data from the FSSP, 2337 penetrations into cumuli were identified. Most of the cumuli occurred at heights below 2500 m, and their median width was on the order of 200 m. The maximum vertical velocity in cloud reached 15 ms-1, whereas the maximum cloud-mean-vertical velocity was 5.5 ms-1. The average and maximum LWC decreased as the PCASP concentration increased, with a median value of 0.078 g m-3 for NPCASP < 740 cm-3, and 0.056 g m-3 for NPCASP > 740 cm-3. Statistical tests describing the complete dependence of the cloud and velocity field on aerosol concentration and implications will be presented at the ASR meeting.