The impacts of aerosols and boundary layer characteristics on the properties of continental shallow cumuli during RACORO

Aerosol-cloud interactions in continental shallow cumuli over the Southern Great Plains (SGP) region in varying meteorological conditions are examined using data obtained during the 2009 Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign. A total of 260 hours of in-situ airborne cloud and aerosol measurements were made over a period of 6 months, with 85 hours of data collected on days when shallow cumuli were present.

Using a multi-scale comprehensive dataset of weather maps, satellite images, radar images, rawinsonde data, and back trajectory analysis, the days with fair weather cumuli were sorted into four categories according to the airmass source regions and meteorological characteristics: 1) continental polar airmass aloft from the Great Basin (cP aloft); 2) modified maritime tropical airmass (mT) from the east under the influence of a tropical cyclone (TC); 3) transition from tropical cyclone to post-tropical cyclone (postTC); and 4) maritime airmass aloft and maritime airmass from the Gulf of Mexico (deeper mT) at the surface.

The category 1 environment was characterized by the presence of an inversion layer and was further sorted into three subcategories depending on the airmass in the low troposphere: A) mT below, B) cP below, and C) the presence of frontal passage (FR). Nine out of seventeen days with cumuli were grouped into Category 1. The average lifting condensation level(LCL) height in Category 1A was 1614 m ± 576 m, compared to 1642 m ± 96 m in Category 1B. In Category 1C, soundings changed significantly after the frontal passages, and therefore the standard deviation was large, 1557 m ± 558 m. The average inversion thickness in Category 1A was 246 m ± 149 m with G = 2.1 ± 1.0 °C / 100 m, compared to 212 m ± 159 m with G = 1.0 ± 0.5 °C / 100 m for Category 1B. Unlike Category 1, no inversion or only very weak inversions were observed for Categories 2, 3, and 4. Even though both Category 2 and Category 4 are influenced by maritime airmasses, Category 2 had cooler and drier air than Category 4. Category 2 air parcels had a long transit from Atlantic, while Category 4 air parcels had shorter transits from the Gulf of Mexico.

The relationship between aerosol concentration and microphysical cloud properties (e.g., liquid water content, total concentration, and effective radius) was investigated for each category, taking into consideration how the properties also varied with vertical velocity and height above cloud base. In this way, it was possible to examine the dependence of aerosol indirect effects on synoptic and mesoscale meteorological conditions. Such relationships were also examined as a function of the averaging scale of the observations. In addition, the impact of the inversion on cloud development was examined.