Saurabh Patil - March 4

Convective Meteorology (Mesoscale Dynamics)

The Impacts of Airmass, Vertical Motion and Mixing on the Microphysical Properties of Continental Boundary Layer Clouds Observed During RACORO

Saurabh Patil

Friday, March 4

03:00 PM

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Boundary layer clouds comprising cumulus, stratus, and stratocumulus cover a large spatial extent and play a key role in regulating Earth’s radiation budget. The microphysical processes that govern the evolution of these cloud properties are usually parameterized in weather and climate prediction models because they occur on smaller scales than can be represented in such models. It is important to understand these processes in order to better represent them in models. In this work, we study the effects of different airmasses, vertical motions and mixing mechanisms on continental boundary layer cloud properties using in-situ measurements in order to hypothesize processes important for their evolution.

Comprehensive measurements of continental boundary layer clouds were obtained by instruments installed on the CIRPAS Twin Otter aircraft during the RACORO field campaign conducted from late January to June 2009 in the vicinity of the Southern Great Plains Atmospheric Radiation Measurement Climate Research facility in Lamont, Oklahoma. The sampled clouds are sorted into five categories based on the origin of the airmass in which they grew, with the origin determined from 72-hour back trajectories produced by the NOAA HYSPLIT model.

In-situ observations show mean cloud droplet number concentrations of around 250 cm-3 with wide variations of up to 2000 cm-3 during the campaign. The mean Liquid Water Content (LWC) was 0.08 g.m-3 but mostly below 0.4 g.m-3. Effective radii of cloud droplets were between 2 to 8 microns indicating condensational growth of droplets could be effective over all cloud heights. Observed cloud data are further categorized according to whether observations were made in the low, middle, or high parts of the cloud to study their vertical structure. There was large variability in LWC at the high parts of clouds. The vertical motions were mostly within +/- 5 m.s-1 at low and mid-levels but span a wider range up to 15 m.s-1 in higher parts of cloud. The scatter plots of cloud droplet effective radius with in-cloud vertical velocities does not show any correlation, suggesting that the condensational growth of droplets has no impact on vertical motions. Overall, our preliminary analysis shows only a minor impact of airmass origin on microphysical properties of continental boundary layer clouds.