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Wayne M. Angevine, S. K. Avery, and G. L. Kok

Abstract

Measurements of the turbulent virtual heat flux in the convective atmospheric boundary layer made with a 915-MHz boundary-layer wind profiler-radio acoustic sounding system (RASS) are compared to flux measurements from a King Air aircraft. The profiler-RASS flux was calculated by a refined eddy correlation technique. The measurements were made during the Rural Oxidants in the Southern Environment II experiment in June 1992. The area over which the measurements were made is primarily pine forest, and the dominant weather conditions were hot with light winds. The profiler-RASS measurements and the aircraft measurements agree well. Even under these light wind conditions, a 2-h-average profiler-RASS measurement may be sufficiently accurate to be useful. The instrumental error is estimated to be less than the uncertainty due to sampling of the turbulence.

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I. R. Paluch, S. McKeen, D. H. Lenschow, R. D. Schillawski, and G. L. Kok

Abstract

A diurnally averaged ozone decay rate of about 0.11 day−1was observed under partly cloudy sky in the boundary layer over the eastern Atlantic during the second Lagrangian experiment in the Atlantic Stratocumulus Transition Experiment. The observed decay rate can be accounted for by the combined effects of ozone loss through photolysis by UV radiation, reaction With H02 radical, and deposition on the sea surface, and effects of ozone production through photooxidation of carbon monoxide and methane in the presence of nitrogen oxides. The main contributor to the ozone decay is photolysis by UV radiation, but the other sources and sinks also make significant contributions.

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I. R. Paluch, D. H. Lenschow, S. Siems, G. L. Kok, R. D. Schillawski, and S. McKeen

Abstract

The mean time rates of change of temperature, total water mixing ratio and ozone along airflow trajectories in the lower troposphere over the eastern Pacific are inferred by comparing aircraft soundings from the First ISCCP Regional Experiment (FIRE) and the Hawaiian Rainband Project (HaRP). Through the use of the estimated mean fluxes of temperature and total water mixing ratio, it is found that the tendency for stratus layers to grow or dissipate is very sensitive to the assumed turbulence structure below the capping inversion. A mixed-layer model that assumes a well-mixed boundary layer up to the capping inversion predicts a solid cloud layer extending all the way to Hawaii, whereas a model that allows decoupling predicts rapid dissipation of the stratus layer. It is concluded that stratus dissipation here is due to the slowdown of turbulent mixing throughout the layer below the capping inversion, not the drying out of a well-mixed layer; hence, the mixed-layer model cannot be expected to predict realistic cloud dissipation. The differences in ozone concentration observed in the boundary layer during HaRP and FIRE suggest a chemical loss of ozone of 3–8ppb day−1, corresponding to a lifetime of 3–9 days. This implies that ozone cannot be treated as a conserved tracer when dealing with ozone budgets over periods of days. The ozone sink is probably of photochemical origin, and it requires further investigation.

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J. Verlinde, J. Y. Harrington, G. M. McFarquhar, V. T. Yannuzzi, A. Avramov, S. Greenberg, N. Johnson, G. Zhang, M. R. Poellot, J. H. Mather, D. D. Turner, E. W. Eloranta, B. D. Zak, A. J. Prenni, J. S. Daniel, G. L. Kok, D. C. Tobin, R. Holz, K. Sassen, D. Spangenberg, P. Minnis, T. P. Tooman, M. D. Ivey, S. J. Richardson, C. P. Bahrmann, M. Shupe, P. J. DeMott, A. J. Heymsfield, and R. Schofield

The Mixed-Phase Arctic Cloud Experiment (M-PACE) was conducted from 27 September through 22 October 2004 over the Department of Energy's Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) on the North Slope of Alaska. The primary objectives were to collect a dataset suitable to study interactions between microphysics, dynamics, and radiative transfer in mixed-phase Arctic clouds, and to develop/evaluate cloud property retrievals from surface-and satellite-based remote sensing instruments. Observations taken during the 1977/98 Surface Heat and Energy Budget of the Arctic (SHEBA) experiment revealed that Arctic clouds frequently consist of one (or more) liquid layers precipitating ice. M-PACE sought to investigate the physical processes of these clouds by utilizing two aircraft (an in situ aircraft to characterize the microphysical properties of the clouds and a remote sensing aircraft to constraint the upwelling radiation) over the ACRF site on the North Slope of Alaska. The measurements successfully documented the microphysical structure of Arctic mixed-phase clouds, with multiple in situ profiles collected in both single- and multilayer clouds over two ground-based remote sensing sites. Liquid was found in clouds with cloud-top temperatures as cold as −30°C, with the coldest cloud-top temperature warmer than −40°C sampled by the aircraft. Remote sensing instruments suggest that ice was present in low concentrations, mostly concentrated in precipitation shafts, although there are indications of light ice precipitation present below the optically thick single-layer clouds. The prevalence of liquid down to these low temperatures potentially could be explained by the relatively low measured ice nuclei concentrations.

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