Measurements in marine stratocumulus over the northeast Pacific help scientists unravel the mysteries of this important cloud regime.T he stratocumulus-topped boundary layer (hereafter the STBL), which prevails in the subtropics in regions where the underlying ocean is much colder than the overlying atmosphere, is thought to be an important component of the climate system. Perhaps most striking is its impact on the radiative balance at the top of the atmosphere. The seasonally averaged net cloud radiative forcing from the STBL has been estimated to be as large as 70 W nr 2 (Stephens and Greenwald 1991), more than an order of magnitude larger than the radiative forcing associated with a doubling of atmospheric C0 2 . This means that even rather subtle sensitivities of the STBL to changes in the properties of the atmospheric aero-
Abstract. We present air-sea fluxes of oxygenated volatile organics compounds (OVOCs) quantified by eddy covariance (EC) during the Atlantic Meridional Transect cruise in 2012. Measurements of acetone, acetaldehyde, and methanol in air as well as in water were made in several different oceanic provinces and over a wide range of wind speeds (1-18 m s −1 ). The ocean appears to be a net sink for acetone in the higher latitudes of the North Atlantic but a source in the subtropics. In the South Atlantic, seawater acetone was near saturation relative to the atmosphere, resulting in essentially zero net flux. For acetaldehyde, the two-layer model predicts a small oceanic emission, which was not well resolved by the EC method. Chemical enhancement of air-sea acetaldehyde exchange due to aqueous hydration appears to be minor. The deposition velocity of methanol correlates linearly with the transfer velocity of sensible heat, confirming predominant airside control. We examine the relationships between the OVOC concentrations in air as well as in water, and quantify the gross emission and deposition fluxes of these gases.
Fast measurements of three scalars, ozone, dimethyl sulfide (DMS), and total water, are used to investigate the entrainment process in the stratocumulus-topped boundary layer (STBL) observed over the eastern subtropical Pacific during the second Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II). Direct measurement of the flux profiles by eddy covariance is used to estimate the entrainment velocity, the average rate at which the boundary layer grows diabatically via incorporation of overlying free tropospheric air. The entrainment velocities observed over the course of the mission, which took place during July 2001, ranged from 0.12 to 0.72 cm s−1, and appear to outpace the estimated large-scale subsidence as the boundary layer advects over warmer sea surface temperatures. Observed entrainment velocities display only a weak correlation with the buoyancy Richardson number defined at the inversion, which suggests that processes other than inversion strength, such as wind shear, might play a larger role in driving entrainment in the STBL than previously recognized. This study is the first to use DMS as an entrainment tracer because the high-rate mass spectrometric technique has only recently been developed. The biogenic sulfur compound shows great promise for such investigations in marine environments because the free tropospheric concentrations are virtually nonexistent, and it therefore serves as an unambiguous marker of boundary layer air. As such, individual mixing events can be analyzed to determine the mixing fraction of boundary layer and free tropospheric air, and in several such cases buoyancy reversal was observed despite the absence of large-scale dissipation of the cloud field as postulated by cloud-top entrainment instability. Moreover, the redundancy attained in using three separate scalars allows for an investigation of the average height scales above the inversion from where air is blended into the STBL, and this tends to be less than 80 m above the mean inversion height, implying that the entrainment process occurs on very small scales.
[1] During the 2007 UK SOLAS Deep Ocean Gas Exchange Experiment in the northeast Atlantic Ocean, we conducted the first ever study of the effect of a deliberately released surfactant (oleyl alcohol) on gas transfer velocities (k w ) in the open ocean. Exchange rates were estimated with the 3 He/SF 6 dual tracer technique and from measured sea-to-air DMS fluxes and surface water concentrations. A total of seven k w estimates derived from 3 He/SF 6 were made, two of which were deemed to be influenced by the surfactant. These exhibited suppression from ∼5% to 55% at intermediate wind speeds (U 10 ) in the range 7.2-10.7 m s −1 . Similarly, k w determined from DMS data (k DMS ) was also depressed by the surfactant; suppression ranged from ∼39% at 5.0 m s −1 to ∼24% at 10.8 m s −1. Surfactant thus has the potential to measurably suppress gas exchange rates even at moderate to high wind speeds.
SUMMARYThe rst research ight of the second Dynamics and Chemistry of Marine Stratocumulus eld study is analysed. This case attracted our interest because it showed a consistently deepening cloud layer despite macroscopic conditions which previous work has suggested should be an indication of cloud thinning or breakup. Detailed analysis of the ight data shows that despite the cloud-top entrainment instability parameter being well beyond its critical value the cloud did indeed deepen through the night. Our best estimates show little indication of rapid changes in cloud top, while cloud base was found to be lowering at a rate of several metres per hour. This evolution, and independent measurements of trace-gas budgets, imply an entrainment rate of 0 0039 0 001 m s 1 . This is compared to entrainment rates from recently proposed parametrizations (forced by the observed forcing of the cloud layer) which range from 0.002 to 0.008 m s 1 . Two of the parametrizations we test reproduce the observed entrainment rates within their stated uncertainties, although subsequent tests show that one of these rules exhibits sensitivities to changes in the environmental conditions which are dif cult to justify. Large-eddy simulation of the observed case was able to reproduce the macroscopic evolution of the layer, but in doing so had some dif culty in maintaining the observed mixing-line structure at cloud top. A comparison of the observed and simulated turbulent structure show these to be broadly consistent, although there is an indication that the structure of the simulated turbulence differs from the observations near the ow boundaries, particularly at cloud top.
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