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-

Clouds are a critical component of the Earth's coupled water and energy cycles. Poor understanding of cloud–radiation–dynamics feedbacks results in large uncertainties in forecasting human-induced climate changes. Better understanding of cloud microphysical and dynamical processes is critical to improving cloud parameterizations in climate models as well as in cloud-resolving models. Airborne in situ and remote sensing can make critical contributions to progress. Here, a new integrated cloud observation capability developed for the University of Wyoming King Air is described. The suite of instruments includes the Wyoming Cloud Lidar, a 183- GHz microwave radiometer, the Wyoming Cloud Radar, and in situ probes. Combined use of these remote sensor measurements yields more complete descriptions of the vertical structure of cloud microphysical properties and of cloud-scale dynamics than that attainable through ground-based remote sensing or in situ sampling alone. Together with detailed in situ data on aerosols, hydrometeors, water vapor, thermodynamic, and air motion parameters, an advanced observational capability was created to study cloud-scale processes from a single aircraft. The Wyoming Airborne Integrated Cloud Observation (WAICO) experiment was conducted to demonstrate these new capabilities and examples are presented to illustrate the results obtained.

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.

No abstract

No abstract

A newly developed technique for airborne dual-Doppler observations with the Wyoming Cloud Radar is used to characterize the velocity fields in vertical planes across cumulus turrets. The clouds sampled were continental in nature, with high bases (near 0°C) and with depths of 2–3 km. Clear evidence was found that the clouds evolved through sequences of bubbles, or thermals, with well-defined toroidal circulations, or vortex rings. The ring core and tube diameters were about 200–600 m, leading to turret sizes of 1–2 km in the horizontal. The largest updraft speeds were observed in the ring centers, but regions of turbulent, ascending air extended behind the thermals to distances comparable with the toroid sizes. Vertical shear of ambient winds, when present, led to a tilting of the updrafts and toroids. Patterns in the reflectivity and velocity fields indicated regions of major intrusions into the thermals, accompanied by entrainment of ambient air, or recycling of larger hydrometeors, depending on their location. In addition, at the upper cloud/environment interface, instability nodes contributed to further entrapment of cloud-free air. The observations presented in this paper constitute clear demonstrations and quantitative characterization of vortical circulations in growing cumulus turrets; they should provide a more reliable basis for the assessment of simulations and of model parameterizations.

No abstract

In the afternoon of 24 May 2002, a well-defined and frontogenetic cold front moved through the Texas panhandle. Detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary. This paper primarily uses reflectivity and Doppler velocity data from an airborne 95-GHz radar, as well as flight-level thermodynamic data, to describe the vertical structure of the cold front as it intersected with the dryline. The prefrontal convective boundary layer was weakly capped, weakly sheared, and about 2.5 times deeper than the cold-frontal density current. The radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail (∼40 m). The echo structure and dual-Doppler-inferred airflow in the vertical plane reveal typical features such as a nose, a head, a rear-inflow current, and a broad current of rising prefrontal air that feeds the accelerating front-to-rear current over the head. The 2D cross-frontal structure, including the frontal slope, is highly variable in time or alongfront distance. Along this slope horizontal vorticity, averaging ∼0.05 s−1, is generated baroclinically, and the associated strong cross-front shear triggers Kelvin–Helmholtz (KH) billows at the density interface. Some KH billows occupy much of the depth of the density current, possibly even temporarily cutting off the head from its trailing body.