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David C. Rogers

Abstract

A few basic experiments were performed with artificial cloud condensation nuclei and ice-nucleating aerosols in a continuous-flow thermal gradient diffusion chamber to investigate the instrument's response. Brief pulses of ice nuclei were input to the chamber and were detected approximately 8 s later as pulses of ice crystals at the outlet. The temporal rise and fall responses were of exponential type, with time constants of about 1 s. These fast response characteristics aye due to the chamber's laminar flow and the central location of the aerosol sample. Changes in the size distribution of crystals at the outlet of the chamber were monitored in response to changes in total airflow through the chamber. The results indicated that the crystals were fewer and smaller as their residence time in the chamber decreased.

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David P. Rogers

Abstract

Aircraft observations obtained during the Frontal Air–Sea Interaction Experiment (FASINEX) are used to investigate the structure of the marine atmospheric boundary layer in the vicinity of an ocean front. A quasi-stationary sea surface temperature (SST) discontinuity of 2°C was maintained across the frontal zone throughout the duration of the experiment The primary response of the atmosphere to changes in the SST was observed in the surface-related turbulence fluxes. In the case of warm air flowing over cold water, the boundary layer appears to develop an internal boundary layer (IBL) in response to the sudden change in the sea surface temperature. The organized updrafts and downdrafts within this layer collapse with entrainment–detrainment processes in these cells dominating the turbulence statistics. The IBL grows in response to the wind shear in this layer, although the surface shear stress is much smaller on the colder side of the front than on the warm. The depth of the IBL, and, in the absence of the IBL, the mixed layer are found to scale with the friction velocity and the Coriolis parameter.

The IBL confines the surface-related turbulent mixing and shear-driven processes to the lower layers of the atmosphere. Thus. the shallow boundary layer cloud field appears to be maintained primarily by radiative transfer within the cloud layer. Multiple cloud-capped mixed layers were frequently observed throughout the experiment. They appear to be directly related to the horizontal variation of the SST with deeper boundary layer and higher cloud levels formed over warmer water.

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Alan Thorpe and David Rogers

Abstract

The Global Weather Enterprise (GWE) encompasses the scientific research, technology, observations, modeling, forecasting, and forecast products that need to come together to provide accurate and reliable weather information and services that save lives, protect infrastructure, and enhance economic output. It is a value chain from weather observations to, ultimately, the creation of actionable analysis-and-forecast weather information of huge benefit to society. The GWE is a supreme exemplar of the value of international cooperation, public–private engagement, and scientific and technological know-how. It has been a successful enterprise, but one that has ever-increasing requirements for continual improvement as population density increases and climate change takes place so that the impacts of weather hazards can be mitigated as far as possible. However, the GWE is undergoing a period of significant change arising, for example, from the growing need for more accurate and reliable weather information, advances coming from science and technology, and the expansion of private sector capabilities. These changes offer real opportunities for the GWE but also present a number of obstacles and risks that could, if not addressed, stifle this development, adversely impacting the societies it aims to serve. This essay aims to catalyze the GWE to address the issues collectively, by dialogue, engagement, and mutual understanding.

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Alan Thorpe and David Rogers
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Darko Koračin and David P. Rogers

Abstract

The effect of a stable internal boundary layer (IBL) on the cloud-capped marine boundary layer is investigated using a one-dimensional second-order closure model. A stable IBL forms if there is a reversal in the surface buoyancy flux when warm air flows over colder water. These conditions exist in the vicinity of Ocean fronts where sea surface temperature discontinuities of about 2°C in 5 km have been observed. There is a balance between the buoyant consumption and inertial production of kinetic energy so that the layer remains weakly turbulent and can deepen due to shear-driven mixing. The stability of the layer limits momentum exchange with the air above so that there is a significant reduction in the surface stress in the IBL and an acceleration of the flow aloft. There are important implications for cloud development in regions of large ocean temperature gradients because a stable IBL can limit the vertical transfer of moisture from the surface to the upper part of the boundary layer.

In addition, solar radiation is found to heat the cloud layer sufficiently to cause decoupling between the cloud and subcloud layers during the daytime. This effect is important in determining the rate at which the cloud layer evaporates.

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David C. Rogers and Gabor Vali

Abstract

Evidence is presented for a process of ice crystal generation in supercooled orographic clouds in contact with snow-covered mountain surfaces. Comparisons of the crystal concentrations at the surface with aircraft sampling indicate that the “anomalous” crystals originate at the interface of the cloud with the surfaces. Crystal concentrations at the surface, over the temperature range −5° to −23°C, were found to be roughly 100 times higher than in the main body of the clouds. Occasionally, the effects extends to altitudes as much as 1 km above the ground in the clouds studied, and indications are that even greater depths of clouds might be influenced over extended mountain ranges. The mechanism of ice crystal generation involved has not yet been firmly established; several possibilities are discussed in the paper. The phenomenon can be expected to have significant implications for the characteristics of low-altitude orographic clouds with respect to their propensity to produce precipitation; radiative, chemical and electric properties; and their suitability for cloud seeding.

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Michael Tjernström and David P. Rogers

Abstract

The average and turbulence structure of two marine stratocumulus layers, from the Atlantic Stratocumulus Transition Experiment, are analyzed. These layers ware in adjacent air masses with different histories: one cloud layer was in a clean air mass with a marine history and the other was in a continental air mass, which had a higher aerosol content. The air masses were brought together by synoptic-scale flow and are separated by a semiclear transition zone.

The clouds were decoupled from the marine surface mixed layer in both air masses. In the transition zone, the marine mixed layer was deeper than that under either cloud layer. The total depth below the main inversion, including the cloud layer was, however, substantially greater than in the semiclear transition zone. The western clean cloud layer was more well mixed than the eastern aerosol-rich cloud layer, and the turbulence analysis shows that the western cloud layer complies to convective scaling. Buoyancy production of turbulence was also positive in the eastern cloud, but here shear production was larger than the buoyancy production by a factor of 4, and convective scaling fails.

One cause of the stability differences may lie in differences in radiative forcing, both external and internal. The difference in external forcing is due to higher humidity aloft to the east, reducing the net cloud-top longwave cooling. The difference in internal forcing is due to differences in the cloud microphysics. The larger number of smaller drops in the eastern cloud, arising from the abundance of aerosol particles, increases both albedo and absorption, and thus solar heating. Increased solar heating balances the longwave cooling at the cloud top, warms the cloud interior, and decreases the depth of the net-cooled layer in the cloud. All these effects decrease the buoyancy in the eastern cloud layer in comparison to the western one.

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David P. Rogers and Darko Korac̆in

Abstract

The effects of longwave and shortwave radiative heating on the coupling between stratocumulus clouds and the boundary layer is investigated using a one-dimensional second-moment turbulence-closure model. The decoupling of a stratiform cloud from the subcloud layer is often a precursor to cloud break up and the transition to scattered cumulus clouds or clear sky. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. In particular, a strong moisture flux can maintain weak coupling between the cloud and subcloud layers so that the lower part of the cloud layer may continue to develop despite the formation of a stable temperature gradient between the top of the subcloud layer and cloud base.The effect of shortwave heating on decoupling is threefold. First, shortwave heating directly offsets the net longwave cooling at cloud top by as much as 30% (in February at latitude 29°N), reducing or eliminating the overall cooling of the cloud layer during part of the day. Second, shortwave heating decreases exponentially from a maximum at cloud top, which tends to stabilize and evaporate the cloud layer. In a deep cloud layer radiative heating is restricted to the upper part of the cloud, which warms at a faster rate than the lower part of the cloud; hence, decoupling can occur within the cloud layer. Vertical mixing in the cloud is limited, and multiple cloud layers may form. Third, the maximum shortwave heating is displaced below the maximum longwave cooling, creating a divergent flux that generates convection in the upper part of the cloud layer that, in turn, promotes entrainment. In a deep cloud layer, shortwave radiative heating can affect the decoupling of a cloud and subcloud layer only if longwave cooling is reduced sufficiently to allow longwave radiative heating of cloud base to warm the lower part of the cloud at a faster rate than the subcloud layer is heated by the sea surface. In a shallow cloud layer, shortwave radiation may penetrate to cloud base to provide an additional heat source to decouple the cloud from the subcloud layer.These results highlight the difficulty of predicting the formation, evolution, and dissipation of marine stratocumulus clouds.

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Lawrence Cheng and David C. Rogers

Abstract

Observational evidence from an Alberta hailstorm was examined in an attempt to demonstrate the link between feeder clouds and hailfalls. Radar data, time resolved surface collections of hail, and cloud photographs from a storm were analyzed. It was found that the streak events in the surface hailfall can he linked to small-scale radar reflectivity maxima in the new growth region of the storm. The results suggest that the hail growth process began with packets of hail embryos in distinct feeder clouds, and that the separation between feeder clouds was eventually manifested as distinct hail streak events at the surface. The feeder clouds formed approximately in a line parallel to the vertical ambient wind shear near the cloud base level. The spacings between feeder clouds were almost equal and estimated to he 3 km. Theoretical predictions indicate that convective spacing in a horizontally uniform atmosphere is determined by environmental wind shear, stability, and depth of the shear layer. The results of this and other observational studies lead to the speculation that the spacing between distinct hail streak events may be controlled by the same factors in the vicinity of the new growth zone of hailstorms.

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Ian M. Brooks and David P. Rogers

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Large-scale horizontal rolls can have a significant influence on turbulent transport across the atmospheric boundary layer. The formation and maintenance of such rolls is dependent on the thermal and dynamic stability of the boundary layer (BL). The authors present aircraft observations of boundary layers, both with and without roll circulations, off the coast of California. The contribution of the rolls to the turbulent fluxes of heat, moisture, and momentum, and the variances of the three velocity components are determined for four cases. The fractional roll contributions to the u and w variances, and the sensible heat and along-wind momentum fluxes, show a near linear increase with altitude, from less than 10% at 30 m to more than 70% at the top of the BL. The variance in υ and crosswind momentum flux are more scattered, although the variance shows a slight increase with altitude from about 40% to 60%. The latent heat flux also shows a great deal of scatter, especially in the lower third of the BL where the total flux is small; above this, values range between about 40% and 85% but show no clear trends. A stability parameter in the form of a bulk Richardson Ri number is calculated for each of 13 profiles through the boundary layer; it is found that the Richardson number successfully identifies those cases where rolls are present, and its value corresponds to some extent with the strength of the rolls. Values close to zero correspond to cases with well-defined rolls; for 0.1 < Ri < 0.25 rolls are found to exist, but they tend to be weak and patchy; and no rolls are found where Ri is greater than the critical value of approximately 0.25. Reynolds numbers are calculated from a number of different definitions and indicate the dynamic instability of the shear dominated boundary layers.

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