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H. Gerber
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H. Gerber

Abstract

This observational study looks at the distribution of some cloud microphysical properties measured from the University of Washington's aircraft in mostly unbroken stratocumulus (Sc) clouds in the vicinity of the Azores Islands during ASTEX. The average behavior of the Sc includes the presence of two drizzle modes. In 85% of the Sc the amount of drizzle LWC (liquid water content in droplets with radius r > 20 µm) is less than 0.01 g m−3, while in the rest, drizzle LWC is much larger than 0.01 g m−3. The microphysics of light-drizzle Sc approach classical conditions, because measured and adiabatic LWC profiles are similar, and droplet spectral dispersions decrease with height. The lognormal function fits approximately measured droplet spectra of light-drizzle Sc. Standard deviations of these spectra remain about constant with height, vary over a small range, and average 1.74-µm radius.

Drizzle LWC > 0.01 g m−3 is found on the average in Sc with spectra that have effective radii r e &ge 16 µm. This heavy-drizzle threshold coincides approximately with the experimental coalescence threshold of 19-µm radius when the largest droplets in the average droplet spectrum are considered. This leads to the conclusion that droplet growth by condensation plays an important role in the formation of heavy-drizzle Sc.

Measurements near Sc cloud top show narrow regions of depleted LWC with unchanging values of re, suggesting the presence of entrainment and inhomogeneous mixing.

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H. Gerber

Abstract

Droplet sizes, larger than expected, and transient water vapor supersaturations were measured in radiation fog. Nongradient turbulent mixing of saturated air parcels at different temperatures and the release of excess vapor by molecular diffusion at the interface between the mixing parcels are suggested as the mechanisms causing the large supersaturations. Approximate agreement is found between calculated rates of change of supersaturation during nongradient mixing and the supersaturation measurements. A stochastic mixing model, based on the supersaturation and other measurements in the fogs, is used to estimate if nongradient mixing and transient supersaturations cause the appearance of large droplets. The model predicts a broadening of the droplet spectra to include no larger than midsized droplets. This study concludes that a form of nonlocal turbulence closure may be required in models to accurately describe microphysics in fogs and clouds when nongradient mixing is important. This mixing causes droplet broadening and activation of cloud condensation nuclei within fogs and clouds; the effect is both proportional to the temperature difference of mixing saturated air parcels and inversely proportional to the droplet integral radius.

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H. Gerber

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A new method is proposed for measuring the liquid water content of fogs and hazes. It consists of a planar circular light sensor placed perpendicular to and coaxial with a narrow collimated light beam of a visible wavelength. The direct light from the beam, which is narrower than the width of the sensor, is blocked near the sensor, so that the sensor sees only the light scattered in the near-forward direction by the droplets in the beam. Calculations with numerous droplet size distributions found in maritime fog, and haze aerosols show that the flux of scattered light is approximately proportional to the liquid water content. Advantages of an instrument using the proposed method would be in situ measurements, rapid response, compactness and simplicity.

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H. E. Gerber

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The bulk single-scattering albedo of atmospheric particles was determined by simultaneously measuring the optical transmittance and the scattering of light in a multipass cell filled with concentrated ambient aerosol. During a naval research cruise in North Atlantic and the Mediterranean, results were obtained which suggested that the absorption of red light by the maritime aerosol was dominated by particles whose origin was the European continent. Absorption extremes approached 50% of light extinction near industrialized harbor areas. Particle size distributions and albedos measured during the cruise were inverted with Mie theory to give the imaginary part of the particles' refractive index. The mean of the imaginary index for the cruise was 0.0224, which is typical of rural continental areas.

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H. E. Gerber

Abstract

The relative humidity, temperature, droplet size distribution and transmittance of light at 632.8 nm were measured in a radiation fog. A new saturation hygrometer capable of measuring relative humidity between ∼95 and 105%, was used for the first time. Excursions of relative humidity into the supersaturation regime were brief and rapid. Estimated peak values of supersaturation were several tenths of 1%. The mean relative humidity in the fog was <100%. The importance of turbulence in controlling the fog was evident in the rapid fluctuations in all the measured variables, and in the fog's quasi-periodic oscillations which had a mean period of 18 min. Evidence suggested that turbulent mixing of nearly saturated eddies was the cause of fog formation and the broadening of the droplet size distribution.

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H. E. Gerber

Abstract

A new saturation hygrometer capable of measuring relative humidity between 95 and 105% is described. The hygrometer uses a thermally thin mirror on which condensation is controlled by limiting the exchange of ambient water vapor to condensation sites consisting of sized submicron salt nuclei. The salt-solution droplets formed on the nuclei are prevented from exceeding a reference size by heating the mirror with infrared diodes which are part of a thermo-optical servo system. The temperature increase of the mirror is directly related to the ambient relative humidity.

The performance of the hygrometer is modeled for the hypothetical case where the instrument is imbedded in an updraft which passes through the base and the lower portion of a typical continental cumulus cloud. The influences of the thermal properties of the mirror, the heat conduction to the mirror support, the radiative energy exchange between the mirror and the environment, the size of the salt nuclei on the mirror, and the interaction with the cloud droplets are evaluated. The hygrometer is tested in the laboratory by measuring temperature increases of the mirror for infrared-heater outputs, and by exposing it to supersaturations produced by a continuous-flow thermal-gradient diffusion chamber.

The theoretical results, the stability of the hygrometer under conditions of supersaturation, its insensitivity to ambient aerosols, and its ease of calibration together indicate that the hygrometer can be developed into a practical sensor for atmospheric use.

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H. Gerber and P. J. DeMott

Abstract

Correction factors C f are derived for ice-crystal volume and effective radius Re, measured by Forward Scattering Spectrometer Probe (FSSP) and Particulate Volume Monitor (PVM) that are known to overestimate both parameters for nonspherical particles. Correction factors are based on ice-crystal volume and the projected area of randomly oriented model ice crystals with column, rosette, capped-column, and dendrite habits described by Takano and Liou. In addition, C f are calculated for oblate and prolate spheroids. To test C f, both probes are compared to small, predominately solid hexagonal ice-crystal plates and columns generated in the Colorado State University (CSU) Dynamic Cloud Chamber (DCC). The tendency of heat released by the PVM (placed inside the chamber) to evaporate ice crystals and the smaller upper size range of the PVM than the size range of the FSSP caused large differences in the probes’ outputs for most comparisons in the DCC. Correction factors improved the accuracy of Re measured by the FSSP for the solid hexagonal crystals, and both probes produced similar results for the projected area and ice water content when crystal sizes fell within the probes’ size ranges. The modification for minimizing ice-crystal shattering and the application of C f for forward scatter probes such as the FSSP suggests the probes’ improved usefulness for measuring small ambient ice crystals.

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H. Gerber, Szymon P. Malinowski, and Haflidi Jonsson

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Buoyancy reversal by evaporative cooling in entrainment holes has a minimal influence on stratocumulus (Sc) observed during the Physics of Stratocumulus Top (POST) aircraft field study held off the California coast in 2008. High-resolution temperature and microphysics measurements show only small differences for Sc with and without buoyancy reversal predicted by mixing fraction analysis that relates mixtures of cloudy air and free-atmospheric air to buoyancies of the mixtures. The reduction of LWC due to evaporation in the holes is a small percentage (average ~12%) of liquid water diluted in the Sc by entrainment from the entrainment interface layer (EIL) located above unbroken cloud top where most mixing, evaporation, and reduction of the large buoyancy jump between the cloud and free atmosphere occur. Entrainment is dominated by radiative cooling at cloud top.

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H. Gerber, Simon Chang, and Teddy Holt

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High-resolution profiles of temperature and wind-speed measurements were made with a tethered baloon in and above the marine boundary layer at San Nicolas Island (SNI) during a period when the cloud-free boundary layer grew from near the sea surface to 450 m in approximately 12 h. Measurements showed the formation of a low-level jet which remained centered at the temperature inversion as the boundary layer grew. The upper limit of the jet coincided with the top of a temperature transition layer that extended from the sharp temperature jump at the inversion to the free atmosphere above.

The experimental evidence suggested that the jet was caused by thermal wind resulting from a specific sea surface temperature gradient, and from horizontal temperature gradients caused by a sloped inversion and the transition layer. Production of mechanical turbulence by wind shear in the jet caused rapid entrainment into the mixed layer of warmer air from above, and the fast growth of the boundary layer.

A quasi-two dimensional (2D) model including turbulence parameterized in terms of turbulent kinetic energy (TKE) and dissipation rate was able to reproduce the main features of the evolving boundary-layer jet and temperature field. The predicted shape, location, and intensity of the jet and the growth of the boundary layer were similar to the observations. The model also predicted realistic heat and momentum fluxes and TKE budgets as judged by comparisons with aircraft measurements by Brost et al. in a similar case off the West Coast. Using a variety of initial conditions, the model further showed that the jet was likely caused by the combined effects of the inertial acceleration of the wind field, the specific temperature gradients, and the sloping inversion.

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