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Jørgen B. Jensen and Henry Granek

Abstract

Measurement of drizzle drop sizes and concentrations are often made using optical probes with linear arrays. Their accuracy is affected by both diffraction of light and response times of the electronics connected to the sensor photodiodes. In this study the behavior of the Particle Measuring Systems, Inc. (PMS), 260X probe (a 62-photodiode system) is studied by calculating the two-dimensional Fresnel diffraction patterns for opaque disks and averaging this optical signal onto areas corresponding to the sizes of the photodiodes. The two-dimensional optical field is then transformed to a one-dimensional optical time-varying field; this simulates the passage of a drop past the photodiode array when the probe is mounted outside an aircraft. The optical field is convolved with an electronic response function to simulate the variation of the electronic signal as a drop passes through the diode array.

For aircraft speeds less than 120 m s−1, one component of the probe sample volume, the depth of field, is found to exceed the manufacturer's values by up to 50% for larger drops (drop radius R > 35 μm), whereas for smaller drops (20–35 μm) the depth of field may be a small fraction of that suggested by the manufacturer. For higher airspeeds, the depth of field tends to be smaller than the manufacturer's values. The present results also show depth of field values that may be more than three times larger than those found in a previous theoretical study using Fresnel diffraction.

Examination of observations from a marine stratocumulus flight with high drizzle rates reveals a problem with the probe. The optoelectronic model predicts that the 260X probe should not be able to register counts in the two smallest-sized bins. Yet the counts in bins 1 and 2 constitute 96%–99% of all the drop counts for 33 1–min segments in cloud or drizzle below cloud. The high number of counts correlates with the mean-volume radius of drizzle drops. One possible explanation that is consistent with the measurements is that counts in bins 1 and 2 occur when large drizzle drops impact on the probe tips, followed by small droplet fragments moving through the laser beam at reduced speed.

Measured drop spectra in the 19–23.5-μm range, using the new calibration, do not match well with another sensor that measures cloud droplets. This suggests that drizzle drop breakup may also contribute artificial counts to bins 3 and larger.

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Jørgen B. Jensen and Sunhee Lee

Abstract

The concentrations and sizes of smaller aerosols (radius smaller than 0.5 μm) in the marine atmosphere vary owing to natural and anthropogenic factors. The concentrations and sizes of giant and ultragiant aerosols vary primarily due to wind-speed-dependent wave breaking. In climate models the formation of warm rain from marine stratocumulus clouds is usually parameterized based on the drops that form on the smaller aerosols. The present process study, using a stochastic Monte Carlo cloud model, shows that the variability of giant sea-salt aerosols and the variability of smaller aerosol cloud condensation nuclei are equally important in determining precipitation flux in marine stratocumulus. This strongly suggests that the effects of giant sea-salt aerosols should be included in the parameterization of warm rain formation in climate and other large-scale models.

The above results are based on highly detailed calculations of droplet growth in an idealized marine stratocumulus cloud; the authors believe that other marine stratus cloud conditions may change the calculated rain rates but that the conclusions regarding the relative importance of small and giant aerosols are robust.

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Jørgen B. Jensen and Marcia B. Baker

Abstract

The mixing of cloudy, saturated air with cloud-free, subsaturated air is examined with a simple one-dimensional model of the mixing process. The model incorporates (i) a one-dimensional parameterization of turbulent deformation, (ii) molecular diffusion, (iii) sedimentation of droplets and (iv) droplet evaporation in the changing humidity field. The model predicts droplet spectral shapes after homogenization of the two air masses.

The turbulent deformation parameterization is based on inertial subrange arguments. It yields a crude estimate of the time development of the vapor and temperature fields in a single characteristic eddy in one dimension rather than the full range of eddy scales in three dimensions.

For the high turbulence levels commonly found in cumuli the evaporation process appears close to that described earlier as homogeneous, which may in part be due to the approximations in our treatment. For all other conditions the calculations show total evaporation of some drops accompanied by some change in droplet spectral shape.

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Jørgen B. Jensen and Graciela B. Raga

Abstract

A number of previous studies have described how airborne temperature-sensor wetting can lead to erroneous temperature measurements under various conditions. During the 1985 Joint Hawaii Warm Rain Project (JHWRP) a large number of trade-wind cumuli were sampled by the University of Wyoming King Air. It was found that both the reverse flow and the Rosemount sensors showed signs of wetting during the project. Therefore, a post-field calibration of the Lyman-α hygrometer was performed to derive the temperature in the cloudy air.

The University of Wyoming Lyman-α sensor is a simple system with a high-pressure emitter tube. Both water vapor and oxygen contribute to the total absorption at the altitudes where the sensor is used. Thus, a highly simplified two-gas model is proposed for its function, and the instrument is calibrated by comparison with the dewpoint temperatures in clear-air soundings. An accuracy of ±0.5°C is estimated for the calibration. Absorption by water vapor and oxygen constitute nearly 70% and 30% of the total absorption, respectively.

Even for moderate liquid water contents (≈1 g m−3), the difference between the temperatures derived from the Lyman-α and from immersion sensors approaches theoretical predictions for fully wetted sensors. The present study shows a somewhat higher degree of wetting of the reverse-flow sensor than recently published results using an Ophir radiometer.

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Deborah J. Abbs and Jørgen B. Jensen

Abstract

A nonhydrostatic mesoscale model is used to simulate the dynamics and microphysics of postfrontal flow in the mountainous region of southeastern Australia. The aim of the paper is to determine if it is possible to use 2D models to simulate the characteristics of the liquid water field upstream from Baw Baw Plateau under postfrontal conditions. Results from both 2D and 3D simulations are compared with aircraft and surface observations taken during the Australian Winter Storms Experiment I, conducted during July and August 1988. The observations and both the 2D and 3D simulations show that under postfrontal conditions, the main feature of the flow is a series of standing lee waves downstream from Baw Baw Plateau. The microphysical fields are characterized by a cap cloud over Baw Baw Plateau and a region of high liquid water content extending at least 50 km upstream from the plateau. Convective elements form upstream from the plateau and are subsequently advected to the northeast. As the convective elements cross Baw Baw Plateau, they precipitate and subsequently evaporate in the drier subsidence region to the lee of the plateau. The main features of the airflow and cloud fields are well simulated by the 2D model runs; however, the 2D runs overestimate the precipitation amounts as compared with the surface observations and the 3D model results.

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Jørgen B. Jensen and Alan M. Blyth

Abstract

No abstract available.

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Jørgen B. Jensen and Alison D. Nugent

Abstract

The most basic aspect of cloud formation is condensational growth onto cloud condensation nuclei (CCN). As such, condensational growth of cloud drops is often assumed to be a well-understood process described by the drop growth equation. When this process is represented in models, CCN activate into cloud drops at cloud base, and it is often assumed that drops consist of pure water or that the hygroscopic contribution after drop activation is small because of the inclusion of only small CCN. Drop growth rate in adiabatic ascent in such models is proportional to supersaturation and assumed to be inversely proportional to the drop radius, thereby making the drop spectrum narrow with altitude. However, the present study demonstrates that drop growth on giant sea-salt aerosol particles (GCCN; dry radius 0.5 m) behaves differently. For typical marine stratocumulus updrafts and for drops grown on GCCN with sizes m, these drops typically remain concentrated salt solutions. Because of this, their condensational growth is accelerated, and they rapidly attain precipitation drop sizes through condensation only. Additionally, drops formed on GCCN may also grow by condensation in cloudy downdrafts. The strong effect of condensation on GCCN is important when carried through to calculating rain-rate contribution as a function of aerosol size. GCCN larger than 2 m account for most of the rainfall rate in the modeled precipitating marine stratocumulus.

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Graciela B. Raga, Jørgen B. Jensen, and Marcia B. Baker

Abstract

We have analyzed aircraft observations from seventeen cumulus cells within cloud bands observed off the east coast of Hawaii during the Joint Hawaii Warm Rain Project (JHWRP) of 1985.

Low level convergence generated by the encounter of the trade winds and the island determines the location of the initial convection. However, the upward momentum below cloud base seems to be less important in the subsequent evolution of the bands than buoyancy associated with latent heat release.

Entrainment into the clouds occurs at all levels, and almost all cloudy parcels below the trade inversion are moving upwards. While evaporative cooling does not seem to enhance entrainment below the inversion, it does play a role in the descent of cloud top air to the bottom of the inversion. Despite the existence of undiluted cores, the average thermodynamic characteristics of the clouds below the inversion appear well described by a very simple, constant lateral entrainment rate parcel model. We have used in-cloud and near environment vertical fluxes of mass, water, heat, and horizontal momentum to estimate the band cloud impacts on vertical profiles of these tracers in the cloud and above cloud layers.

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Matthew Hayman, Katie J. McMenamin, and Jørgen B. Jensen

Abstract

Two-dimensional optical array probes are commonly used for imaging raindrops and ice particles on research aircraft. The ability of these probes to accurately measure particle concentration and size partly depends on the response characteristics of the detection system. If the response characteristics are too slow, then small particles are less likely to be detected and the associated effective sample volume decreases. In an effort to better understand the sample volumes of optical array probes at the National Center for Atmospheric Research, the temporal response of the Fast-2D optical array probe detector board from optical input on the detector to digitization was characterized. The analysis suggests that the board electronics have a response time constant consistently near 50 ns. However, there is also a slow decay term that conforms to a decay rate. The amplitude of this slow function can impact the probe response, varying the minimum detectable pulse width between 60 and 150 ns. Also, the amplitude of the slow function is largely dictated by the illumination angle of incidence. The effects of the response time characteristics are analyzed using a simulator for a 2D cloud (2D-C) probe with 25-μm photodiode spacing. The results show the greatest sensitivity to response time characteristics when particles are smaller than 150 μm, where 10% uncertainty in the slow fraction is likely to produce sample volume uncertainties near 10%. Ignoring response time effects may bias sample volume estimates in the small size regime by as much as 25%.

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Piotr Dziekan, Jørgen B. Jensen, Wojciech W. Grabowski, and Hanna Pawlowska

Abstract

The impact of giant sea salt aerosols released from breaking waves on rain formation in marine boundary layer clouds is studied using large eddy simulations (LES). We perform simulations of marine cumuli and stratocumuli for various concentrations of cloud condensation nuclei (CCN) and giant CCN (GCCN). Cloud microphysics are modeled with a Lagrangian method that provides key improvements in comparison to previous LES of GCCN that used Eulerian bin microphysics. We find that GCCN significantly increase precipitation in stratocumuli. This effect is strongest for low and moderate CCN concentrations. GCCN are found to have a smaller impact on precipitation formation in cumuli. These conclusions are in agreement with field measurements. We develop a simple parameterization of the effect of GCCN on precipitation, accretion, and autoconversion rates in marine stratocumuli.

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