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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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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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H. Gerber, J. B. Jensen, A. B. Davis, A. Marshak, and W. J. Wiscombe

Abstract

Aircraft measurements of liquid water content (LWC) made at sampling frequencies of 1 and 2 kHz with a particle volume monitor (PVM) probe from horizontal traverses in stratocumulus clouds during the Southern Ocean Cloud Experiment and cumulus clouds during the Small Cumulus Microphysics Study are described. The spectral density of the LWC measurements is calculated and compared to the −5/3 scaling law. The effect of PVM sampling noise is found to be small in most cases. Most measurements follow approximately the −5/3 law until cloud scales decrease below about 5 m in length. Below this length LWC variance can exceed that predicted by the −5/3 law. It is suggested that the enhanced LWC variance at small scales is related to entrainment of environmental air into the clouds, which changes primarily the droplet concentration.

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K. N. Bower, T. W. Choularton, J. Latham, J. Nelson, M. B. Baker, and J. Jensen

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

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J. B. Jensen, P. H. Austin, M. B. Baker, and A. M. Blyth

Abstract

The analysis of Paluch suggests that some cumuli contain cloudy air from only two sources: cloud base and cloud top. A framework is presented for the investigation of droplet spectral evolution in clouds composed of air from only these two sources. The key is the investigation of the dependence of droplet concentration N on the fraction of cloud base air F in a sample of cloudy air. This N-vs-F analysis is coupled with an investigation of droplet spectral parameters to infer the types and scales of entrainment and mixing events.

The technique is used in a case study of a small, nonprecipitating continental cumulus cloud which was sampled during the 1981 CCOPE project in eastern Montana. The mixing between cloudy and entrained air in this cloud often appears to occur without total removal of droplets, although there is evidence that total evaporation occurs in some regions with low liquid water content. The observed droplet spectra are compared with those calculated from an adiabatic parcel model. The spectral comparison and the results of the N-vs-F analysis support the hypothesis that cloudy and environmental air interact on fairly large scales with subsequent homogenization of the large-scale regions. This description is consistent with recent models of mixing in turbulent flows.

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P. H. Austin, M. B. Baker, A. M. Blyth, and J. B. Jensen

Abstract

We have analyzed small-scale fluctuations in microphysical, dynamical and thermodynamical parameters measured in two warm cumulus clouds during the Cooperative Convective Precipitation Experiment (CCOPE) project (1981) in light of predictions of several recent models. The measurements show the existence at all levels throughout the sampling period of two statistically distinct kinds of cloudy regions, termed “variable” and “steady,” often separated by transition zones of less than ten meters. There is some evidence for microphysical variability induced by local fluctuations in thermodynamic and dynamic parameters; however, the predominant variations are of a nature consistent with laboratory evidence suggesting that mixing is dominated by large structures. Entrainment appears to occur largely near cloud top but the data presented here do not permit identification of a mechanism for transport of the entrained air throughout the cloud.

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Patrick Hamill, Eric J. Jensen, P. B. Russell, and Jill J. Bauman

This paper describes the life cycle of the background (nonvolcanic) stratospheric sulfate aerosol. The authors assume the particles are formed by homogeneous nucleation near the tropical tropopause and are carried aloft into the stratosphere. The particles remain in the Tropics for most of their life, and during this period of time a size distribution is developed by a combination of coagulation, growth by heteromolecular condensation, and mixing with air parcels containing preexisting sulfate particles. The aerosol eventually migrates to higher latitudes and descends across isentropic surfaces to the lower stratosphere. The aerosol is removed from the stratosphere primarily at mid- and high latitudes through various processes, mainly by isentropic transport across the tropopause from the stratosphere into the troposphere.

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Frederick Sanders, Alan L. Adams, Norma J. B. Gordon, and Wade D. Jensen

Abstract

To enable use of aircraft winds and satellite cloud-motion vectors in the SANBAR model for prediction of tropical storm tracks, we have derived regression equations for estimating the tropospherically averaged flow from information at one, two or three levels. Two-level results represent an improvement over climatology and a third level yields substantial further improvement. We find from a study of the 1975 season in the Atlantic Basin that reduction in initial position and track-velocity errors can produce substantial improvement in position-forecast accuracy out to 72 h range. We recommend a new procedure for evaluating and using wind observations within the region influenced by the storm circulation. The new method has the potential for substantial reduction of present forecast error for storms within 24 h of landfall.

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Michael L. Banner, Wei Chen, Edward J. Walsh, Jorgen B. Jensen, Sunhee Lee, and Chris Fandry

Abstract

The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air–sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered atmospheric turbulence data in the marine boundary layer and sea surface topography data over the Southern Ocean for a wide range of wind speeds. The aim was to increase present knowledge of severe sea state air–sea interaction. This first paper presents an overview of the experiment and a detailed discussion of the methodology and mean results. A companion paper describes the findings on variability of the wind speed and wind stress and their relationship to variations in the underlying sea surface roughness.

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Wei Chen, Michael L. Banner, Edward J. Walsh, Jorgen B. Jensen, and Sunhee Lee

Abstract

The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air–sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered marine boundary-layer atmospheric turbulence data and sea surface roughness data over the Southern Ocean, particularly for gale-force wind conditions. In this paper analysis and findings are presented on key aspects of the coupled variability of the wind field, the wind stress, and the underlying sea surface roughness. This study complements the overview, methodology, and mean results published in Part I.

Weakly unstable atmospheric stratification conditions prevailed during SOWEX, with wind speeds ranging from gale force to light and variable. Throughout the SOWEX observational period, the wind field was dominated by large-scale atmospheric roll-cell structures, whose height scale was comparable with the thickness of the marine atmospheric boundary layer (MABL). Well above the sea surface, these coherent structures provide the dominant contribution to the downward momentum flux toward the sea surface. Closer to the sea surface, these organized large-scale structures continued to make significant contributions to the downward momentum flux, even within a few tens of meters of the sea surface.

At the minimum aircraft height, typical cumulative stress cospectra indicated that 10-km averages along crosswind tracks appeared adequate to close the stress cospectrum. Nevertheless, a large-scale spatial inhomogeneity in the wind stress vector was observed using 10- and 20-km spatial averaging intervals on one of the strongest wind days when the mean wind field was close to being spatially uniform. This indicates a departure from the familiar drag coefficient relationship and implies large-scale transverse modulations in the MABL with an effective horizontal to vertical aspect ratio of around 20.

A high visual correlation was found between mean wind speed variations and collocated sea-surface mean square slope (mss) variations, averaged over 1.9 km. A comparable plot of the 10-km running average of the downward momentum flux, observed at heights from 30 to 90 m, showed appreciably lower visual correlation with the wind speed variations and mss variations. The 10–20 km averaging distance needed to determine the wind stress was larger than the local scale of variation of the mss roughness variations. It also exceeded the scale of the striations often observed in synthetic aperture radar imagery under unstable atmospheric conditions and strong wind forcing. This highlights an overlooked intrinsic difficulty in using the friction velocity as the wind parameter in models of the wind wave spectrum, especially for the short wind wave scales.

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