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Steven A. Stage

Abstract

Equations for the evolution of the atmospheric boundary layer during cold air outbreaks are examined and analytic solutions are obtained which apply to the region prior to cloud formation whenever the soundings of potential temperature and water vapor mixing ratio at the shore may be treated as linear. The solutions have non-dimensionalized boundary layer depth as the independent parameter. Layer depth, potential temperature, mixing ratio, dewpoint, lifting condensation level, distance from shore (fetch), net sensible heat input, and not latent heat input are found as explicit functions of a nondimensional mixed-layer height. A perturbation method is used to derive solutions when divergence is significant and is shown to be a good approximation for realistically strong divergences.

The solution is implicit for the problem of finding the fetch at which clouds will form for given sea surface temperature and soundings at the shore but can readily be solved using numerical techniques. For cases in which divergence is negligible, cloud formation can be found using simple nomograms.

A test example based on a cold air outbreak off New York is studied and it is shown that the prediction for cloud edge agrees well with satellite observations. Analysis of the solution indicates that the sensible and latent heat fluxes per unit travel can be modeled by transfer coefficients times the difference in virtual temperature of the air at the shore and the sea surface and by the mixing ratio difference between the air at the shore and the sea surface. Methods of using the solution in analysis of satellite observations are discussed.

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Steven A. Stage and Joost A. Businger

Abstract

The model for the cloud-topped marine boundary layer presented by Stage and Businger (1981) is discussed and compared with previous models. Our model gives a considerably different interpretation of the energetics of the layer and indicates that a much higher fraction (20%) of the layer turbulence kinetic energy production is available to drive entrainment than previously supposed. In a test case, the Lilly (1968) minimum entrainment model gives entrainment rates similar to ours; however, this model is based on physically unrealistic assumptions about layer energetics. It is noted that two soundings from the International Field Year for the Great Lakes (IFYGL) exhibit behavior not allowed by Deardorff’s (1976) model. In these cases our model gives a good fit to the data and Deardorff’s model predicted a boundary layer much deeper than observed. The depth of the layer of radiative cooling at cloud top is shown to be important only if it is a significant fraction of the mixed-layer depth zB. Layer energetics are shown to prevent the cloud-top entrainment instability condition from causing much difference in theentrainment rate

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Steven A. Stage and Joost A. Businger

Abstract

A model is presented for the growth and evolution of a cloud-topped marine boundary layer. In this model the entrainment rate is determined from the turbulence kinetic energy (TKE) budget. It is assumed that the TKE budget can be partitioned according to whether each process produces TKE or converts it into potential energy, and that dissipation is proportional to production. This leads to an entrainment relationship which is considerably different than used in previous cloud-topped models.

This model is used to study an episode of cold-air outbreak over Lake Ontario during the International Field Year for the Great Lakes (IFYGL). The model reproduces changes in potential temperature and dew point as the air crossed the lake and the associated time variation of these parameters at the down-wind shore with an accuracy of better than 1°C. Model and measured soundings closely match, especially with respect to the presence and location of such features as cloud layers. Depth of the mixed layer also was generally well modeled. Use of divergences measured by the lakewide IFYGL buoy network did not give good agreement with the data. It is believed that this indicates that mixed-layer depth is sensitive to divergences at a smaller scale than the size of the lake.

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Steven A. Stage and Robert A. Weller

The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the response of the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spring of 1985/86 with an intensive period in February and March 1986 in the vicinity of 27°N, 70°W, where sea-surface-temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. This article gives a brief history of FASINEX and presents its scientific goals. An article in next month's Bulletin will describe the experimental plan.

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Steven A. Stage and Robert A. Weller

The Frontal Air-Sea Interaction Experiment (FASINEX) is a study of the response of the upper ocean to atmospheric forcing in the vicinity of an oceanic front in the subtropical convergence zone southwest of Bermuda, the response of the lower atmosphere in that vicinity to the oceanic front, and the associated two-way interaction between ocean and atmosphere. FASINEX is planned for the winter and spring of 1985/86 with an intensive period in February and March 1986 in the vicinity of 27°N, 70°W, where sea-surface-temperature fronts are climatologically common. Measurements will be made from buoys, ships, aircraft, and spacecraft. A previous article gave a brief history of FASINEX and presented its scientific goals. This article describes the FASINEX experimental plan.

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