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Howard P. Hanson

Abstract

Research aircraft measurements of a well-developed marine stratocumulus cloud-topped boundary layer, made in June 1981 off the coast of California, are analyzed using the saturation point method developed by Betts. Estimates of the cloud-top entrainment rate made from the measurements permit construction of a mixing diagram in which the physics of the layer collapse to a single mixing line when diabatic processes and nonstationarity are accounted for. This is possible because the vertically-integrated (mixed-layer) budget equations balance to within measurement uncertainty.

The mixing diagram allows calculation of cloud-top entrainment from the geometry of surface, mixed-layer and above-inversion parcel saturation points (corrected for diabatic processes) and the cloud-top cooling rate. This method, basically an inversion of the thermodynamic budgets, can also be used to calculate surface fluxes. It should be adaptable to routine meteorological data.

While no insight is given into model parameterization of entrainment, it is concluded that, for stratocumulus layers such as the one measured in the data presented here, a mixed-layer model is likely to adequately represent the thermodynamic interactions. However, the data also indicate that the criterion for breakup of a stratocumulus deck used in such models has not been adequately developed.

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Howard P. Hanson

Abstract

Mixing processes associated with tropical marine boundary-layer cumuli are examined with a one-dimensional, steady-state model of the cloud and inversion layers which explicitly differentiates between cloud-scale and subcloud-scale motions. Physically, the clouds are modeled as an ensemble average of well-mixed turbulent bursts into an otherwise quiescent, subsiding environment. The cloud-base vertical velocity distribution is found from a new closure based on a simplified vertical momentum budget. The clouds are described by two free parameters, fractional area and a detrainment time scale.

Faster detrainment rates are shown to be associated with undiluted clouds, in the sense that the cloud entrainment rate becomes zero. For this case, the cloud-scale fluxes dominate the boundary-layer mixing. Conversely, slower detrainment and large entrainment imply a diluted cloud, with the boundary-layer mixing dominated by the subcloud-scale turbulence.

Simple qualitative arguments show that the vertically averaged temperature perturbation of the clouds is very small and that increases in cloud-base moist static energy fluxes lead to enhancement of the cloud-top entrainment instability and vertical mixing associated with increasingly larger scales.

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Howard P. Hanson

Abstract

The entrainment closure for the turbulence energy budgetcommonly used in unsaturated mixed layer models is re-derivedfrom the perspective of potential energy changes, and it isconcluded that the correct physical interpretation of theenergetics based on experimental evidence is that the turbulenceform entrainment at about the 20% efficiency level(not the 4% level). In the case of a cloud-toppedmixed layer, this physical interpretation leads to are-examination of the condition for stratocumulus cloud-topentrainment instability and the conclusion that recentcorrections for liquid water loading may yet be toorestrictive.

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Howard P. Hanson

Abstract

The differential absorption and emission of radiation with height inside clouds creates sources and sinks of buoyancy and thus can be an important factor in the turbulence-maintaining and dissipating processes of the clouds. This paper is concerned with the roles that solar and infrared radiation play in the turbulence budget of layer clouds, with primary emphasis on marine stratocumulus and inferential discussion of other layer cloud systems.

Physically realistic parameterizations of solar and infrared (IR) fluxes are used to show how the turbulence generation by cloud-top IR cooling can be more than offset by stabilization due to absorption of sunlight, and how the role of cloud-base IR warming depends crucially on the height of the cloud base. In the context of a mixed-layer model, these effects can be cast entirely in terms of the height of the layer's center of mass relative to the net heating and/or cooling due to the radiative transfer. Implications for the diurnal cycle and for a thin-cloud instability are discussed.

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Howard P. Hanson
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Howard P. Hanson

Results from an analysis of 11 years of the AMS Employment Announcements newsletter are discussed. The strongest signals in the record of various job categories are the annual cycle in advertisements of tenure-track faculty positions and the interannual variations in the overall market, which are dominated by private-sector positions. On average, there are 23±9 new positions announced each month, with maxima in March and November and the minimum in July. The overall market grew throughout the mid-1980s, but has declined in recent years.

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Howard P. Hanson

Abstract

Changes in propagation of free linear waves on the equatorial β-plane associated with air-sea heat exchange are in investigated here. By using a mixed-layer model, with the waves considered as perturbations on a specified basic state, the usual separability problems are avoided and the sea surface temperature is carried as a prognostic variable. The heat exchange is limited to that associated with turbulent fluxes, and a simplified air-sea transfer function allows analytic solutions of the various equatorial modes.

The problem is reduced to the classical solutions for a single vertical mode with the air-sea heat flux and mixed layer entrainment feedback effects cast in terms of three adjustments scales: an atmospheric adjustment length scale and two oceanic adjustment time scales, one for the response to surface fluxes and one for the response to entrainment. In order for the feedback to have any effect, both surface fluxes and entrainment must be included.

Propagation speeds of the equatorial waves are affected significantly by the presence of feedback. For an assumed easterly wind, the Kelvin wave speed is decreased by as much as 15% and the Rossby wave speeds are increased by as much as 50%, depending on the magnitude of the feedback parameters. In addition, the feedback increases (decrease) the wave-related SST amplitude for downwind (upwind) propagating waves over that for the no-feedback case. This is not a positive feedback, because the dissipative nature of the feedback causes the solutions to decay.

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Howard P. Hanson
and
Baosen Long

Abstract

Extratropical cyclogenesis occurs off the coast of China, over the East China Sea and surrounding areas, as a result of cold-air outbreaks from the mainland about every five days, on the average, during winter months. Many of these storms subsequently propagate northeastward into the Pacific, and in the process they have a substantial impact on the weather of highly populated areas of northeast China, the Koreas and Japan.

This paper presents a climatological analysis of this cyclogenesis for the period 1899–1962 based on U.S. Weather Bureau historical weather maps. The averaged seasonal variability during the period is discussed in some detail with comparisons to sea-surface temperature variability, taken from the Historical Sea‐Surface Temperature Project data, revealing that the meridional gradient of sea surface temperature across the East China Sea plays a significant role in the annual cycle of cyclogenesis. In contrast, wind speed, air–sea temperature difference and averaged heat fluxes show little relationship to the cyclogenesis. This suggests that the surface baroclinicity associated with this temperature gradient triggers baroclinic instability on the subtropical jet stream as it enters and merges with the Aleutian Low to the east.

Interannual variability of frequency of storm formation is shown to be linked to variations in the surface pressure at Darwin, Australia and, by inference, to the Southern Oscillation. This appears to be a manifestation of the West Pacific teleconnection in which warm equatorial temperatures and anomalous deep convection patterns in the Central Pacific are associated with a deepening and broadening to the west of the Aleutian Low, thus increasing the potential for storm formation over the East China Sea. Variability of storm formation at the frequency of the quasi‐biennial oscillation is also suggestive of this mechanism.

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Howard P. Hanson
and
Vernon E. Derr

Abstract

The vertical structure of radiative flux profiles within clouds can have a significant impact on the thermodynamic processes that maintain and dissipate the clouds, particularly in the case of marine stratus and stratocumulus. However, dynamic models of these and other cloud systems have tended to include radiative transfer physics in only the most rudimentary fashion. This has caused potentially important feedback processes in the clouds to be neglected.

We present here simple formulations for the vertical structure of solar and infrared radiative fluxes within a layer cloud overlying a partially reflective surface. The parameterized profile shapes are analytic, with governing parameters derived from bulk radiative properties and more physically based radiative transfer models. The bulk cloud, subcloud layer and surface radiative properties are assumed to be known. The parameterizations are based on exponential functions of height, with decay scales related to cloud liquid water content. Although the results presented here are based on very simple assumptions about the cloud structure in the vertical, the method used is applicable to more general cases as well as to various other analytic and/or numerical radiative transfer calculations.

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Howard P. Hanson
and
Patricia L. Gruber

Abstract

The mixed-layer stratocumulus model first developed by Lilly is extended to include liquid-water-dependent solar optical properties and infrared radiative fluxes. The ocean-surface heat budget under these clouds is discussed as a function of ocean temperature, wind speed and large-scale divergence.

Comparison of diurnally-varying solar forcing with daily-averaged forcing indicates the importance of the nonlinear effect of the clouds becoming thin during mid-day, when the sun is strongest. Absorption of solar energy by the cloud is responsible for this: it tends to cut off turbulent entrainment, and the cloud top becomes lower; it heats the layer, and the cloud base rises.

The ocean-surface heat budget is generally negative (oceanic heating) under these clouds, and tends to become positive as the ocean temperature is raised. The climatic implications of this negative feedback, and a similar feedback at the cloud-top level are discussed.

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