An observational study of the structure of the nocturnal boundary layer

In an effort to describe the basic vertical structure of the nocturnal boundary layer, observations from four experiments are analyzed. During the night, the depth of significant cooling appears to increase with time while the depth of the turbulence and height of the low level wind maximum tend to remain constant or decrease with time. Since the inversion layer extends above the low level wind maximum and shear is small in the region of the low level jet, the Richardson number reaches a maximum at the jet level and then decreases again with height. As a result, turbulence is observed to be a minimum at the height of the low level wind maximum and then increases again above this height.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Part of this work was performed while a visiting scientist at Oregon State University.     

The sea surface temperature deviation and the heat flow at the sea-air interface

The deviation of the sea surface temperature from the water temperature below is calculated as a function of the heat flow through the air-sea interface, using wind tunnel determinations of the effective thermal diffusivity in a boundary layer. The influence ofQ, shortwave radiation, andH, latent and sensible heat transfer plus effective back radiation, and U, wind speed, can be described by:T ₀ –T _w =C ₁ ·H/U +C ₂ ·Q/U. The calculated coefficients vary slightly with reference depth, Tables II and III. They are in good agreement with independent observations.On leave at Department of Oceanography, Oregon State University, Corvallis, Oregon in 1969–70.     

Surface currents and equatorial thermocline in a coupled upper ocean-atmosphere GCM

The Oregon State University coupled upper ocean-atmosphere GCM is evaluated in terms of the simulated winds, ocean currents and thermocline depth variations. Although the zonal wind velocities in the model are underestimated by a factor of about three and the zonal current velocities are underestimated by a factor of about five, the model is seen to qualitatively simulate the major features of the gyral scale currents, and the phases of the seasonal variation of the principal equatorial currents are in reasonable agreement with observations. The simulated tropical currents are dominated by Ekman transport and the eastern boundary currents do not penetrate far enough equatorward, while the western boundary currents do not penetrate far enough poleward. The subtropical trade wind belt and the mid-latitude westerlies are displaced equatorward of observations; hence, the mid-latitude eastward currents, principally the Kuroshio-North Pacific Drift and the Gulf Stream-North Atlantic Current are displaced equatorward. In spite of these shortcomings the surface current simulation of this two-layer upper ocean model is comparable with that of other ocean GCMs of coarse resolution. The coupled model successfully simulates the deepening of the thermocline westward across Pacific as a consequence of the prevailing Walker circulation. The region of most intense simulated surface forcing is located in the western Pacific due to a southwestward displacement of the northeast trade winds relative to observations; hence the equatorial Pacific is dominated by eastward propagation of thermocline depth variations. The excessively strong Ekman divergence and upwelling in the western Pacific cools the local warm pool, while incorrectly simulated westerlies in the eastern Pacific suppress upwelling and inhibit cooling from below. These features reduce the simulated trans-Pacific sea-surface temperature gradient, weakening the Walker circulation and the anomalies associated with the simulated Southern Oscillation. Offprint requests to: KR Sperber     

Diurnal variations of winds over an upwelling region off Oregon

The diurnal variation of surface winds off the coast of Oregon is described and compared with a recent analysis of winds off the coast of Peru. The Oregon wind speeds have a distinct 24-h periodicity, while the Peru wind speeds were reported to have an irregular 12-h variation. The long-and trans-shore components of both winds exhibit 24-h periodicities; the ratio of the long-shore to trans-shore diurnal amplitudes off Oregon is 2.8, twice the ratio found off Peru. Although meteorological conditions off Oregon were quasi-stationary during the period investigated, there were considerable day-to-day variations in diurnal amplitudes and phases. Diurnal amplitudes were found to be correlated with the daily mean long-shore winds.     

A load-based CO2 cap in the power sector

Oregon's governor has proposed a load-based cap and trade programme that limits carbon dioxide (CO₂) emissions to 10% below 1990 levels by 2020. A load-based programme is different from the source-based European Union Emissions Trading Scheme (EU ETS), as it regulates emissions sources, located outside the state, that serve Oregon's electricity load. This article describes the stakeholder process that developed the legislative proposal for the load-based cap. The Oregon Clean Energy Planning Model©, a modified capacity expansion model of annual load resource balances, is used to estimate programme costs. The net present value of the climate policy to Oregon ranges from a $518 million benefit to a $414 million cost under various load growth scenarios. Programme benefits are possible under low and medium load growth because the societal returns of energy efficiency exceed its cost over the life of the programme. CO₂ allowance prices in 2017–2020 are estimated in the medium case at approximately $21 per tonne. Low energy efficiency deployment could raise allowance costs to $36, while an aggressive efficiency programme could reduce them to $13.50. Competition for Northwest renewable resources could increase allowance prices in final phase to $37, indicating the interdependence in programme design among state climate policies.     

The depth of the daytime mixed layer at two coastal sites: A model and its validation

A mathematical model of mixed-layer depth based on the thermodynamic analysis of Tennekes (1973) is generalized to include advection and subsidence. The effects of advection on mixed-layer depth have been modelled by setting the model equations in a Lagrangian frame, performing an approximate first integral in order to derive the spatial dependence of the model variables, and using these spatial forms to give a set of Eulerian equations. The effects of subsidence have been modelled by imposing a subsidence velocity on the top of the mixed layer as well as allowing subsidence-induced warming above that layer.The model thus derived consists of a system of non-linear differential equations which may be numerically solved to elucidate the temporal behaviour of mixed-layer depth. The boundary conditions necessary for such a solution are drawn from field studies at two coastal sites: one with a relatively simple coastline and essentially flat land under agricultural use, the other with a considerably more complex coastline, rolling relief and mixed land use (agricultural, parkland and urban). In both cases the modelled evolution of mixed-layer depth is in good agreement with the measured depth.The sensitivity of the model to all the input variables is investigated by examining the dependence of the maximum mixed-layer depth on each of these variables in an artificial set.     

Application of global snow model for the estimation of snow depth in the UK

Microwave imagery can be used successfully for mapping of snow and estimation of snow pack characteristics under almost all weather conditions. This research is a contribution to the field of space borne remote sensing of snow by means of passive microwave data imagery. The satellite data are acquired from the Special Sensor Microwave Imager (SSM/I). The SSM/I is a four frequency seven channels dual polarization (except 22 GHz which is only vertically polarized) scanning radiometer with channels located at 19, 22, 37, and 85 GHz frequencies. A radiative transfer theory based model is used to estimate the snow cover characteristics of different snow pack types in the UK. A revised form of the Chang et al. (Nord Hydrol 16:57–66, 1987) model is used for this purpose. The revised Chang model was calibrated for global snow monitoring and takes into account forest fractional coverage effects. Snow cover characteristics have significant effects on up-welling naturally emitted microwave radiation through the processes of forward scattering. The up-welling signal is more complex for snow covers that consist of free liquid water content. The aim of this study is to test the global snow depth model for the UK snow cover. The Chang model predicted snow depth bias results for January, February, and March 1995 are ?1.26, ?0.35, and ?0.63 cm, respectively. Similarly, the Chang model Mean Absolute Error (MAE) for January, February, and March 1995 have values 2.88, 2.38, and 1.91 cm, respectively. These results show that the Chang model underestimates the snow depth prediction for all the case studies. The results of this study led us to the conclusion that the global snow models (Chang model) when applied for the retrieval of local snow depth estimation (UK snow cover) underestimate snow depth.     

Predicting Snow Depth in a Forest–Tundra Landscape using a Conceptual Model Allowing for Snow Redistribution and Constrained by Observations from a Digital Camera

Estimation of snow depth in the forest–tundra landscape remains a challenge because of a lack of reliable and frequent observations on precipitation and snow depth. Snow models forced by gridded meteorological datasets are often the only option available for assessing snow depth at the local scale. Unfortunately, these models generally do not take into account the snow redistribution process between open and forested areas which frequently occurs in the forest–tundra landscape. A simple modification to an existing snow accumulation and melt model is proposed in order to allow for snow redistribution. Along with a technique for taking advantage of snow depth observations obtained from a digital camera, the model is shown to provide accurate predictions of snow depth at the local scale when forced with precipitation data from Environment Canada's Canadian Precipitation Analysis. Results from this study suggest that instrumenting automated weather stations with a digital camera, together with small modifications to an existing model used operationally for snow depth prediction, could result in significant improvements to snow depth prediction and analysis in this environment. Further testing at sites where snow water equivalent of the snowpack is available should, however, be performed to fully validate the method.     

An applied model for the height of the daytime mixed layer and the entrainment zone

A model is presented for the height of the mixed layer and the depth of the entrainment zone under near-neutral and unstable atmospheric conditions. It is based on the zero-order mixed-layer height model of Batchvarova and Gryning (1991) and the parameterization of the entrainment zone depth proposed by Gryning and Batchvarova (1994). However, most zero-order slab type models of mixed-layer height may be applied. The use of the model requires only information on those meteorological parameters that are needed in operational applications of ordinary zero-order slab type models of mixed-layer height: friction velocity, kinematic heat flux near the ground and potential temperature gradient in the free atmosphere above the entrainment zone. When information is available on the horizontal divergence of the large-scale flow field, the model also takes into account the effect of subsidence, although this is usually neglected in operational models of mixed-layer height owing to lack of data. Model performance is tested using data from the CIRCE experiment.     

A Case Study on the Effects of Heterogeneous Soil Moisture on Mesoscale Boundary-Layer Structure in the Southern Great Plains, U.S.A. Part I: Simple Prognostic Model

The atmospheric boundary-layer (ABL) depth was observed by airborne lidar and balloon soundings during the Southern Great Plains 1997 field study (SGP97). This paper is Part I of a two-part case study examining the relationship of surface heterogeneity to observed ABL structure. Part I focuses on observations. During two days (12–13 July 1997) following rain, midday convective ABL depth varied by as much as 1.5 km across 400 km, even with moderate winds. Variability in ABL depth was driven primarily by the spatial variation in surface buoyancy flux as measured from short towers and aircraft within the SGP97 domain. Strong correlation was found between time-integrated buoyancy flux and airborne remotely sensed surface soil moisture for the two case-study days, but only a weak correlation was found between surface energy fluxes and vegetation greenness as measured by satellite. A simple prognostic one-dimensional ABL model was applied to test to what extent the soil moisture spatial heterogeneity explained the variation in north–south ABL depth across the SGP97 domain. The model was able to better predict mean ABL depth and variations on horizontal scales of approximately 100 km using observed soil moisture instead of constant soil moisture. Subsidence, advection, convergence/divergence and spatial variability of temperature inversion strength also contributed to ABL depth variations. In Part II, assimilation of high-resolution soil moisture into a three-dimensional mesoscale model (MM5) is discussed and shown to improve predictions of ABL structure. These results have implications for ABL models and the influence of soil moisture on mesoscale meteorology     

An advective mixed-layer model for heat and moisture incorporating an analytic expression for moisture entrainment

A slab mixed-layer model with zero-order entrainment for both temperature and humidity is developed in order to examine the relative magnitude of advective and turbulence flux convergence effects. The model formulation provides an analytic function for the ratio of surface-layer to entrainment-layer humidity flux. Model results are compared with measured mixed-layer properties over one day at a coastal location. It is concluded that the model is highly successful at simulating the mixed-layer depth, and mean mixed-layer humidity. It is suggested that a first-order model may be more appropriate for the latter half of the day when the mixed-layer depth is decreasing due to the dominance of advection over vertical turbulence flux convergence.     

An embedded mixed-layer-ocean circulation model

The rationale and numerical technique of embedding an oceanic bulk mixed-layer model with a multi-level primitive equation model is presented. In addition to the usual prognostic variables that exist in a multi-level primitive equation model, the embedded model predicts the depth of the well-mixed layer as well as the jumps in temperature and velocity that occur at the base of that layer. The depth of the mixed layer need not coincide with any of the fixed-model levels used in the primitive equations calculations.In addition to advective changes, the mixed layer can deepen by entrainment and it can reform at a shallower depth in the absence of entrainment. When the mixed layer reforms at a shallower depth, the vertical profile of temperature below the new, shallower mixed layer is adjusted to fit the fixed-level structure used in the primitive equations calculations using a method which conserves heat, momentum and potential energy. Finally, a dynamic stability condition, which includes a consideration of both the vertical current shear and the vertical temperature gradient, is introduced in place of the traditional ‘convective adjustment’.A two-dimensional version of the model is used to test the embedded model formulations and to study the response of the ocean to a stationary axisymmetric hurricane. The model results indicate a strong interdependence between vertical turbulent mixing and advection of heat.     

Drag coefficient modeling for the near coastal zone

Using the JONSWAP spectrum for describing the surface wave state in the near coastal zone, models for the roughness length and the drag coefficient are used to simulate the dependence of the wind stress on fetch and depth. The results of each model are then compared with a compiled set of past investigations of the neutral drag coefficient over a variety of conditions. It is found that the models of Donelan, Hsu, and Kitaigorodskii correctly predict the trends in the drag coefficient with fetch and depth. Although it did not account for all the observed variations in the neutral drag coefficient. Kitaigorodskii's model, when incorporating the JONSWAP spectrum, more accurately simulated the slopes of the various C_DN regressions against windspeed.     

A Note on a Parameterization of the Sea Drag

A new parameterization of the sea drag is based on a wind-over-wavescoupling theory. The parameterization accounts for the wind speed, wave ageand finite depth dependencies of the sea drag. The latter two are introducedthrough the integral parameters of the wind-wave field: the dominant waveheight and the wavenumber at the spectral peak, and the water depth. Theparameterization is checked against the wind-over-waves model results andtwo field datasets obtained in a wide range of the wind speed and wave age.The comparison is encouraging. The parameterization is aimed for use inoperational ocean-state and atmosphere models.     

Validation of A One-Dimensional Snow-Land Surface Model at the Sleepers River Watershed

A one-dimensional land surface model, based on conservations of heat and water substance inside the soil and snow, is presented. To validate the model, a stand-alone experiment is carried out with five years of meteorological and hydrological observations collected from the NOAA-ARS Cooperative Snow Research Project (1966–1974) at the Sleepers River watershed in Danville, Vermont, U.S.A. The numerical results show that the model is capable of reproducing the observed soil temperature at different depths during the winter as well as a rapid increase of soil temperature after snow melts in the spring. The model also simulates the density, temperature, thickness, and equivalent water depth of snow reasonably well. The numerical results are sensitive to the fresh snow density and the soil properties used in the model, which affect the heat exchange between the snowpack and the soil.     

Application of sodar sounding to atmospheric dispersion—Mixing depth and concentration at the ground

It is the intent of this paper to illustrate how to apply acoustic radar data on the variation of mixing depth in the study of atmospheric dispersion. The box model, as an example of the routine usage of acoustic sounding, has been modified. A case of the development of the structure of mixed layer, resulting from some synoptic process is discussed and the results show that the ordinary model calculations regarding atmospheric dispersion will mislead the assessment of air quality if no remote techniques, such as the acoustic radar, are associated with.     

Coastal trapped waves,with application to the northeast pacific ocean

The dispersion relation is derived for long coastal trapped waves of sub‐inertial frequency that propagate along a single‐step continental shelf in a two‐layer fluid. When the internal (Rossby) deformation radius is smaller than the shelf width, we show that the dispersion relation can be factored exactly, giving two possible modes: i. an internal Kelvin wave modified by topography;

ii. a continental shelf wave modified by the stratification.

A detailed discussion of the eigen‐functions associated with each of these modes is presented. Then the shelf wave dispersion relation is plotted for parameters applicable to the Oregon‐Washington coast. Theoretical values for the periods and wavelengths predicted from these plots are shown to agree favorably with observed values for this region.     

一个混合层高度发展的数值模拟试验

本文利用在ＨＥＩＦＥ期间戈壁地区采集的资料，并用一个考虑由地表热力及动力强的害和高空下沉等因子控制的零阶跳跃模式，对该地区白天混层高度增长的机制进行了数值模拟分析，结果表明：白天混合层高的增长受来自祁连山强烈的下沉冷空气的影响较大，使其高度的增长受到抑制，且最大值出现的时间超前，其结果与声雷达回波强度确定的混合层高度吻合较好。     

The influence of geostrophic shear on the cross-isobar angle of the surface wind

A Simple slab model of the planetary boundary layer is extended to include vertical shear of the geostrophic wind. The layer depth is assumed to be determined by a Richardson number criterion. The cross-isobar angle for the surface wind is given in terms of the drag coefficient, the Froude number of the layer, and the angle between the thermal wind and the surface isobars. The theoretical results resemble the observations rather well.