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Jielun Sun
and
L. Mahrt

Abstract

This study examines the bulk aerodynamic method for estimating surface fluxes of heat and moisture using the surface radiative temperature. The surface rediative temperature is often the only available surface temperature from field measurements. Models typically predict heat fluxes from the surface radiative temperature computed from the surface energy balance. In this study, the corresponding radiometric exchange coefficient and radiometric roughness height are computed from tower- and low-level aircraft data taken during four different field programs. The data analysis shows that the radiometric exchange coefficient does not increase with increasing instability. This is because the radiometric exchange coefficient must compensate for the large vertical temperature difference resulting from use of the surface radiative temperature.

The data analysis and scaling arguments indicate that the radiometric exchange coefficient for heat in the bulk aerodynamic formulation is closely related to θ*/Δθ for both stable and unstable conditions, where Δθ is the difference between the surface radiative temperature and the air temperature and θ* is the negative of the heat flux divided by the surface friction velocity. Application of Monin–Obukhov similarity theory with surface radiative temperature also reduces to a relatively circular internal relationship between the radiometric roughness height and θ*/Δθ. This roughness height is flow dependent and not systematically related to the roughness height for momentum.

As an additional complication, the observed radiometric exchange coefficient for heat depends on the relationship between the measured surface radiative temperature and the microscale distribution of surface radiative temperature in the footprint of the heat flux measurement. Analogous problems affect the prediction of the moisture flux based on the saturation vapor prssure at the surface radiative temperature.

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Ronald B. Smith
and
Jielun Sun

Abstract

Nonlinear steady-state solutions for stratified two-layer flow over a ridge are found. In parameter space, these solutions lie between the interfacial case of Long and the constant stratification case of Smith. The solutions predict the depth of the layer which will descend and accelerate. Qualitative agreement with Bora and Boulder observations is found.

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Jielun Sun
and
Jeffrey R. French

Abstract

Air–sea interactions are investigated using the data from the Coupled Boundary Layers Air–Sea Transfer experiment under low wind (CBLAST-Low) and the Surface Wave Dynamics Experiment (SWADE) over sea and compared with measurements from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) over land. Based on the concept of the hockey-stick transition (HOST) hypothesis, which emphasizes contributions of large coherent eddies in atmospheric turbulent mixing that are not fully captured by Monin–Obukhov similarity theory, relationships between the atmospheric momentum transfer and the sea surface roughness, and the role of the sea surface temperature (SST) and oceanic waves in the turbulent transfer of atmospheric momentum, heat, and moisture, and variations of drag coefficient C d (z) over sea and land with wind speed V are studied.

In general, the atmospheric turbulence transfers over sea and land are similar except under weak winds and near the sea surface when wave-induced winds and oceanic currents are relevant to wind shear in generating atmospheric turbulence. The transition of the atmospheric momentum transfer between the stable and the near-neutral regimes is different over land and sea owing to the different strength and formation of atmospheric stable stratification. The relationship between the air–sea temperature difference and the turbulent heat transfer over sea is dominated by large air temperature variations compared to the slowly varying SST. Physically, C d (z) consists of the surface skin drag and the turbulence drag between z and the surface; the increase of the latter with decreasing V leads to the minimum C d (z), which is observed, but not limited to, over sea.

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Jielun Sun
,
Steven K. Esbensen
, and
L. Mahrt

Abstract

The authors reconsider the problem of estimating the sensible heat transfer at the earth's surface from direct measurements of turbulent fluxes in the atmospheric boundary layer. For simplicity, only horizontally homogeneous conditions are considered for a thin atmospheric layer containing no liquid water, adjacent to the earth's ground surface. Applying the first law of thermodynamics to the thin interfacial layer, an expression is obtained for thermal conduction at the surface in terms of the traditionally defined sensible heat flux by turbulence and a set of correction terms including the so-called moisture correction term. A scale analysis is presented to suggest that the magnitudes of the miscellaneous correction terms are usually negligible. Previous literature on estimation of the sensible heat flux is critically reviewed in light of the new result.

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Jielun Sun
,
Donald H. Lenschow
,
Larry Mahrt
, and
Carmen Nappo

Abstract

Relationships among the horizontal pressure gradient, the Coriolis force, and the vertical momentum transport by turbulent fluxes are investigated using data collected from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). Wind toward higher pressure (WTHP) adjacent to the ground occurred about 50% of the time. For wind speed at 5 m above the ground stronger than 5 m s−1, WTHP occurred about 20% of the time. Focusing on these moderate to strong wind cases only, relationships among horizontal pressure gradients, Coriolis force, and vertical turbulent transport in the momentum balance are investigated. The magnitude of the downward turbulent momentum flux consistently increases with height under moderate to strong winds, which results in the vertical convergence of the momentum flux and thus provides a momentum source and allows WTHP.

In the along-wind direction, the horizontal pressure gradient is observed to be well correlated with the quadratic wind speed, which is demonstrated to be an approximate balance between the horizontal pressure gradient and the vertical convergence of the turbulent momentum flux. That is, antitriptic balance occurs in the along-wind direction when the wind is toward higher pressure. In the crosswind direction, the pressure gradient varies approximately linearly with wind speed and opposes the Coriolis force, suggesting the importance of the Coriolis force and approximate geotriptic balance of the airflow. A simple one-dimensional planetary boundary layer eddy diffusivity model demonstrates the possibility of wind directed toward higher pressure for a baroclinic boundary layer and the contribution of the vertical turbulent momentum flux to this phenomenon.

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Jielun Sun
,
Larry Mahrt
,
Robert M. Banta
, and
Yelena L. Pichugina

Abstract

An investigation of nocturnal intermittent turbulence during the Cooperative Atmosphere–Surface Exchange Study in 1999 (CASES-99) revealed three turbulence regimes at each observation height: 1) regime 1, a weak turbulence regime when the wind speed is less than a threshold value; 2) regime 2, a strong turbulence regime when the wind speed exceeds the threshold value; and 3) regime 3, a moderate turbulence regime when top-down turbulence sporadically bursts into the otherwise weak turbulence regime. For regime 1, the strength of small turbulence eddies is correlated with local shear and weakly related to local stratification. For regime 2, the turbulence strength increases systematically with wind speed as a result of turbulence generation by the bulk shear, which scales with the observation height. The threshold wind speed marks the transition above which the boundary layer approaches near-neutral conditions, where the turbulent mixing substantially reduces the stratification and temperature fluctuations. The preference of the turbulence regimes during CASES-99 is closely related to the existence and the strength of low-level jets. Because of the different roles of the bulk and local shear with regard to turbulence generation under different wind conditions, the relationship between turbulence strength and the local gradient Richardson number varies for the different turbulence regimes. Turbulence intermittency at any observation height was categorized in three ways: turbulence magnitude oscillations between regimes 1 and 2 as wind speed varies back and forth across its threshold value, episodic turbulence enhancements within regime 1 as a result of local instability, and downbursts of turbulence in regime 3.

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L. Mahrt
,
Jielun Sun
,
S. P. Oncley
, and
T. W. Horst

Abstract

Drainage of cold air down a small valley and associated near-surface wind maxima are examined from 20 stations with sonic anemometers at 1 m and from a 20-m tower that includes six sonic anemometers in the lowest 5 m, deployed in the Shallow Cold Pool Experiment (SCP). The small valley is about 270 m wide and 12 m deep with a downvalley slope of 2%–3%. The momentum budget indicates that the flow is driven by the buoyancy deficit of the flow and opposed primarily by the stress divergence while the remaining terms are estimated to be at least an order of magnitude smaller. This analysis also reveals major difficulties in quantifying such a budget due to uncertainties in the measurements, sensitivity to choice of averaging time, and sensitivity to measurement heights.

Wind maxima occur as low as 0.5 m in the downvalley drainage flow—the lowest observational level. The downvalley cold air drainage and wind maxima are frequently disrupted by transient modes that sometimes lead to significant vertical mixing. On average, the downvalley drainage of cold air occurs with particularly weak turbulence with stronger turbulence above the drainage flow. The momentum flux profile responds to the shear reversal at the wind maximum on a vertical scale of 1 m or less, suggesting the important role of finescale turbulent diffusion.

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Jielun Sun
,
Larry Mahrt
,
Carmen Nappo
, and
Donald H. Lenschow

Abstract

The authors investigate atmospheric internal gravity waves (IGWs): their generation and induction of global intermittent turbulence in the nocturnal stable atmospheric boundary layer based on the new concept of turbulence generation discussed in a prior paper by Sun et al. The IGWs are generated by air lifted by convergence forced by the colliding background flow and cold currents near the ground. The buoyancy-forced IGWs enhance wind speed at the wind speed wave crests such that the bulk shear instability generates large coherent eddies, which augment local turbulent mixing and vertically redistribute momentum and heat. The periodically enhanced turbulent mixing, in turn, modifies the air temperature and flow oscillations of the original IGWs. These turbulence-forced oscillations (TFOs) resemble waves and coherently transport momentum and sensible heat. The observed momentum and sensible heat fluxes at the IGW frequency, which are due to either the buoyancy-forced IGWs themselves or the TFOs, are larger than turbulent fluxes near the surface. The IGWs enhance not only the bulk shear at the wave crests, but also local shear over the wind speed troughs of the surface IGWs. Temporal and spatial variations of turbulent mixing as a result of this wave-induced turbulent mixing change the mean airflow and the shape of the IGWs.

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L. Mahrt
,
Jielun Sun
,
Dean Vickers
,
J. I. Macpherson
,
J. R. Pederson
, and
R. L. Desjardins

Abstract

Repeated aircraft runs at about 33 m over heterogeneous terrain are analyzed to study the spatial variability of the mesoscale flow and turbulent fluxes. An irrigated area, about 12 km across, generates a relatively cool moist inland breeze. As this air flows out over the warmer, drier surrounding land surface, an internal boundary layer develops within the inland breeze, which then terminates at a well-defined inland breeze front located about 1½ km downstream from the change of surface conditions. This front is defined by horizontal convergence, rising motion, and sharp spatial change of moisture, carbon dioxide, and ozone.

Both a scale analysis and the observations suggest that the overall vertical motion associated with the inland breeze is weak. However, the observations indicate that this vertical motion and attendant vertical transport are important in the immediate vicinity of the front, and the inland breeze does lead to significant modification of the turbulent flux. In the inland breeze downstream from the surface wetness discontinuity, strong horizontal advection of moisture is associated with a rapid increase of the turbulent moisture flux with height. This large moisture flux appears to be partly due to mixing between the thin moist inland breeze and overlying drier air.

As a consequence of the strong vertical divergence of the flux in the transition regions, the fluxes measured even as low as a few tens of meters are not representative of the surface fluxes. The spatial variability of the fluxes is also interpreted within the footprint format. Attempts are made to reconcile predictions by footprint and internal boundary-layer approaches.

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Jielun Sun
,
James F. Howell
,
Steven K. Esbensen
,
L. Mahrt
,
Christine M. Greb
,
Robert Grossman
, and
M. A. LeMone

Abstract

The goal of this study is to examine the horizontal scale dependence of vertical eddy flux in the tropical marine surface boundary layer and how this scale dependence of flux relates to the bulk aerodynamic relationship and the parameterization of subgrid-scale flux. The fluxes of heat, moisture, and momentum are computed from data collected from 27 NCAR Electra flight legs in TOGA COARE (The Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment) with flight elevations lower than 40 m and flight runs longer than 60 km. The dependence of the fluxes on two length scales are studied: the cutoff length scale, defining the averaging length over which mean components are obtained in order to partition field variables into mean and perturbation components; and the flux averaging length scale, defining the length over which products of perturbations are averaged in order to estimate vertical fluxes. Based on the characteristics of the scale dependence of fluxes, the total flux of each flight leg is partitioned into “turbulent,” “large eddy,” and “mesoscale” fluxes due to motions smaller than 1 km, between 1 and 5 km, and between 5 km and the flight leg length, respectively.

The results show that fluxes are sensitive to the choice of cutoff length scale in the presence of significant mesoscale activity and in the weak wind case where the turbulent fluxes are small. The turbulent momentum flux decreases with increasing flux averaging length scale due to mesoscale modulation of the turbulent stress vector.

Mesoscale heat, moisture, and momentum fluxes for individual flight legs can reach 20% of the turbulent fluxes in the presence of well-organized convective cloud systems even at 35 m above the sea surface. The mesoscale flux is less correlated to the wind speed and bulk air-sea difference than turbulent fluxes. The local mesoscale flux can be upward or downward, and therefore, its average value is reduced when averaging over a single flight leg and reduced further when compositing over all of the legs. The mesoscale momentum flux is less systematic than the turbulent stress and is more sensitive to the flux averaging scale than the turbulent stress. Sampling and instrumentation problems are briefly discussed, particularly with respect to mesoscale motions.

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