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H. K. Johnson
and
H. J. Vested

Abstract

With the goal of improving the formulation of the wind shear stress used in numerical current modeling, a new method for including the effect of water waves on sea roughness is presented. The method is a hybrid of earlier sea roughness models by Kitaigorodskii and Volkov and Donelan, with two distinctions: 1) the roughness height is assumed to be proportional to a wave height representing high-frequency waves (k>1.5k p ) in the spectrum, and 2) the hybrid model is calibrated to give drag coefficients obtained by Smith and Banke for their site conditions and verified by comparisons with other independent datasets. In deep water, this model gives a sea roughness that increases with wave age (C p /U *) in the early stages of wave growth (up to C p /U *=5), after which the sea roughness decreases with wave age. This method yields wave-modified C d values for several test cases and can be used directly in numerical current models.

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A. K. Wåhlin
and
H. L. Johnson

Abstract

The Atlantic overturning circulation has conventionally been pictured in the meridional–vertical plane, but a significant densification of the water masses involved also occurs as the surface branch of the circulation flows in boundary currents around the subpolar gyre and northern marginal seas. Here an analytical model of the heat and salt budget for an idealized coastal boundary current in a marginal sea is presented. The boundary current exchanges heat and freshwater with the atmosphere as well as with the interior of the basin through eddy and Ekman transports. Its along-coast volume transport is assumed to be constant and independent of buoyancy; it is set, for example, by the wind forcing. Because the atmospheric fluxes of heat and freshwater are different, the temperature and salinity of the boundary current adjust on different length scales. The size of these length scales compared with the circumference of the basin determines the properties of the water that flows over the sill. Furthermore, the relative size of the two length scales determines the evolution of the density as the current moves around the basin. If temperature and salinity adjust on the same length scale (or if the density forcing is represented by a single component), then the density will increase or decrease monotonically from the inflow to the outflow. However, when the adjustment length scale for temperature is shorter than that for salinity, a warm and salty inflow can cool significantly before it freshens. As a result, the density first increases to a local maximum before decreasing again. Therefore, when salinity as well as temperature is included in the buoyancy forcing, the outflow from the basin can be significantly denser than for the equivalent single-component density forcing and can be more sensitive to the forcing parameters. The relevance and implications for the Nordic seas are discussed.

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James H. Ruppert Jr.
,
Richard H. Johnson
, and
Angela K. Rowe

Abstract

The diurnal cycle of the local circulation, rainfall, and heat and moisture budgets is investigated in Taiwan's heavy rain (mei-yu) season using data from the 2008 Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). Comparisons are made between an undisturbed (UNDIST; 22–29 May) and disturbed period (DIST; 31 May–4 June). Many aspects of the diurnal evolution in surface flows and rainfall were similar during both periods. At night and during early morning hours, the low-level southwesterly flow was deflected around Taiwan's main topographic barrier, the Central Mountain Range (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and land-breeze flows. During the day, the flow switched to onshore and upslope, rainfall shifted inland, and deep convection developed along the coastal plains and windward slopes. Atmospheric budget analysis indicates a day-to-evening transition of convective structure from shallow to deep to stratiform. Evaporation associated with the evening/nighttime stratiform precipitation likely assisted the nocturnal katabatic flow.

Though the flow impinging on Taiwan was blocked during both periods, a very moist troposphere and strengthened low-level oncoming flow during DIST resulted in more widespread and intense rainfall that was shifted to higher elevations, which resembled a more weakly blocked regime. Correspondingly, storm cores were tilted upslope during DIST, in contrast to the more erect storms characteristic of UNDIST. There were much more lofted precipitation-sized ice hydrometeors within storms during DIST, the upslope advection of which led to extensive stratiform rain regions overlying the CMR peaks, and the observed upslope shift in rainfall.

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H. K. Johnson
,
H. J. Vested
,
Hans Hersbach
,
J. Højstrup
, and
S. E. Larsen

Abstract

The reliability of the wave model (WAM, cycle 4) for predicting waves and wind stress in restricted fetches is investigated using measured data obtained during the Risø Air–Sea Experiment (RASEX) at Vindeby, Denmark. The WAM model includes Janssen’s theory for calculating sea roughness as a function of wave spectra. RASEX is characterized by being located in relatively shallow waters (depths of about 3 to 4 m in an area where the waves are predominantly fetch limited, with a maximum fetch of about 20 km).

Comparison between WAM results and measured data (integral wave parameters and friction velocities) shows fair agreement for moderate winds (U 10 ≃ 10 m s−1) but significant overprediction for strong winds. Analysis of the WAM results for sea roughness yields a trend of increasing dimensionless roughness with inverse wave age, as obtained from field data; however, the WAM values are generally higher than that obtained from field data.

It is shown that inclusion of depth-induced wave breaking does not explain the overprediction of measured wind stress and associated wave heights. Furthermore, it is shown that using the measured wind friction velocities to force the WAM model significantly reduces the wave height overprediction for strong winds.

These investigations indicate that further improvements are required before the WAM model can be reliably used in shallow and fetch-limited areas, such as Vindeby.

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H. K. Johnson
,
J. Højstrup
,
H. J. Vested
, and
S. E. Larsen

Abstract

The influence of wind waves on the momentum transfer (wind stress) between the atmosphere and sea surface was studied using new measured data from the RASEX experiment and other datasets compiled by Donelan et al.

Results of the data analysis indicate that errors in wind friction velocity u∗ of about ±10% make it difficult to conclude on the trend in z ch using measured data from a particular dataset. This problem is solved by combining different field data together. This gives a trend of decreasing z ch with wave age, expressed as: z ch = 1.89(c p /u∗)−1.59.

Furthermore, it is shown that calculations of the wind friction velocities using the wave-spectra-dependent expression of Hansen and Larsen agrees quite well with measured values during RASEX. It also gives a trend in Charnock parameter consistent with that found by combining the field data. Last, calculations using a constant Charnock parameter (0.018) also give very good results for the wind friction velocities at the RASEX site.

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B. Lange
,
H. K. Johnson
,
S. Larsen
,
J. Højstrup
,
H. Kofoed-Hansen
, and
M. J. Yelland

Abstract

The wave age dependency of the nondimensional sea surface roughness (also called the Charnock parameter) is investigated with data from the new field measurement program at Rødsand in the Danish Baltic Sea. An increasing Charnock parameter with inverse wave age is found, which can be described by a power-law relation. Friction velocity is a common quantity in both the Charnock parameter and wave age. Thus self-correlation effects are unavoidable in the relation between them. The significance of self-correlation is investigated by employing an artificial “dataset” with randomized wave parameters. It is found that self-correlation severely influences the relation. For the Rødsand dataset the difference between real and randomized “data” was found to be within the measurement uncertainty. By using a small subset of the data it was found that the importance of self-correlation increases for a narrower range of wave age values. This supports the conclusion of Johnson et al. that because of the scatter and self-correlation problems the coefficients of the power-law relation can only be obtained from the analysis of an aggregated dataset with a wide wave age range combining measurements from several sites. The dependency between wave age and sea roughness has been discussed extensively in the literature with different and sometimes conflicting results. A wide range of coefficients has been found for the power-law relation between the Charnock parameter and wave age for different datasets. It is shown that self-correlation contributes to such differences, since it depends on the range of wave age values present in the datasets. Also, data are often selected for rough flow conditions with the Reynolds roughness number. It is shown that for datasets with large scatter this can lead to misleading results with regard to the relationship between wave age and Charnock parameter. Two different methods to overcome this problem are presented.

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Tom H. Zapotocny
,
Allen J. Lenzen
,
Donald R. Johnson
,
Todd K. Schaack
, and
Fred M. Reames

Abstract

Five- and 10-day inert trace constituent distributions prognostically simulated with the University of Wisconsin (UW) hybrid isentropic–sigma (θσ) model, the nominally identical UW sigma (σ) model, and the National Center for Atmospheric Research Community Climate Model 2 (CCM2) are analyzed and compared in this study. The UW θσ and σ gridpoint models utilize the flux form of the primitive equations, while CCM2 is based on the spectral representation and uses semi-Lagrangian transport (SLT) for trace constituents. Results are also compared against a version of the CCM that uses spectral transport for the trace constituent. These comparisons 1) contrast the spatial and temporal evolution of the filamentary transport of inert trace constituents simulated with the UW θσ and σ models against a “state of the art” GCM under both isentropic and nonisentropic conditions and 2) examine the ability of the models to conserve the initial trace constituent maximum value during 10-day integrations.

Results show that the spatial distributions of trace constituent evolve in a similar manner, regardless of the transport scheme or model type. However, when compared to the UW θσ model’s ability to simulate filamentary structure and conserve the initial trace constituent maximum value, results from the other models in this study indicate substantial spurious dispersion. The more accurate conservation demonstrated with the UW θσ model is especially noticeable within extratropical amplifying baroclinic waves, and it stems from the dominance of two-dimensional, quasi-horizontal isentropic exchange processes in a stratified baroclinic atmosphere. This condition, which largely precludes spurious numerical dispersion associated with vertical advection, is unique to isentropic coordinates. Conservation of trace constituent maxima in sigma coordinates suffers from the complexity of, and inherent need for, resolving three-dimensional transport in the presence of vertical wind shear during baroclinic amplification, a condition leading to spurious vertical dispersion. The experiments of this study also indicate that the shape-preserving SLT scheme used in CCM2 further reduces conservation of the initial maximum value when compared to the spectral transport of trace constituents, although the patterns are more coherent and the Gibbs phenomenon is eliminated.

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D. E. Johnson
,
W-K. Tao
,
J. Simpson
, and
C-H. Sui

Abstract

Interactions between deep tropical clouds over the western Pacific warm pool and the larger-scale environment are key to understanding climate change. Cloud models are an extremely useful tool in simulating and providing statistical information on heat and moisture transfer processes between cloud systems and the environment, and can therefore be utilized to substantially improve cloud parameterizations in climate models. In this paper, the Goddard Cumulus Ensemble (GCE) cloud-resolving model is used in multiday simulations of deep tropical convective activity over the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Large-scale temperature and moisture advective tendencies, and horizontal momentum from the TOGA COARE Intensive Flux Array region, are applied to the GCE version that incorporates cyclical boundary conditions. Sensitivity experiments show that the horizontal extent (size) of the domain produces the largest response to domain-mean temperature and moisture deviations, as well as cloudiness, in comparison with grid horizontal or vertical resolution, and advection scheme. It is found that a domain size of at least 512 km is needed to adequately contain the convective cloud features and to replicate both the eastward and westward movements of the observed precipitating systems. The control experiment shows that the atmospheric heating and moistening is primarily a response to cloud latent processes of condensation/evaporation, and deposition/sublimation. Air–sea exchange of heat and moisture is found to be of secondary importance, while the net radiational heating–cooling is small except above cloud tops. A convective–stratiform breakdown of the precipitating systems shows that while 55% of the total rainfall occurs in convective regions, 90% of the total rainfall coverage occurs in stratiform regions. The simulated rainfall and atmospheric heating and moistening rates agree very well with observations, and the results compare favorably to other models simulating this case.

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Donald R. Johnson
,
Allen J. Lenzen
,
Tom H. Zapotocny
, and
Todd K. Schaack

Abstract

A challenge common to weather, climate, and seasonal numerical prediction is the need to simulate accurately reversible isentropic processes in combination with appropriate determination of sources/sinks of energy and entropy. Ultimately, this task includes the distribution and transport of internal, gravitational, and kinetic energies, the energies of water substances in all forms, and the related thermodynamic processes of phase changes involved with clouds, including condensation, evaporation, and precipitation processes.

All of the processes noted above involve the entropies of matter, radiation, and chemical substances, conservation during transport, and/or changes in entropies by physical processes internal to the atmosphere. With respect to the entropy of matter, a means to study a model’s accuracy in simulating internal hydrologic processes is to determine its capability to simulate the appropriate conservation of potential and equivalent potential temperature as surrogates of dry and moist entropy under reversible adiabatic processes in which clouds form, evaporate, and precipitate. In this study, a statistical strategy utilizing the concept of “pure error” is set forth to assess the numerical accuracies of models to simulate reversible processes during 10-day integrations of the global circulation corresponding to the global residence time of water vapor. During the integrations, the sums of squared differences between equivalent potential temperature θ e numerically simulated by the governing equations of mass, energy, water vapor, and cloud water and a proxy equivalent potential temperature e numerically simulated as a conservative property are monitored. Inspection of the differences of θ e and e in time and space and the relative frequency distribution of the differences details bias and random errors that develop from nonlinear numerical inaccuracies in the advection and transport of potential temperature and water substances within the global atmosphere.

A series of nine global simulations employing various versions of Community Climate Models CCM2 and CCM3—all Eulerian spectral numerics, all semi-Lagrangian numerics, mixed Eulerian spectral, and semi-Lagrangian numerics—and the University of Wisconsin—Madison (UW) isentropic-sigma gridpoint model provides an interesting comparison of numerical accuracies in the simulation of reversibility. By day 10, large bias and random differences were identified in the simulation of reversible processes in all of the models except for the UW isentropic-sigma model. The CCM2 and CCM3 simulations yielded systematic differences that varied zonally, vertically, and temporally. Within the comparison, the UW isentropic-sigma model was superior in transporting water vapor and cloud water/ice and in simulating reversibility involving the conservation of dry and moist entropy. The only relative frequency distribution of differences that appeared optimal, in that the distribution remained unbiased and equilibrated with minimal variance as it remained statistically stationary, was the distribution from the UW isentropic-sigma model. All other distributions revealed nonstationary characteristics with spreading and/or shifting of the maxima as the biases and variances of the numerical differences of θ e and e amplified.

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Richard H. Johnson
,
Paul E. Ciesielski
,
Brian D. McNoldy
,
Peter J. Rogers
, and
Richard K. Taft

Abstract

The 2004 North American Monsoon Experiment (NAME) provided an unprecedented observing network for studying the structure and evolution of the North American monsoon. This paper focuses on multiscale characteristics of the flow during NAME from the large scale to the mesoscale using atmospheric sounding data from the enhanced observing network.

The onset of the 2004 summer monsoon over the NAME region accompanied the typical northward shift of the upper-level anticyclone or monsoon high over northern Mexico into the southwestern United States, but in 2004 this shift occurred slightly later than normal and the monsoon high did not extend as far north as usual. Consequently, precipitation over the southwestern United States was slightly below normal, although increased troughiness over the Great Plains contributed to increased rainfall over eastern New Mexico and western Texas. The first major pulse of moisture into the Southwest occurred around 13 July in association with a strong Gulf of California surge. This surge was linked to the westward passages of Tropical Storm Blas to the south and an upper-level inverted trough over northern Texas. The development of Blas appeared to be favored as an easterly wave moved into the eastern Pacific during the active phase of a Madden–Julian oscillation.

On the regional scale, sounding data reveal a prominent sea breeze along the east shore of the Gulf of California, with a deep return flow as a consequence of the elevated Sierra Madre Occidental (SMO) immediately to the east. Subsidence produced a dry layer over the gulf, whereas a deep moist layer existed over the west slopes of the SMO. A prominent nocturnal low-level jet was present on most days over the northern gulf. The diurnal cycle of heating and moistening (Q 1 and Q 2) over the SMO was characterized by deep convective profiles in the mid- to upper troposphere at 1800 LT, followed by stratiform-like profiles at midnight, consistent with the observed diurnal evolution of precipitation over this coastal mountainous region. The analyses in the core NAME domain are based on a gridded dataset derived from atmospheric soundings only and, therefore, should prove useful in validating reanalyses and regional models.

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