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William Perrie and Liangming Wang

Abstract

The authors present a simple model for the dynamics that couple the atmospheric boundary layer and wind-generated waves. The model is empirically motivated by parameterizations for the sea state-dependent drag coefficient and sea surface roughness derived by Smith et al. from HEXOS measurements. Estimates are made for the effect the coupling dynamics has on predicted sea state parameters such as spectral wave energy and the air–sea flux of momentum. Results are verified with observations collected during the CAL/VAL experiment of Dobson and Vachon. The authors demonstrate that inclusion of the coupling dynamics systematically improves wave modeling. The effect of the coupling dynamics is particularly important for young waves in the presence of high wind speeds. A tendency to improve estimates of maximum wave heights is achieved.

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William Perrie and Liangming Wang

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Junhong Wang and William B. Rossow

Abstract

A method is described to use rawinsonde data to estimate cloud vertical structure, including cloud-top and cloud-base heights, cloud-layer thickness, and the characteristics of multilayered clouds. Cloud-layer base and top locations are identified based on three criteria: maximum relative humidity in a cloud of at least 87%, minimum relative humidity of at least 84%, and relative humidity jumps exceeding 3% at cloud-layer top and base, where relative humidity is with respect to liquid water at temperatures greater than or equal to 0°C and with respect to ice at temperatures less than 0°C. The analysis method is tested at 30 ocean sites by comparing with cloud properties derived from other independent data sources. Comparison of layer-cloud frequencies of occurrence with surface observations shows that rawinsonde observations (RAOBS) usually detect the same number of cloud layers for low and middle clouds as the surface observers, but disagree more for high-level clouds. There is good agreement between the seasonal variations of RAOBS-determined top pressure of the highest cloud and that from the International Satellite Cloud Climate Project (ISCCP) data. RAOBS-determined top pressures of low and middle clouds agree better with ISCCP, but RAOBS often fail to detect very high and thin clouds. The frequency of multilayered clouds is qualitatively consistent with that estimated from surface observations. In cloudy soundings at these ocean sites, multilayered clouds occur 56% of the time and are predominately two layered. Multilayered clouds are most frequent (≈70%) in the Tropics (10°S–10°N) and least frequent at subtropical eastern Pacific stations. The frequency of multilayered clouds is higher in summer than in winter at low-latitude stations (30°S–30°N), but the opposite variation appears at the two subtropical stations. The frequency distributions of cloud top, cloud base, and cloud-layer thickness and cloud occurrence as a function of height are also presented. The lowest layer of multilayered cloud systems is usually located in the atmospheric boundary layer.

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Guohui Wang and William K. Dewar

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A quasigeostrophic point vortex numerical model is used to explore interactions of eddies and seamounts. The ultimate objective of this study is to assess the role of meddy–seamount interaction as an input to Mediterranean salt tongue maintenance. Secondary objectives are to clarify the dynamics of meddy–seamount interaction. The results suggest that meddies survive seamount collisions with 60%–70% of their initial cores remaining intact as coherent vortices. Given observed formation rates, it appears meddies, in their interactions with seamounts, inject between one-quarter and one-half of the salt anomaly necessary to sustain the Mediterranean salt tongue. Other considerations suggest the anomalous mass flux by meddies is comparable to that due to the mean flow. In summary, meddies are important to the maintenance of the salt tongue, although other mechanisms are needed. Coherent vortex transport, of which meddies are one example, is a mesoscale process not well described by the downgradient mixing algorithms normally employed in general circulation models. More sophisticated mesoscale models are thus suggested by this study. In particular, survival by meddies of collisions with seamounts emerges as a potentially important limiting effect on the Mediterranean salt tongue. This effect has climatically significant implications for ocean simulations.

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Junhong Wang, William B. Rossow, and Yuanchong Zhang

Abstract

A global cloud vertical structure (CVS) climatic dataset is created by applying an analysis method to a 20-yr collection of twice-daily rawinsonde humidity profiles to estimate the height of cloud layers. The CVS dataset gives the vertical distribution of cloud layers for single and multilayered clouds, as well as the top and base heights and layer thicknesses of each layer, together with the original rawinsonde profiles of temperature, humidity, and winds. The average values are cloud-top height = 4.0 km above mean sea level (MSL), cloud-base height = 2.4 km MSL, cloud-layer thickness = 1.6 km, and separation distance between consecutive layers = 2.2 km. Multilayered clouds occur 42% of the time and are predominately two-layered. The lowest layer of multilayered cloud systems is usually located in the atmospheric boundary layer (below 2-km height MSL). Clouds over the ocean occur more frequently at lower levels and are more often formed in multiple layers than over land. Latitudinal variations of CVS also show maxima and minima that correspond to the locations of the intertropical convergence zone, the summer monsoons, the subtropical subsidence zones, and the midlatitude storm zones. Multilayered clouds exist most frequently in the Tropics and least frequently in the subtropics; there are more multilayered clouds in summer than in winter. Cloud layers are thicker in winter than in summer at mid- and high latitudes, but are thinner in winter in Southeast Asia.

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William B. Rossow, Yuanchong Zhang, and Junhong Wang

Abstract

To diagnose how cloud processes feed back on weather- and climate-scale variations of the atmosphere requires determining the changes that clouds produce in the atmospheric diabatic heating by radiation and precipitation at the same scales of variation. In particular, not only the magnitude of these changes must be quantified but also their correlation with atmospheric temperature variations; hence, the space–time resolution of the cloud perturbations must be sufficient to account for the majority of these variations. Although extensive new global cloud and radiative flux datasets have recently become available, the vertical profiles of clouds and consequent radiative flux divergence have not been systematically measured covering weather-scale variations from about 100 km, 3 h up to climate-scale variations of 10 000 km, decadal inclusive. By combining the statistics of cloud layer occurrence from the International Satellite Cloud Climatology Project (ISCCP) and an analysis of radiosonde humidity profiles, a statistical model has been developed that associates each cloud type, recognizable from satellite measurements, with a particular cloud vertical structure. Application of this model to the ISCCP cloud layer amounts produces estimates of low-level cloud amounts and average cloud-base pressures that are quantitatively closer to observations based on surface weather observations, capturing the variations with latitude and season and land and ocean (results are less good in the polar regions). The main advantage of this statistical model is that the correlations of cloud vertical structure with meteorology are qualitatively similar to “classical” information relating cloud properties to weather. These results can be evaluated and improved with the advent of satellites that can directly probe cloud vertical structures over the globe, providing statistics with changing meteorological conditions.

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Hailong Wang, William C. Skamarock, and Graham Feingold

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In the Advanced Research Weather Research and Forecasting Model (ARW), versions 3.0 and earlier, advection of scalars was performed using the Runge–Kutta time-integration scheme with an option of using a positive-definite (PD) flux limiter. Large-eddy simulations of aerosol–cloud interactions using the ARW model are performed to evaluate the advection schemes. The basic Runge–Kutta scheme alone produces spurious oscillations and negative values in scalar mixing ratios because of numerical dispersion errors. The PD flux limiter assures positive definiteness but retains the oscillations with an amplification of local maxima by up to 20% in the tests. These numerical dispersion errors contaminate active scalars directly through the advection process and indirectly through physical and dynamical feedbacks, leading to a misrepresentation of cloud physical and dynamical processes. A monotonic flux limiter is introduced to correct the generally accurate but dispersive solutions given by high-order Runge–Kutta scheme. The monotonic limiter effectively minimizes the dispersion errors with little significant enhancement of numerical diffusion errors. The improvement in scalar advection using the monotonic limiter is discussed in the context of how the different advection schemes impact the quantification of aerosol–cloud interactions. The PD limiter results in 20% (10%) fewer cloud droplets and 22% (5%) smaller cloud albedo than the monotonic limiter under clean (polluted) conditions. Underprediction of cloud droplet number concentration by the PD limiter tends to trigger the early formation of precipitation in the clean case, leading to a potentially large impact on cloud albedo change.

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William M. Frank, Houjun Wang, and John L. McBride

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During the 120 days of the TOGA COARE intensive observation period, there was an enhanced network of rawinsonde stations covering a large portion of the equatorial West Pacific. These soundings were of sufficient quality and frequency to permit computation of line integral beat and moisture budgets over a variety of large-scale arrays. In this study an enhanced operational dataset is used to compute rainfall, surface beat, and moisture fluxes, and vertical profiles of diabatic and/or subgrid-scale heating and moistening over these arrays.

Time series of daily rainfall computed from beat and moisture budgets are presented over seven arrays, including the intensive flux array, outer sounding array, and large-scale array. Vertical profiles of apparent beat source and apparent moisture sink are analyzed and presented for different arrays and for different rainfall rates.

The mean budget-derived rainfall ranged from 4 to 12 mm day−1 over the various arrays, with the most rain occurring within the intensive flux array and the least over Papua New Guinea. Correlations between convective indicators, low-level winds, and surface fluxes indicate that convection tends to precede or be coincident with increased surface fluxes in the more active regions south of the equator but not in the less convectively active regions.

Convective heating in this region tends to be vertically distributed in a dominant single mode, apparently a characteristic blend of convective and stratiform rain heating, with a broad peak in the midtroposphere around 400–500 mb. This distribution varies surprisingly little from day to day or with rainfall intensity. In contrast, convection over Papua New Guinea differs from the maritime convection. The convection over this large island produces more beating at upper-tropospheric levels than does the surrounding maritime convection. This indicates a fundamental difference between maritime and island rainfall that may well have significant effects on global-scale circulations.

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Yi Jin, William T. Thompson, Shouping Wang, and Chi-Sann Liou

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The impact of dissipative heating on tropical cyclone (TC) intensity forecasts is investigated using the U.S. Navy’s operational mesoscale model (the Coupled Ocean/Atmosphere Mesoscale Prediction System). A physically consistent method of including dissipative heating is developed based on turbulent kinetic energy dissipation to ensure energy conservation. Mean absolute forecast errors of track and surface maximum winds are calculated for eighteen 48-h simulations of 10 selected TC cases over both the Atlantic basin and the northwest Pacific. Simulation results suggest that the inclusion of dissipative heating improves surface maximum wind forecasts by 10%–20% at 15-km resolution, while it has little impact on the track forecasts. The resultant improvement from the inclusion of the dissipative heating increases to 29% for the surface maximum winds at 5-km resolution for Hurricane Isabel (2003), where dissipative heating produces an unstable layer at low levels and warms a deep layer of the troposphere. While previous studies depicted a 65 m s−1 threshold for the dissipative heating to impact the TC intensity, it is found that dissipative heating has an effect on the TC intensity when the TC is of moderate strength with the surface maximum wind speed at 45 m s−1. Sensitivity tests reveal that there is significant nonlinear interaction between the dissipative heating from the surface friction and that from the turbulent kinetic energy dissipation in the interior atmosphere. A conceptualized description is given for the positive feedback mechanism between the two processes. The results presented here suggest that it is necessary to include both processes in a mesoscale model to better forecast the TC structure and intensity.

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William J. Gutowski Jr., David S. Gutzler, and Wei-Chyung Wang

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We examine surface energy balances simulated by three general circulation models for current climatic boundary conditions and for an atmosphere with twice current levels of CO2. Differences between model simulations provide a measure of uncertainty in the prediction of surface temperature in a double-CO2 climate, and diagnosis of the energy balance suggests the radiative and thermodynamic processes responsible for these differences. The scale dependence of the surface energy balance is examined by averaging over a hierarchy of spatial domains ranging from the entire globe to regions encompassing just a few model grid points.

Upward and downward longwave fluxes are the dominant terms in the global-average balance for each model and climate. The models product nearly the same global-average surface temperature in their current climate simulations, so their upward longwave fluxes are nearly the same, but in the global-average balance their downward longwave fluxes, absorbed solar radiation, and sensible and latent heat fluxes have intermodel discrepancies that are larger than respective flux changes associated with doubling CO2. Despite the flux discrepancies, the globally averaged surface flux changes associated with CO2 doubling are qualitatively consistent among the models, suggesting that the basic large-scale mechanisms of greenhouse warming are not very sensitive to the precise surface balance of heat occurring in a model's current climate simulation.

The net longwave flux at the surface has small spatial variability, so global-average discrepancies in surface longwave fluxes are also manifested in the regional-scale balances. For this reason, increasing horizontal resolution will not improve the consistency of regional-scale climate simulations in these models unless discrepancies in global-average longwave radiation are resolved. Differences between models in simulating effects of moisture in the atmosphere and in the ground appear to be an important cause of differences in surface energy budgets on all scales.

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