Abstract

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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.

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.

Abstract

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.

Abstract

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.

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.

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.

Abstract

The macroscale cloud vertical structure (CVS), including cloud-base and -top heights and layer thickness, and characteristics of multilayered clouds, is studied at Porto Santo Island during the Atlantic Stratocumulus Transition Experiment (ASTEX) by using rawinsonde, radar, ceilometer, and satellite data. The comparisons of CVS parameters obtained from four different approaches show that 1) by using the method developed by Wang and Rossow rawinsonde observations (raob’s) can sample all low clouds and determine their boundaries accurately, but oversample low clouds by about 10%, mistaking clear moist layers for clouds; 2) cloud-base heights less than 200 m in the radar data are ambiguous, but can be replaced by the values measured by ceilometer; and 3) the practical limit on the accuracy of marine boundary layer cloud-top heights retrieved from satellites appears to be about 150–300 m mainly due to errors in specifying the atmospheric temperature and humidity in the inversion layer above the cloud. The vertical distribution of clouds at Porto Santo during ASTEX is dominated by low clouds below 3 km, a cloud-free layer between 3 and 4 km, and ∼20% high clouds with a peak occurrence around 7–8 km. Low clouds have mean base and top heights of 1.0 km and 1.4 km, respectively, and occur as single layers 90% of the time. For double-layered low clouds, the tops of the uppermost layers and the bases of the lowermost layers have similar distributions as those of single-layered clouds. The temporal variations of low clouds during ASTEX are apparently dominated by advecting mesoscale (20–200 km) horizontal variations. Coherent time variations are predominately synoptic (timescale 4.5–6.8 days) and diurnal variability. On the diurnal timescale, all cloud properties show maxima in the early morning (around 0530 LST) decreasing to minima in the late afternoon. Diurnal variations appear to be altered when high clouds are present above low clouds. The general characteristics of CVS in three ASTEX and the First ISCCP Regional Experiment (FIRE87) regions derived from a 20-yr rawinsonde dataset are also presented. The results suggest that CVS characteristics obtained from data collected at Porto Santo during ASTEX (June 1992) are not representative of other marine stratiform cloud regions.

Abstract

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.

Abstract

Empirical orthogonal function (EOF) analyses are performed of time–height series of zonal and meridional winds and of cumulonimbus heating and drying in the Tropics. The data are from a rawinsonde array in the western Pacific located between the equator and 10°S during the intensive observation period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The EOF analyses are performed by applying a window of 20 days to the data and thus calculating the EOFs of the time development of the vertical structure.

The wind time series is found to be well represented by two pairs of EOFs, each representing an oscillation. The first oscillation has a period of approximately 40 days, is predominantly in the zonal wind component, and has a first internal mode vertical structure with westerly anomalies in the lower troposphere corresponding to easterly perturbations in the upper troposphere. This pair describes 48.3% of the variance. A second EOF pair in the wind is a zonal variation that occurs predominantly in the upper troposphere. It has a period of approximately 24 days and describes 13.9% of the variance.

The heating–drying series is described by a dominant oscillation of period 40 days representing 41% of the variance. The structure is maximum in the middle troposphere and is associated with the same physical phenomeon as the dominant (u, υ) oscillation. The second EOF pair for heating–drying has a period of 13 days, so there is a large time separation in periodiocities for heating–drying compared to that for winds. The second (13 day) oscillation in heating–drying has the same vertical structure as the dominant (40 day) oscillation.