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J. Pedlosky and P. Klein

Abstract

The nonlinear dynamics of a slightly unstable baroclinic wave is studied for a two-layer f-plane system in which the basic flow is strongly sheared in the horizontal direction. The basic flow is purely baroclinic, i.e., equal and opposite in each layer. In addition, the basic flow vanishes on the channel walls containing the flow. Weakly nonlinear theory predicts that for small supercriticality, the basic wave eigenfunction has the same horizontal structure as the basic flow although it is vertically barotropic. Moreover, weakly nonlinear theory predicts growth of the wave amplitudes, which is unrestrained by wave–mean flow interaction. This prediction is verified by direct numerical calculation. The numerical calculations further reveal the manner by which the wave eventually equilibrates. The strongly growing wave cascades energy to higher zonal harmonics. These harmonics alter the meridional structure of the fundamental that allows wave–mean flow interaction to operate, leading finally to equilibration. If the cascade to higher zonal wavenumbers is artificially blocked by truncating the numerical model to a single zonal wavenumber, equilibration artificially requires the annihilation of the basic shear.

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P. Klein and J. Pedlosky

Abstract

The effect of three different parameterizations of dissipation on the nonlinear dynamics of unstable baroclinic waves is studied. The model is the two-layer f-plane model and the dynamics is quasigeostrophic. The dissipation mechanisms are 1) dissipation due to Ekman layers at the horizontal boundary surfaces, 2) the addition of interfacial Ekman friction, or 3) dissipation proportional to the perturbation potential vorticity.

We find, as anticipated by weakly nonlinear theory, a strong effect on the nonlinear amplitude dynamics for supercriticalities as large as four times the threshold value for instability. The use of interfacial friction or potential vorticity damping expunges the vacillating behavior common to the system with type 1 dissipation.

At high supercriticality a barotropic vacillation involving the mean flow and harmonics of the fundamental is superimposed on the basic baroclinic wave dynamics. Examination of the critical transition for the emergence of the barotropic oscillation reveals that the enhanced linear instability of the higher harmonics is responsible for the self-sustained vacillation.

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G. Lapeyre and P. Klein

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In this study, the relation between the interior and the surface dynamics for nonlinear baroclinically unstable flows is examined using the concepts of potential vorticity. First, it is demonstrated that baroclinic unstable flows present the property that the potential vorticity mesoscale and submesoscale anomalies in the ocean interior are strongly correlated to the surface density anomalies. Then, using the invertibility of potential vorticity, the dynamics are decomposed in terms of a solution forced by the three-dimensional (3D) potential vorticity and a solution forced by the surface boundary condition in density. It is found that, in the upper oceanic layers, the balanced flow induced only by potential vorticity is strongly anticorrelated with that induced only by the surface density with a dominance of the latter. The major consequence is that the 3D balanced motions can be determined from only the surface density and the characteristics of the basin-scale stratification by solving an elliptic equation. These properties allow for the possibility to reconstruct the 3D balanced velocity field of the upper layers from just the knowledge of the surface density by using a simpler model, that is, an “effective” surface quasigeostrophic model. All these results are validated through the examination of a primitive equation simulation reproducing the dynamics of the Antarctic Circumpolar Current.

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P. Klein and A. M. Treguier

Abstract

The dynamics of the mixed layer in the presence of an embedded geostrophic jet has been investigated using a simple 1½-layer model and a two-dimensional primitive equation model. The jet vorticity induces a spatial variability of the wind-driven inertial motions that can have some important consequences on the mixed-layer dynamics. With a steady wind stress parallel to the front, the main effect is the generation of steady upwellings and downwellings due to the divergence of the mean Ekman drift (as reported by Niiler). With a cross-front wind, however, a dramatic exponential amplification of the inertial oscillations caused by an inertial resonance mechanism is found: this mechanism can increase the inertial waves amplitude by a factor up to 10 within ten inertial periods. Competition between this resonance mechanism and the dispersion mechanisms (mainly the horizontal and vertical propagation of inertial waves) that can limit its effects has been assessed. A consequence of horizontal propagation is that energetic waves can propagate well away from the jet while continuing to absorb energy from the wind. Downward propagation disperses this energy to a depth of at least 500 m in a few days.

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P. Klein and A. M. Treguier

Abstract

No abstract available

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Cédric P. Chavanne and Patrice Klein

Abstract

A quasigeostrophic model is developed to diagnose the three-dimensional circulation, including the vertical velocity, in the upper ocean from high-resolution observations of sea surface height and buoyancy. The formulation for the adiabatic component departs from the classical surface quasigeostrophic framework considered before since it takes into account the stratification within the surface mixed layer that is usually much weaker than that in the ocean interior. To achieve this, the model approximates the ocean with two constant stratification layers: a finite-thickness surface layer (or the mixed layer) and an infinitely deep interior layer. It is shown that the leading-order adiabatic circulation is entirely determined if both the surface streamfunction and buoyancy anomalies are considered. The surface layer further includes a diabatic dynamical contribution. Parameterization of diabatic vertical velocities is based on their restoring impacts of the thermal wind balance that is perturbed by turbulent vertical mixing of momentum and buoyancy. The model skill in reproducing the three-dimensional circulation in the upper ocean from surface data is checked against the output of a high-resolution primitive equation numerical simulation.

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A. M. G. Klein Tank and G. P. Können

Abstract

Trends in indices of climate extremes are studied on the basis of daily series of temperature and precipitation observations from more than 100 meteorological stations in Europe. The period is 1946–99, a warming episode. Averaged over all stations, the indices of temperature extremes indicate “symmetric” warming of the cold and warm tails of the distributions of daily minimum and maximum temperature in this period. However, “asymmetry” is found for the trends if the period is split into two subperiods. For the 1946–75 subperiod, an episode of slight cooling, the annual number of warm extremes decreases, but the annual number of cold extremes does not increase. This implies a reduction in temperature variability. For the 1976–99 subperiod, an episode of pronounced warming, the annual number of warm extremes increases 2 times faster than expected from the corresponding decrease in the number of cold extremes. This implies an increase in temperature variability, which is mainly due to stagnation in the warming of the cold extremes.

For precipitation, all Europe-average indices of wet extremes increase in the 1946–99 period, although the spatial coherence of the trends is low. At stations where the annual amount increases, the index that represents the fraction of the annual amount due to very wet days gives a signal of disproportionate large changes in the extremes. At stations with a decreasing annual amount, there is no such amplified response of the extremes.

The indices of temperature and precipitation extremes in this study were selected from the list of climate change indices recommended by the World Meteorological Organization–Commission for Climatology (WMO–CCL) and the Research Programme on Climate Variability and Predictability (CLIVAR). The selected indices are expressions of events with return periods of 5–60 days. This means that the annual number of events is sufficiently large to allow for meaningful trend analysis in ∼50 yr time series. Although the selected indices refer to events that may be called “soft” climate extremes, these indices have clear impact relevance.

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Jerome P. Charba and William H. Klein

All known long-term records of forecasting performance for different types of precipitation forecasts in the National Weather Service were examined for relative skill and secular trends in skill. The largest upward trends were achieved by local probability of precipitation (PoP) forecasts for the periods 24–36 h and 36–48 h after 0000 and 1200 GMT. Over the last 13 years, the skill of these forecasts has improved at an average rate of 7.2% per 10-year interval. Over the same period, improvement has been smaller in local PoP skill in the 12–24 h range (2.0% per 10 years) and in the accuracy of “Yes/No” forecasts of measurable precipitation. The overall trend in accuracy of centralized quantitative precipitation forecasts of ≥0.5 in and ≤1.0 in has been slightly upward at the 0–24 h range and strongly upward at the 24–48 h range. Most of the improvement in these forecasts has been achieved from the early 1970s to the present. Strong upward accuracy trends in all types of precipitation forecasts within the past eight years are attributed primarily to improvements in numerical and statistical centralized guidance forecasts.

The skill and accuracy of both measurable and quantitative precipitation forecasts is 35–55% greater during the cool season than during the warm season. Also, the secular rate of improvement of the cool season precipitation forecasts is 50–110% greater than that of the warm season. This seasonal difference in performance reflects the relative difficulty of forecasting predominantly stratiform precipitation of the cool season and convective precipitation of the warm season.

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William H. Klein, Frank Lewis, and George P. Casely

Abstract

An objective method of forecasting maximum and minimum surface temperatures for periods from 12 to 60 hr in advance is discussed and illustrated. The method makes use of multiple regression equations derived for 108 cities in the United States and 11 cities in Canada from 16 years of daily data stratified by 2-month periods. The predictors are selected by screening (by pairs) the following parameters:

a) 700-mb height and 700–1000 mb thickness observed at 67 grid points in North America about 12 hr before the valid time of the prognostic temperature;

b) maximum and minimum temperatures observed at the network of 119 cities about 12 or 24 hr before the prognostic valid time; and

c) the day of the year.

On the average approximately ¾ of the temperature variance is explained by 5 variables, and the standard error of estimate is about 4F.

The method has been applied in an iterative fashion twice daily at the National Meteorological Center at Suitland, Md., since September 1965. The first forecast, for 12 hr in advance, uses only observed values of height, thickness, and temperature as input to the multiple regression equations. Subsequent forecasts utilize numerical prognoses of height and thickness, as upper air input, and preceding automated forecasts of maximum and minimum, prepared by the system, as temperature input. Verification statistics are presented for a year's operation, and the resulting objective forecasts appear to be almost as good as subjective forecasts and superior to persistence or an older objective method.

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Neil P. Lareau, Yunyan Zhang, and Stephen A. Klein

Abstract

The boundary layer controls on shallow cumulus (ShCu) convection are examined using a suite of remote and in situ sensors at ARM Southern Great Plains (SGP). A key instrument in the study is a Doppler lidar that measures vertical velocity in the CBL and along cloud base. Using a sample of 138 ShCu days, the composite structure of the ShCu CBL is examined, revealing increased vertical velocity (VV) variance during periods of medium cloud cover and higher VV skewness on ShCu days than on clear-sky days. The subcloud circulations of 1791 individual cumuli are also examined. From these data, we show that cloud-base updrafts, normalized by convective velocity, vary as a function of updraft width normalized by CBL depth. It is also found that 63% of clouds have positive cloud-base mass flux and are linked to coherent updrafts extending over the depth of the CBL. In contrast, negative mass flux clouds lack coherent subcloud updrafts. Both sets of clouds possess narrow downdrafts extending from the cloud edges into the subcloud layer. These downdrafts are also present adjacent to cloud-free updrafts, suggesting they are mechanical in origin. The cloud-base updraft data are subsequently combined with observations of convective inhibition to form dimensionless “cloud inhibition” (CI) parameters. Updraft fraction and liquid water path are shown to vary inversely with CI, a finding consistent with CIN-based closures used in convective parameterizations. However, we also demonstrate a limited link between CBL vertical velocity variance and cloud-base updrafts, suggesting that additional factors, including updraft width, are necessary predictors for cloud-base updrafts.

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