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Alan J. Thorpe
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Alan J. Thorpe

Abstract

Ate balanced flow structure of various classic synopitc-scale disturbances is reviewed using the invertibility principle for isentropic potential vorticity (IPV) distributions. Complete solutions are shown for cold and warm core structures of various types. The basic model imagines the tropopause to be the interface between the lower potential vorticity of the troposphere and the approximately six-fold larger value typical of the lower stratosphere. The sensitivity of the structure of the potential temperature variation along the tropopause and at the surface is described. Results are presented in diagrammatic form to allow easy diagnosis of the vortex structure from synoptic data available at perhaps only a few levels. The point is made that upper air IPV and surface potential temperature distributions are often the most crucial in accounting for the balanced flow structure.

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Walter Fernandez and Alan J. Thorpe

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Raymond's (1975) wave-CISK model is applied to several tropical convective storms observed in Venezuela, the eastern Atlantic and West Africa to predict their propagation velocity. Similar calculations are carried out with Moncrieff and Miller's (1976) analytical model for tropical cumulonimbus and squall lines. A comparison of the model predictions with the observed values is made. In some cases the models give good predictions, but not in others. In general, Raymond's model underestimates the propagation speed of the storms, while the Moncrieff-Miller model overestimates it. Raymond's model is poor when the cloud bases are very low. This result indicates that over tropical oceans wave-CISK models cannot give good results unless the mass flux due to the plumes, which is equated to the mass flux across cloud base, is treated in a more realistic way. The Moncrieff-Miller model gives better results if the mean wind component along the direction of motion is used rather than the mid-level wind.

The wave-CISK model and steady-state models of storm motion are then considered in conditions of constant wind shear. In particular, their predictions are compared over a wide range of shear values, using realistic thermodynamic soundings. Despite the obvious differences between the models, it is found that, for Richardson number small (R<1) and very large, they give comparable predictions for the storm velocity. It appears that a very good approximation for the wave-CISK model over the entire R range is to put the storm speed proportional to the shear, plus a constant.

An important conclusion is that the ability of storms to propagate relative to the environmental flow can be reproduced in the linear wave-CISK model and thus may not be a fundamentally nonlinear effect. It is therefore crucial to further examine forcing mechanisms of convective overturning and, in particular, to clarify the relationship between CISK and the implicit forcing involved in the steady model.

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Ming Xue and Alan J. Thorpe

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A nonhydrostatic numerical model suitable for simulating mesoscale meteorological phenomena is developed and described here. The model is the first to exploit the nonhydrostatic equation system in σ (normalized pressure) coordinates. In addition to the commonly recognized advantages of σ-coordinate models, this model is potentially advantageous in nesting with large-scale σ-coordinate models. The equation system does not support sound waves but it presents the internal gravity waves accurately. External gravity waves are the fastest wave modes in the system that limit the integration time step. However, since short nonhydrostatic external waves are much slower than the speed of shallow-water waves and because fast hydrostatic long waves imposes less severe restriction on the time step when they are resolved by many grid points, a large time step (compared to that determined by the speed of hydrostatic shallow-water waves) can be used when horizontal grid spacing is on the order of 1 km.

The system is solved in a way analogous to the anelastic system in terrain-following height coordinates. The geopotential height perturbation is diagnosed from an elliptic equation. Conventional finite-differencing techniques are used based on Arakawa C grid, The flux-corrected transport (FCT) scheme is included as an option for scalar advection.

The model has been used to study a variety of problems and here the simulations of dry mountain waves are presented. The resists of simulations of the 11 January 1972 Boulder severe downslope windstorm are reported and the wave development mechanism discussed.

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Meral Demirtas and Alan J. Thorpe

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A new method is described to interpret satellite water vapor (WV) imagery in dynamical terms using potential vorticity (PV) concepts. The method involves the identification of mismatches between the WV imagery and a numerical weather prediction model description of the upper-level PV distribution at the analysis time. These mismatches are usually associated with horizontal positioning errors in the tropopause location in the oceanic storm-track region in midlatitudes. The PV distribution is locally modified to minimize this mismatch, and PV inversion is carried out to provide dynamically consistent additional initial data with which to reinitialize the numerical forecast.

One of the advantages of using this method is that it is possible to generate wind and temperature data suitable for inclusion as initial data for numerical weather forecasts. By using PV additional data can be inferred that cannot otherwise be simply derived from the WV data. In this way dynamical concepts add considerable value to the WV imagery, which by themselves would probably not have as significant a forecast impact.

Several examples of the use of this method are given here including cases of otherwise poorly forecast North Atlantic cyclones. In cases where the analysis errors occur at upper levels of the troposphere, the method leads to a significant improvement in the short-range forecast skill. In general, it is useful in highlighting where forecast problems are arising.

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Hannah R. Pomroy and Alan J. Thorpe

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The existence and production of reduced upper-tropospheric potential vorticity (RUPV) by heating is considered. An objective technique is used that identifies anomalies of PV arising from a particular physical process (here latent heat release). The evolution of two RUPV anomalies and a related diabatically increased lower-tropospheric PV (ILPV) anomaly occurring during Intensive Observing Period One of the cyclones from the Fronts and Atlantic Storm Track Experiment (FASTEX) is examined using model analyses, sounding data, and trajectory calculations. Three distinct airflows are identified emanating from the ILPV anomaly each with a different evolution. Results show that RUPV anomalies exist in the atmosphere and, in a weaker form, in numerical models.

The dynamical role of RUPV anomalies is examined using a nonlinear balance PV inversion and reruns of the U.K. Meteorological Office Limited Area Model. This shows that instantaneously the flow and temperature perturbations associated with RUPV anomalies are of at least comparable magnitude and extent to those induced by a similar positive anomaly. Over time one RUPV anomaly is seen to have a significant effect upon the development of its parent low. This low is more compact and more rapidly developing in the absence of the anomaly. The effect of the positive anomaly is also significant, but removing it has only a short-term effect as the anomaly quickly reforms. These results show that it is important to consider the role of RUPV in the PV model of a midlatitude cyclone.

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Linus Magnusson, Jean-Raymond Bidlot, Simon T. K. Lang, Alan Thorpe, Nils Wedi, and Munehiko Yamaguchi

Abstract

On 30 October 2012 Hurricane Sandy made landfall on the U.S. East Coast with a devastating impact. Here the performance of the ECMWF forecasts (both high resolution and ensemble) are evaluated together with ensemble forecasts from other numerical weather prediction centers, available from The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) archive. The sensitivity to sea surface temperature (SST) and model resolution for the ECMWF forecasts are explored. The results show that the ECMWF forecasts provided a clear indication of the landfall from 7 days in advance. Comparing ensemble forecasts from different centers, the authors find the ensemble forecasts from ECMWF to be the most consistent in the forecast of the landfall of Sandy on the New Jersey coastline. The impact of the warm SST anomaly off the U.S. East Coast is investigated by running sensitivity experiments with climatological SST instead of persisting the SST anomaly from the analysis. The results show that the SST anomaly had a small effect on Sandy’s track in the forecast, but the forecasts initialized with the warm SST anomaly feature a more intense system in terms of the depth of the cyclone, wind speeds, and precipitation. Furthermore, the role of spatial resolution is investigated by comparing four global simulations, spanning from TL159 (150 km) to TL3999 (5 km) horizontal resolution. Forecasts from 3 and 5 days before the landfall are evaluated. While all resolutions predict Sandy’s landfall, at very high resolution the tropical cyclone intensity and the oceanic wave forecasts are greatly improved.

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