Forecasting in the Tropics With a Barotropic Atmospheric Model (original) (raw)
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An Empirical Model of Barotropic Atmospheric Dynamics and Its Response to Tropical Forcing
Journal of Climate, 1998
A linear empirical model of barotropic atmospheric dynamics is constructed in which the streamfunction tendency field is optimally predicted using the concurrent streamfunction state as a predictor. The prediction equations are those resulting from performing a linear regression between tendency and state vectors. Based on the formal analogy between this model and the linear nondivergent barotropic vorticity equation, this empirical model is applied to problems normally addressed with a conventional model based on physical principles. It is found to qualitatively represent the horizontal dispersion of energy and to skillfully predict how a general circulation model will respond to steady tropical heat sources. Analysis of model solutions indicates that the empirical model's dynamics include processes that are not represented by conventional nondivergent linear models. Most significantly, the influence of internally generated midlatitude divergence anomalies and of anomalous vorticity fluxes by high-frequency transients associated with low-frequency anomalies are automatically incorporated into the empirical model. The results suggest the utility of empirical models of atmospheric dynamics in situations where estimates of the response to external forcing are needed or as a standard of comparison in efforts to make models based on physical principles more complete.
Monthly Weather Review, 1999
The technique of ensemble forecasting is applied to the problem of tropical cyclone motion prediction. Three methods of generating perturbations for the environmental flow, Monte Carlo forecast (MCF), lagged-average forecast (LAF), and the breeding of growing modes (BGM), are tested with a barotropic model using 66 cases from the Tropical Cyclone Motion (TCM-90) Experiment. For the MCF, the ensemble mean forecast is almost identical to that without any perturbation. The other two methodologies are verified both under the perfect model assumption and using the best tracks. On average, in about half of the cases improvement in forecast can be demonstrated in the former verification. A high degree of correlation (with linear correlation coefficient Ͼ0.9) is also found between the spread of the ensemble and the root-mean-square forecast error. In the best-track verification, improvement in forecasts can also be obtained in 36% (42%) of all the cases using the LAF (BGM) technique. The spread-skill correlation is still significant (correlation coefficients vary from ϳ0.4 to 0.7 for different forecast times). An examination of the synoptic flow associated with cases in which the forecast is improved suggests some favorable conditions for the application of ensemble forecasting. These include a tropical cyclone (TC) making a transition from one synoptic region to another, an apparent break in the subtropical ridge (STR), a rapid strengthening/weakening of the STR, recurvature of a TC, and multiple-TC situations.
The Equivalent Barotropic Structure of Waves in the Tropical Atmosphere in the Western Hemisphere
Journal of the Atmospheric Sciences
Tropical waves are generally considered to have a baroclinic structure. However, analysis of ERA-Interim and NOAA OLR data for the period 1979–2010 shows that in the equatorial and Northern Hemisphere near-equatorial regions in the tropical Western Hemisphere (WH), westward- and eastward-moving transients, with zonal wavenumbers 2–10 and periods of 2–30 days, have little tilt in the vertical and can be said to be equivalent barotropic. The westward-moving transients in the equatorial region have large projections onto the westward mixed Rossby–gravity (WMRG) wave and those in the near-equatorial region project onto the gravest Rossby wave and also the WMRG. The eastward-moving transients have large projections onto the Doppler-shifted eastward-moving versions of these waves. To examine how such equivalent barotropic structures are possible in the tropics, terms in the vorticity equation are analyzed. It is deduced that waves must have westward intrinsic phase speeds and can exist in...
Terrestrial, Atmospheric and Oceanic Sciences, 2014
This study used the barotropic kinetic energy conversion to record the active eddy-mean flow interaction between the TC/sub-monthly wave pattern (TSM) and the intraseasonal oscillation (ISO) in the western North Pacific (WNP). Overall, the TSM extracted (lost) kinetic energy from (to) the cyclonic (anticyclonic) circulation of the ISO, which is located in the South China Sea and the Philippine Sea, during the ISO westerly (easterly) phase. The phase change in barotropic energy conversion was due to the opposite background flow set up by the ISO. When the climatological-mean southwesterly was retained as part of the background flow in both ISO westerly and easterly phases as in previous studies, the ISO along with the low-frequency background flow always provided kinetic energy to the TSM regardless of the phase. The stronger (weaker) southwesterly in the ISO westerly (easterly) phase, the stronger (weaker) energy conversion to the TSM. Climatological mean flow exclusion showed an upscale feedback in the TSM to the ISO during the easterly phase. However, this feedback was weaker than the downscale conversion from the ISO to the TSM during the westerly phase.
A Comparison of the Downstream Predictability Associated with ET and Baroclinic Cyclones
Monthly Weather Review, 2017
The impact of the extratropical transition (ET) of tropical cyclones and baroclinic cyclogenesis in the western North Pacific (WNP), Atlantic, and southern Indian Ocean (SIO) basins on the predictability of the downstream midlatitude flow is assessed using 30 years of cases from the Global Ensemble Forecast System (GEFS) Reforecast, version 2. In all three basins, ET is associated with statistically larger 500-hPa geopotential height forecast standard deviation (SD) compared to the forecast climatology. The higher SD values originate from where the TC enters the midlatitudes and spread downstream at the group velocity of the associated wave packet. Of the three basins, WNP ET is associated with the largest amplitude and longest-lasting SD anomalies. Forecasts initialized 2–4 days prior to the onset of ET have larger SD anomalies compared to forecasts initialized during or after the onset of ET. By contrast, the region of positive SD anomaly associated with winter baroclinic cyclones...
A Simple Atmospheric Model of the Local and Teleconnection Responses to Tropical Heating Anomalies
Journal of Climate, 2009
A minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated. Two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected. The model is linearized about some idealized and realistic background wind fields and forced with a localized heating for illustration. In the tropics, the baroclinic responses are familiar from the Matsuno–Gill model; these excite barotropic responses by advective interactions with vertical background wind shear. The barotropic signals are in turn transmitted to high latitudes only in the presence of barotropic background westerly winds. For an El Niño–like equatorial heating, the barotropic response has anticyclones to the north and south of the heating reinforcing (opposing) the anticyclonic (cyclonic) baroclinic gyres in the upper (lower) troposphere. With realistic background flows, the model reproduces the hemispheric asymmetry of ENSO teleconnections. Further experiments show that the winter hemisphere is favored mainly because the summer hemispheric subtropical jet is farther from the heating latitude, suggesting that the summer hemisphere can still host robust stationary Rossby waves if the heating occurs in the vicinity of the jet. As an example, it is shown that summer heating over the Atlantic warm pool (AWP) can have a remote influence on the summer climate of North America and Europe.
Quarterly Journal of the Royal Meteorological Society, 1991
The motion of an initially symmetric vortex in a spatially-varying large-scale flow on a beta plane is investigated using a nondivergent, barotropic numerical model. The calculations extend those carried out for the case of zero basic flow in Part I. The large-scale flows are provided by meridionally-varying zonal flows, or single-mode, stationary, finite-amplitude planetary waves in a channel. Interest is focused on the evolution of vortex asymmetries and their role in determining vortex motion relative to the basic large-scale flow. As in Part I, the calculations are used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data. It is shown that averaging over an annular region centred on the vortex is, in principle, more accurate when there is a basic flow, in comparison with the case of zero basic flow.
Journal of the Atmospheric Sciences, 2001
A stochastic barotropic model linearized about the 850-mb flow is used to investigate the relationship between wind variations associated with the Madden-Julian oscillation (MJO) and eddy kinetic energy variations in the Tropics. Such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the MJO and the suppression of eddy activity during the easterly phase of the MJO. The concentration of eddy energy during the westerly phase results from the strong east-west and north-south gradients of the large-scale wind fields. The model shows that barotropic wave propagation and wave meanflow interaction tend to concentrate small-scale Rossby wave energy in regions of convergence, which may be an important mechanism for organizing convection into tropical cyclones. The structure and barotropic energetics of the wave activity are similar to those observed, but the modeled eddies are smaller in scale and do not move westward as do the observed eddies. The eddies that dominate the observed correlations are heavily modified by convection, but barotropic processes can explain the localization of eddy energy by the MJO that is observed.