Extratropical Troposphere Research Articles

10hPa面以下の領域におけるトレンド分布に関するノート : 北半球中高緯度域の場合

The trend in the zonally averaged temperature, 1964-1993, was positive in most of the extratropical troposphere. It was negative in the annual mean in the stratosphere. In January a positive trend appeared in the middle stratosphere in the arctic which progressed downward through the winter. This is reflected in a reversal of the trends in the geopotential heights at 30 hPa and 10 hPa in late winter. The trends in the annual mean geopotential heights in both troposphere and stratosphere were positive south of about 60°N and negative to the north. The zonally averaged geostrophic westerly wind between middle and high latitudes therefore increased during the period.

A PV View of the Zonal Mean Distribution of Temperature and Wind in the Extratropical Troposphere

The dependence of the temperature and wind distribution of the zonal mean flow in the extratropical troposphere on the gradient of pontential vorticity along isentropes is examined. The extratropics here refer to the region outside the Hadley circulation. Of particular interest is whether the distribution of temperature and wind corresponding to a constant potential vorticity (PV) along isentropes resembles the observed, and the implications of PV homogenization along isentropes for the role of the tropics. With the assumption that PV is homogenized along isentropes, it is found that the temperature distribution in the extratropical troposphere may be determined by a linear, first-order partial differential equation. When the observed surface temperature distribution and tropical lapse rate are used as the boundary conditions, the solution of the equation is close to the observed temperature distribution except in the upper troposphere adjacent to the Hadley circulation, where the troposphere with no PV gradient is considerably colder. Consequently, the jet is also stronger. It is also found that the meridional distribution of the balanced zonal wind is very sensitive to the meridional distribution of the tropopause temperature. The result may suggest that the requirement of the global momentum balance has no practical role in determining the extratropical temperature distribution. The authors further investigated the sensitivity of the extratropical troposphere with constant PV along isentropes to changes in conditions at the tropical boundary (the edge of the Hadley circulation). It is found that the temperature and wind distributions in the extratropical troposphere are sensitive to the vertical distribution of PV at the tropical boundary. With a surface distribution of temperature that decreases linearly with latitude, the jet maximum occurs at the tropical boundary and moves with it. The overall pattern of wind distribution is not sensitive to the change of the position of the tropical boundary. Finally, the temperature and wind distributions of an extratropical troposphere with a finite PV gradient are calculated. It is found that the larger the isentropic PV gradient, the warmer the troposphere and the weaker the jet.

中緯度対流圏上部に卓越する中間規模東進波

中緯度対流圏上部に卓越する中間規模東進波

One-to-Two Month Oscillations: Observed High-Latitude Tropospheric and Stratospheric Response to Tropical Forcing

Abstract Careful spectral, correlation and coherence analyses of low-frequency fluctuations in global geopotential height data are presented. Attention is paid to proper statistical assessments. The main points are: 1) one-to-two month oscillating quasi-stationary wavetrains have recently been reported in the extratropical Southern Hemisphere troposphere, as far south as the edge of Antarctica. However, only weak correlations were observed with the supposed tropical forcing region, leading to the question of whether the wavetrain is a response to tropical forcing or possibly due to in situ instabilities on 1–2 month time scales. The present paper clears up this enigma with analyses of other tropical datasets which reveal clear correlation between low latitude source regions and the SH extratropical troposphere. 2) An earlier investigation found strong correlations between 1–2 month oscillations in the upper stratosphere and tropical troposphere, yet no vertical propagation was found directly above the tro...

Extended-range predictions with ECMWF models: Influence of horizontal resolution on systematic error and forecast skill

The influence of horizontal resolution on model systematic error and skill scores are examined for both winter and summer seasons from a set of extended-range integrations of the European Centre for Medium Range Weather Forecasts (ECMWF) numerical weather prediction model. the forecast data are composed of 24 30-day integrations, two per month from successive 12 GMT operational analyses, from April 1985 to March 1986 at four (triangular truncation) horizontal spectral resolutions of T21, T42, T63 and T106. Systematic errors in wind, temperature, heat and momentum fluxes are discussed, both in the tropics and extratropics. Dynamical reasons for differences between various model resolutions and between simulations and observations are proposed. Diagnosis of model systematic error is aided by comparing zonal-mean diagnostics of the observed flow in the northern hemisphere winter and southern hemisphere winter. the impact of resolution on regional simulations of rainfall during the Indian and African monsoons is also discussed. In the extratropical troposphere, the behaviour of the T21 model is quite distinct from the higher-resolution models. the southern hemisphere flow is weak, and in both hemispheres horizontal momentum fluxes are severely underestimated. the systematic errors of the T42, T63 and T106 models are quite similar to each other in the extratropical troposphere. Errors at these higher resolutions resemble the observed difference (in zonal-mean wind, temperature and eddy fluxes) between diagnostics in the northern hemisphere winter and southern hemisphere winter, suggesting that orographic forcing in this version of the ECMWF model is inadequate, despite the inclusion of envelope orography. In the extratropical stratosphere, the behaviours of the T21 and T42 models resemble each other, both having a cold polar bias, apparently associated with inadequate dynamical heating and therefore relaxation to radiative equilibrium. This in turn is related to insufficient Rossby wave focusing into the polar vortex at T21 and T42. the T63 and T106 resolution integrations have much smaller stratospheric cold-pole biases. In the tropics, there are serious systematic errors at all resolutions; however, some of these mean errors increase with increasing resolution. These include the global divergent and nondivergent wind errors, and more regional simulations such as the monsoon flow, where simulation of diabatic heating associated with convective activity is crucial. Since the local values of moisture flux convergence at T106 are effectively much noisier than the larger-scale values of such fluxes used to drive the convection scheme at T21, it is speculated that diabatic heating fields are more likely to project onto the relevant tropical meteorological modes at low model resolution. On the other hand, it is shown that the ability to resolve local orographic features is important in accurately simulating local tropical precipitation maxima over land. Despite having smaller systematic errors in the northern hemisphere, 10-day mean T21 model anomaly correlation coefficient scores were worse than those of higher-resolution models, at least up to day 20. However, there was no indication that the higher-resolution models had significantly different extended-range skill characteristics. On the other hand, the increase of tropical systematic error with increasing horizontal resolution may indicate that the potential of higher-resolution models in extended-range prediction may be underestimated in this study. the asymptotic spread between forecasts initialized 24 hours apart indicated that all models substantially underestimated low-frequency variability (by up to 50% in the extended summer period). Bearing in mind the computational burden of longer timescale integrations with complex numerical models of the atmosphere, the evidence presented in this paper suggests that the T106 resolution may be unnecessarily high for both extended-range forecasting, and for climate simulation studies. However, at present, the cost of integrating at T63 resolution may also be excessive for many climate studies. It would appear from our analysis, that in the troposphere the behaviour of the T42 model is comparable with the higher-resolution models, and is a satisfactory compromise for many purposes, particularly bearing in mind the impact of resolution on tropical systematic errors. However, it would appear that the extreme sensitivity of the 1985/86 ECMWF model climate drift to resolution in the range between T21 and T42, and the apparent inability of the ECMWF T21 model to simulate the correct internal nonlinear dynamics of the extratropics, make questionable its use for climate studies at resolution lower than T42.

Extended‐range predictions with ecmwf models: Influence of horizontal resolution on systematic error and forecast skill

AbstractThe influence of horizontal resolution on model systematic error and skill scores are examined for both winter and summer seasons from a set of extended‐range integrations of the European Centre for Medium Range Weather Forecasts (ECMWF) numerical weather prediction model. the forecast data are composed of 24 30‐day integrations, two per month from successive 12 GMT operational analyses, from April 1985 to March 1986 at four (triangular truncation) horizontal spectral resolutions of T21, T42, T63 and T106.Systematic errors in wind, temperature, heat and momentum fluxes are discussed, both in the tropics and extratropics. Dynamical reasons for differences between various model resolutions and between simulations and observations are proposed. Diagnosis of model systematic error is aided by comparing zonal‐mean diagnostics of the observed flow in the northern hemisphere winter and southern hemisphere winter. the impact of resolution on regional simulations of rainfall during the Indian and African monsoons is also discussed.In the extratropical troposphere, the behaviour of the T21 model is quite distinct from the higher‐resolution models. the southern hemisphere flow is weak, and in both hemispheres horizontal momentum fluxes are severely underestimated. the systematic errors of the T42, T63 and T106 models are quite similar to each other in the extratropical troposphere. Errors at these higher resolutions resemble the observed difference (in zonal‐mean wind, temperature and eddy fluxes) between diagnostics in the northern hemisphere winter and southern hemisphere winter, suggesting that orographic forcing in this version of the ECMWF model is inadequate, despite the inclusion of envelope orography.In the extratropical stratosphere, the behaviours of the T21 and T42 models resemble each other, both having a cold polar bias, apparently associated with inadequate dynamical heating and therefore relaxation to radiative equilibrium. This in turn is related to insufficient Rossby wave focusing into the polar vortex at T21 and T42. the T63 and T106 resolution integrations have much smaller stratospheric cold‐pole biases.In the tropics, there are serious systematic errors at all resolutions; however, some of these mean errors increase with increasing resolution. These include the global divergent and nondivergent wind errors, and more regional simulations such as the monsoon flow, where simulation of diabatic heating associated with convective activity is crucial. Since the local values of moisture flux convergence at T106 are effectively much noisier than the larger‐scale values of such fluxes used to drive the convection scheme at T21, it is speculated that diabatic heating fields are more likely to project onto the relevant tropical meteorological modes at low model resolution. On the other hand, it is shown that the ability to resolve local orographic features is important in accurately simulating local tropical precipitation maxima over land.Despite having smaller systematic errors in the northern hemisphere, 10‐day mean T21 model anomaly correlation coefficient scores were worse than those of higher‐resolution models, at least up to day 20. However, there was no indication that the higher‐resolution models had significantly different extended‐range skill characteristics. On the other hand, the increase of tropical systematic error with increasing horizontal resolution may indicate that the potential of higher‐resolution models in extended‐range prediction may be underestimated in this study. the asymptotic spread between forecasts initialized 24 hours apart indicated that all models substantially underestimated low‐frequency variability (by up to 50% in the extended summer period).Bearing in mind the computational burden of longer timescale integrations with complex numerical models of the atmosphere, the evidence presented in this paper suggests that the T106 resolution may be unnecessarily high for both extended‐range forecasting, and for climate simulation studies. However, at present, the cost of integrating at T63 resolution may also be excessive for many climate studies. It would appear from our analysis, that in the troposphere the behaviour of the T42 model is comparable with the higher‐resolution models, and is a satisfactory compromise for many purposes, particularly bearing in mind the impact of resolution on tropical systematic errors.However, it would appear that the extreme sensitivity of the 1985/86 ECMWF model climate drift to resolution in the range between T21 and T42, and the apparent inability of the ECMWF T21 model to simulate the correct internal nonlinear dynamics of the extratropics, make questionable its use for climate studies at resolution lower than T42.

Numerical forecasts of stratospheric warming events using a model with a hybrid vertical coordinate

AbstractA numerical forecast model based on use of a hybrid vertical coordinate which is terrain‐following at low levels but which tends continuously to a pressure coordinate at upper levels is introduced. Results from this model are first presented for 10‐day forecasts (extending up to 10mb) of the major stratospheric circulation changes which occurred during the winter of 1979. A considerable degree of success is illustrated, not only in predicting a major wavenumber‐two warming up to 10 days ahead, but also in the forecasting of a preceding development of wavenumber one and a subsequent decay of the wavenumber two perturbation. Diagnosis using Eliassen‐Palm cross‐sections confirms previous observational diagnoses and aids understanding of the performance of the forecast model. Computations of the meridional gradient of zonal‐mean potential vorticity are also discussed.The sensitivity of tropospheric forecasts to stratospheric resolution and the choice of vertical coordinate is also examined. These forecasts are in fact characterized by a marked insensitivity to such aspects. Thus lowering the top forecast level from 10mb to 50mb has little effect on objective forecast scores averaged over the extratropical troposphere of the Northern Hemisphere, although planetary‐wave structures in high latitudes are affected to a small degree. The hybrid coordinate gives results similar to those obtained using the usual sigma coordinate, but such differences as are found tend to favour the use of the hybrid representation. Idealized analyses of the stability of the semi‐implicit time scheme for hybrid coordinates are confirmed in practice.

Quasi-Biennial Oscillations in Tropospheric Energy

Abstract Evidence is presented for an approximate year-to-year variation of the zonal available potential (AZ), eddy available potential (AE), zonal kinetic (KZ) and eddy kinetic energy (KE) in the extra-tropical troposphere between 1962 and 1971. A modification of the oscillations occurs after 1968 due to as yet undetermined causes, but prior to this there is a fairly significant correlation between these oscillations and the quasi-biennial zonal wind variation in the low tropical stratosphere. The results indicate a relative in-phase relationship between the AZ, KZ and AE, KE terms and a relative out-of-phase relationship between the AZ, AE and KZ, KE terms.

A MERIDIONAL AEROLOGICAL CROSS SECTION IN THE SOUTHWEST PACIFIC

From recent aerological information a meridional section has been drawn through the southern hemisphere in the region of longitude 150°E. Its most striking feature, the coldness of the extratropical troposphere, is mainly ascribed to the higher albedo of the southern hemisphere. A break in the height of the southern troposphere between 30°S and 40°S is established statistically. In winter the average tropopause descends very little between 35°S and 55°S. The poleward slope of the mean isobars, and consequently the westerly component of the mean geostrophic wind, is stronger in summer than in the northern hemisphere summer; in the winter seasons the difference between the hemispheres is less. The bulk of the southern troposphere in temperate and subpolar latitudes is marked by small differences between summer and winter. The jet stream appears clearly in the seasonal wind fields for both summer and winter. Its intensity and low latitude in winter is surprising; but the latter feature is confirmed by the available radar wind observations. In winter a somewhat problematical minimum of the zonal wind component is found in temperate latitudes.