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Adrian J. Simmons and Brian J. Hoskins

Abstract

nonlinear behavior of baroclinic waves is studied by means of numerical integrations of the primitive equations for initial zonal-mean states with almost identical temperature fields but differing barotropic flow components. The amplitude of the wave motion, and to a lesser extent that of the eddy transfers of heat and momentum, differ substantially from case to case. Some of this sensitivity is traced to distinctly barotropic effects which arise during the nonlinear evolution of the waves.

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Adrian J. Simmons and Brian J. Hoskins

Abstract

The response of a baroclinically unstable atmosphere to a localized initial perturbation is examined. In a preliminary numerical experiment, using a nonlinear primitive-equation model with spherical geometry, new disturbances grow regularly downstream in broad agreement with a particular type of observed development. The initial disturbance moves downstream, while smaller scale disturbances develop upstream of it—each forming at about the same longitude. Later downstream disturbances have upper level amplitudes significantly larger than found in nonlinear integrations using normal-mode initial conditions. Similar development is found in a quasi-geostrophic model with spherical geometry, and the rate of spreading of the instability is insensitive to the amplitude of the initial perturbation.

For the baroclinic instability model considered by Eady the downstream disturbances appear first at the upper surface at a position which moves with a speed close to that of the zonal-mean flow at this surface, while the upstream disturbances all form close to a fixed location it the surface mean flow is zero. New disturbances possess a short horizontal length scale which subsequently expands toward that of the most unstable normal mode, and their amplitude increases at a rate initially much in excess of normal-mode growth rates. Phase speed is close to that of normal modes. Diagnosis is performed both using a form of the omega equation and through an asymptotic analysis of the solution represented by a sum of normal modes of different wavenumbers.

Further calculations have been performed for a linearized β-plant model. Inclusion of a mean density variation with height and the β-effect slows the downstream spread of the instability by up to 15%, and this downstream development occurs on a longer length scale. The upstream disturbances quite generally develop close to a fixed location when the mean surface flow is zero, and possess a relatively short zonal scale if the low-level mean flow has vertical shear.

An additional primitive-equation integration confirms the results of the preliminary nonlinear experiment.

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Richard J. Reed and Adrian J. Simmons

Abstract

The effect of surface sensible and latent heat fluxes on explosive cyclogenesis has been found to be quite variable. Sensitivity tests reveal little or no influence in some cases and dramatic effects in others. However, in nearly all cases where little impact was found, the predicted deepening rate was significantly less than the observed rate, opening the possibility that the unexplained deepening might be a consequence of a shortcoming of the flux formulation.

We present here the results of a test of the impact of the fluxes in a case of extreme deepening that was accurately predicted by the ECMWF T106 operational model. In the test the fluxes were switched off during the 24-h period of most rapid deepening. The result was to change the predicted deepening rate of 48 mb by only 1 mb. This confirms that in certain circumstances the concurrent fluxes can have negligible effect on the deepening despite their great importance in other situations.

It is shown that downward heat and moisture fluxes (cooling and drying) occurred in the warm sector close to the low center, whereas upward heat and moisture fluxes (warming and moistening) occurred in the cold air well to the rear of the low. It is known from theory that this pattern is not conducive to cyclonic development. The cause of the vast difference in the effect of the fluxes on cyclone development is discussed in terms of the stage of development, the geographical location, and the degree of atmospheric preconditioning.

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Adrian J. Simmons and Brian J. Hoskins

Abstract

Some aspects of the nonlinear behavior of mid-latitude baroclinic waves are investigated by means of a series of integrations of the primitive equations with spherical geometry. Each integration has as initial conditions a balanced zonal flow perturbed by a small-amplitude disturbance of normal-mode form. Results are presented in detail for several zonal flows and perturbations which are confined initially to either zonal wavenumber 6 or zonal wavenumber 9.

In each case a disturbance grows by baroclinic instability and develops a structure in some agreement with the usual synoptic picture of an occluding system. Its growth rate at low levels decreases more rapidly than that at higher levels, as found by Gall using a more severely truncated model, and upper-level amplitudes become larger relative to surface values than in the initial linear mode. This is more marked for wavenumber 6 than for wavenumber 9, and differences in linear structure are thus enhanced in the nonlinear regime.

Barotropic processes become important during the occlusion of the disturbance as the forcing of vertical motion by thermal advection decreases in importance, although the vorticity actually changes at about half the rate that would occur in a barotropic fluid. In these examples the barotropic effects bring about a decay of the wave at a rate similar to that of its earlier baroclinic growth, and a well-defined life cycle exists.

Large-scale eddy fluxes of heat and momentum averaged over this life cycle have a structure that is substantially different from that given by linear stability analyses, and agreement with observation is improved. Net changes to the zonal-mean temperature gradient are largely confined to the lower troposphere and, to a lesser extent, the lower stratosphere. The change in surface zonal-mean flow is much as suggested by linear theory but at upper levels the westerly jet is strengthened as the disturbance decays.

Additional barotropic integrations have been performed to examine the changes in structure of longer wavelength disturbances at upper levels. Predominantly poleward momentum fluxes result from latitudinal variations in phase speed, and movement at a particular latitude is found to be governed largely by the zonal-mean velocity and vorticity gradient at that latitude. Additional baroclinic experiments provide an example of interactions involving a slower growing, longer wavelength component, and examples of some truncation errors that may result from use of lower resolution models. The sensitivity of results to the inclusion of dissipative processes is also examined.

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Frédéric Chevallier, Graeme Kelly, Adrian J. Simmons, Sakari Uppala, and Angeles Hernandez

Abstract

The reanalysis programs of numerical weather prediction (NWP) centers provide global, comprehensive descriptions of the atmosphere and of the earth’s surface over long periods of time. The high realism of their representation of key NWP parameters, like temperature and winds, implies some realism for less emblematic parameters, such as cloud cover, but the degree of this realism needs to be documented.

This study aims to evaluate the high clouds over open oceans in the ECMWF 15- and 45-yr reanalyses. The assessment is based on a new 23-yr climatology of monthly frequencies of high-cloud occurrence retrieved from the infrared radiances measured by operational polar satellites. It is complemented by data from the International Satellite Cloud Climatology Project.

It is shown that the 45-yr ECMWF reanalysis dramatically improves on the previous 15-yr reanalysis for the realism of seasonal and interannual variations in high clouds, despite remaining systematic errors. More than 60% of the observed anomalies during the January 1979–February 2002 period over large oceanic basins are captured by the latest reanalysis. However the realism of the analyses in the areas and in the years with sparse observations appears to be poor. Consequently, the interannual variations may not be reliable before January 1979 in most parts of the world. Possible improvements of the handling of assimilated satellite observations before and after this date are suggested.

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Gloria L. Manney, Douglas R. Allen, Kirstin Krüger, Barbara Naujokat, Michelle L. Santee, Joseph L. Sabutis, Steven Pawson, Richard Swinbank, Cora E. Randall, Adrian J. Simmons, and Craig Long

Abstract

Several meteorological datasets, including U.K. Met Office (MetO), European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP), and NASA’s Goddard Earth Observation System (GEOS-4) analyses, are being used in studies of the 2002 Southern Hemisphere (SH) stratospheric winter and Antarctic major warming. Diagnostics are compared to assess how these studies may be affected by the meteorological data used. While the overall structure and evolution of temperatures, winds, and wave diagnostics in the different analyses provide a consistent picture of the large-scale dynamics of the SH 2002 winter, several significant differences may affect detailed studies. The NCEP–NCAR reanalysis (REAN) and NCEP–Department of Energy (DOE) reanalysis-2 (REAN-2) datasets are not recommended for detailed studies, especially those related to polar processing, because of lower-stratospheric temperature biases that result in underestimates of polar processing potential, and because their winds and wave diagnostics show increasing differences from other analyses between ∼30 and 10 hPa (their top level). Southern Hemisphere polar stratospheric temperatures in the ECMWF 40-Yr Re-analysis (ERA-40) show unrealistic vertical structure, so this long-term reanalysis is also unsuited for quantitative studies. The NCEP/Climate Prediction Center (CPC) objective analyses give an inferior representation of the upper-stratospheric vortex. Polar vortex transport barriers are similar in all analyses, but there is large variation in the amount, patterns, and timing of mixing, even among the operational assimilated datasets (ECMWF, MetO, and GEOS-4). The higher-resolution GEOS-4 and ECMWF assimilations provide significantly better representation of filamentation and small-scale structure than the other analyses, even when fields gridded at reduced resolution are studied. The choice of which analysis to use is most critical for detailed transport studies (including polar process modeling) and studies involving synoptic evolution in the upper stratosphere. The operational assimilated datasets are better suited for most applications than the NCEP/CPC objective analyses and the reanalysis datasets (REAN/REAN-2 and ERA-40).

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Erik Andersson, Peter Bauer, Anton Beljaars, Frederic Chevallier, Elías Hólm, Marta Janisková, Per Kållberg, Graeme Kelly, Philippe Lopez, Anthony McNally, Emmanuel Moreau, Adrian J. Simmons, Jean-Noël Thépaut, and Adrian M. Tompkins

Several new types of satellite instrument will provide improved measurements of Earth's hydrological cycle and the humidity of the atmosphere. In an effort to make the best possible use of these data, the modeling and assimilation of humidity, clouds, and precipitation are currently the subjects of a comprehensive research program at the European Centre for Medium-Range Weather Forecasts (ECMWF). Impacts on weather prediction and climate reanalysis can be expected. The preparations for cloud and rain assimilation within ECMWF's four-dimensional variational data assimilation system include the development of linearized moist physics, the development of fast radiative transfer codes for cloudy and precipitating conditions, and a reformulation of the humidity analysis scheme.

Results of model validations against in situ moisture data are presented, indicating generally good agreement—often to within the absolute calibration accuracy of the measurements. Evidence is also presented of shortcomings in ECMWF's humidity analysis, from the operational data assimilation and forecasting system in 2002, and from the recently completed ERA-40 reanalysis project. Examples are shown of biases in the data and in the model that lead to biased humidity analyses. Although these biases are relatively small, they contribute to an overprediction of tropical precipitation and to an overly intense Hadley circulation at the start of the forecast, with rapid adjustments taking place during the first 6–12 h. It is shown that with an improved humidity analysis this long-standing “spindown” problem can be reduced.

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Shu-peng Ho, Richard A. Anthes, Chi O. Ao, Sean Healy, Andras Horanyi, Douglas Hunt, Anthony J. Mannucci, Nicholas Pedatella, William J. Randel, Adrian Simmons, Andrea Steiner, Feiqin Xie, Xinan Yue, and Zhen Zeng

Abstract

Launched in 2006, the Formosa Satellite Mission 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) was the first constellation of microsatellites carrying global positioning system (GPS) radio occultation (RO) receivers. Radio occultation is an active remote sensing technique that provides valuable information on the vertical variations of electron density in the ionosphere, and temperature, pressure, and water vapor in the stratosphere and troposphere. COSMIC has demonstrated the great value of RO data in ionosphere, climate, and meteorological research and operational weather forecasting. However, there are still challenges using RO data, particularly in the moist lower troposphere and upper stratosphere. A COSMIC follow-on constellation, COSMIC-2, was launched into equatorial orbit in 2019. With increased signal-to-noise ratio (SNR) from improved receivers and digital beam steering antennas, COSMIC-2 will produce at least 5,000 high-quality RO profiles daily in the tropics and subtropics. In this paper, we summarize 1) recent (since 2011 when the last review was published) contributions of COSMIC and other RO observations to weather, climate, and space weather science; 2) the remaining challenges in RO applications; and 3) potential contributions to research and operations of COSMIC-2.

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Lennart Bengtsson, Phil Arkin, Paul Berrisford, Philippe Bougeault, Chris K. Folland, Chris Gordon, Keith Haines, Kevin I. Hodges, Phil Jones, Per Kallberg, Nick Rayner, Adrian J. Simmons, Detlef Stammer, Peter W. Thorne, Sakari Uppala, and Russell S. Vose
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Paul Poli, Hans Hersbach, Dick P. Dee, Paul Berrisford, Adrian J. Simmons, Frédéric Vitart, Patrick Laloyaux, David G. H. Tan, Carole Peubey, Jean-Noël Thépaut, Yannick Trémolet, Elías V. Hólm, Massimo Bonavita, Lars Isaksen, and Michael Fisher

Abstract

The ECMWF twentieth century reanalysis (ERA-20C; 1900–2010) assimilates surface pressure and marine wind observations. The reanalysis is single-member, and the background errors are spatiotemporally varying, derived from an ensemble. The atmospheric general circulation model uses the same configuration as the control member of the ERA-20CM ensemble, forced by observationally based analyses of sea surface temperature, sea ice cover, atmospheric composition changes, and solar forcing. The resulting climate trend estimations resemble ERA-20CM for temperature and the water cycle. The ERA-20C water cycle features stable precipitation minus evaporation global averages and no spurious jumps or trends. The assimilation of observations adds realism on synoptic time scales as compared to ERA-20CM in regions that are sufficiently well observed. Comparing to nighttime ship observations, ERA-20C air temperatures are 1 K colder. Generally, the synoptic quality of the product and the agreement in terms of climate indices with other products improve with the availability of observations. The MJO mean amplitude in ERA-20C is larger than in 20CR version 2c throughout the century, and in agreement with other reanalyses such as JRA-55. A novelty in ERA-20C is the availability of observation feedback information. As shown, this information can help assess the product’s quality on selected time scales and regions.

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