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M. Christoph
,
U. Ulbrich
, and
U. Haak

Abstract

Murakami's recursive filter technique is suitable for computing storm tracks with reduced needs in data length, and it decreases computing time by the factor 3.5. It is shown that the storm tracks differ only slightly from the ones obtained using the conventional Blackmon filtering approach. A problem identified with respect to the exact frequency of the 1.0 response of Murakami's filter appears to be of minor importance.

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M. Christoph
,
T. P. Barnett
, and
E. Roeckner

Abstract

A phenomenon called the Antarctic Circumpolar Wave (ACW), suggested earlier from fragmentary observational evidence, has been simulated realistically in an extended integration of a Max Planck Institute coupled general circulation model. The ACW both in the observations and in the model constitutes a mode of the coupled ocean–atmosphere–sea-ice system that inhabits the high latitudes of the Southern Hemisphere. It is characterized by anomalies of such climate variables as sea surface temperature, sea level pressure, meridional wind, and sea ice that exhibit intricate and evolving spatial phase relations to each other.

The simulated ACW signal in the ocean propagates eastward over most of the high-latitude Southern Ocean, mainly advected along in the Antarctic Circumpolar Current. On average, it completes a circuit entirely around the Southern Ocean but is strongly dissipated in the South Atlantic and in the southern Indian Ocean, just marginally maintaining statistical significance in these areas until it reaches the South Pacific where it is reenergized. In extreme cases, the complete circumpolar propagation is more clear, requiring about 12–16 yr to complete the circuit. This, coupled with the dominant zonal wavenumber 3 pattern of the ACW, results in the local reappearance of energy peaks about every 4–5 yr.

The oceanic component of the mode is forced by the atmosphere via fluxes of heat. The overlying atmosphere establishes patterns of sea level pressure that mainly consist of a standing wave and are associated with the Pacific–South American (PSA) oscillation described in earlier works. The PSA, like its counterpart in the North Pacific, appears to be a natural mode of the high southern latitudes. There is some ENSO-related signal in the ACW forced by anomalous latent heat release associated with precipitation anomalies in the central and western tropical Pacific. However, ENSO-related forcing explains at most 30% of the ACW variance and, generally, much less.

It is hypothesized that the ACW as an entity represents the net result of moving oceanic climate anomalies interacting with a spatially fixed atmospheric forcing pattern. As the SST moves into and out of phase with the resonant background pattern it is selectively amplified or dissipated, an idea supported by several independent analyses. A simplified ocean heat budget model seems to also support this idea.

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M. Venkat Ratnam
,
G. Tetzlaff
, and
Christoph Jacobi

Abstract

Global analyses of gravity wave (GW) activity in the stratosphere are presented using radio occultation data from the Challenging Minisatellite Payload (CHAMP) satellite. Temperature profiles obtained from CHAMP/ GPS radio occultations are first compared with ground-based instruments. In general, good agreement is found between these different techniques. Monthly mean values of potential energy E p , being a measure of GW activity, which is estimated with radiosonde observations, are compared with CHAMP/GPS data and it is found that radiosonde-observed E p values are slightly higher than those estimated with radio occultations. Strong diurnal variation of GW activity has been found. From the global morphology of GW activity, large E p values are noticed, besides at tropical latitudes, even at midlatitudes during winter, but not during equinoxes. This suggests that wave activity at stratospheric heights is not only modulated due to orography (mountain/lee waves) but mainly depends on seasonal variations at the respective latitudes. Significant correlations are found between GW activity and the outgoing longwave radiation (OLR) observations, OLR being a proxy for tropical deep convection. Gravity wave activity is found to be high in the zones of deep convection confirming that convection is the major source of GW generation in the Tropics. Latitudinal and vertical variations of GW activity reveal the existence of large E p values below 25 km and low values between 25 and 30 km in all the seasons near the equator. During the Southern Hemisphere winter, large values are noticed. Large values are also found during equinoxes, and these values are nearly the same in Northern and Southern Hemispheres (NH and SH, respectively) at midlatitudes. During solstices, the E p distribution involves a larger hemispheric asymmetry at middle and higher latitudes. The latitudinal range is wide (±30° latitude in both hemispheres) with large E p values in all seasons. Large values of E p are noticed during the major stratospheric sudden warming that occurred over Antarctica during September 2002.

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M. Christoph
,
U. Ulbrich
, and
P. Speth

Abstract

The seasonal cycle of the North Pacific and the North Atlantic storm track activity is investigated on the basis of daily National Meteorological Center (now known as NCEP) upper-air analyses (1946–89) and of data from the ECHAM3 T42 atmospheric general circulation model.

Emphasis is put on the midwinter suppression of the Pacific storm track. This feature of seasonal variability is not sensitive to a particular definition of midlatitude synoptic wave activity, as is shown by applying a common definition of area mean storm track intensity.

The suppression is reproduced by the atmospheric model with very similar characteristics. It is attributed to a negative correlation between the storm track intensity and the speed of the subtropical jet at 250 hPa for average zonal winds exceeding the threshold of approximately 45 m s−1, contrasting with a positive correlation below this value. The lack of an analogous behavior over the Atlantic may be assigned to the lower wind speeds there. In a 3·CO2 time-slice experiment with the ECHAM3 model, very intense jet streams occur more often in winter and the suppression becomes more pronounced. At the same time, the level of climatological storm track activity over the Pacific during winter is higher than in the control run. This is explained by the fact that the time-slice experiment produces statistically higher levels of activity for every given jet intensity.

The suppression is dominated by a decrease in synoptic-scale wave activity. Two possible reasons for this decrease were investigated but had to be rejected: there is neither a seasonal shift in the energy spectrum to frequencies that are outside the range sampled by the typical bandpass filter, nor evidence that the suppression is attributed to Pacific blocking activity occurring preferably during midwinter.

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M. Christoph
,
U. Ulbrich
,
J. M. Oberhuber
, and
E. Roeckner

Abstract

Variability at all timescales, including low-frequency variability, is found in the North Atlantic sector in a 300-yr control integration of the coupled ocean–atmosphere general circulation model (CGCM) ECHAM4/OPYC3. The atmospheric variability is dominated by the North Atlantic Oscillation (NAO). Only rather weak spectral peaks are superimposed on the “white noise” power spectrum of sea level pressure and on the essentially “red noise” spectrum of SST in highly active regions. Replacing the full ocean model with a 50-m fixed-depth mixed layer ocean (MLO) and coupling it to the atmosphere yields qualitatively and quantitatively very similar power spectra of the NAO index.

Net surface heat fluxes, describing the coupling between the ocean and the atmosphere for the long-term variations (>10 yr) of the NAO are much weaker in the MLO model, but show general agreement in both simulations regarding spatial distributions. This spatial agreement with respect to NAO variability occurs even though the associated SST anomaly pattern in the CGCM is shifted northward by about 10° relative to its position in the run without the dynamical ocean. This fact is mainly attributed to advection in the full ocean model.

There is evidence for the existence of ocean–cryosphere–atmosphere coupling in the CGCM. From the fact that we found only weak spectral peaks it appears that the role of a fully coupled ocean with respect to long-term NAO variability is limited to a shift in SST variability and to a moderate increase of the atmosphere’s long-term variability over most part of the domain. In view of the subordinate relevance of ocean–atmosphere coupling for the NAO it is suggested that the CGCM presented in this study mainly follows the stochastic climate model concept, that is, the ocean integrates over the chaotic forcing imposed by the atmosphere, leaving the NAO rather unpredictable on decadal and longer timescales.

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Christoph Erath
,
Peter H. Lauritzen
, and
Henry M. Tufo

Abstract

It is the purpose of this short article to analyze mass conservation in high-order rigorous remapping schemes, which contrary to flux-based methods, relies on elaborate integral constraints over overlap areas and reconstruction functions. For applications on the sphere these integral constraints may be violated primarily as a result of inexact or ill-conditioned integration and the authors propose a generic, local, and multitracer efficient method that guarantees that the integral constraints are satisfied in discrete space irrespective of the accuracy of the numerical integration method and slight inaccuracies in the computation of overlap areas. The authors refer to this method as enforcement of consistency as it is based on integral constraints valid in continuous space. The consistency enforcement method is illustrated in idealized transport tests with the Conservative Semi-Lagrangian Multitracer scheme (CSLAM) in the High Order Method Modeling Environment (HOMME) where the analytic integrals, which were found to be ill conditioned at certain resolutions and flow conditions, have been replaced with robust quadrature. This violates mass conservation; however, with the consistency enforcement method, mass conservation is inherent even with low-order quadrature and renders rigorous remap schemes such as CSLAM (which was previously limited to gnomonic cubed-sphere grids) mass conservative on any spherical grid.

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Pedro M. Sousa
,
Alexandre M. Ramos
,
Christoph C. Raible
,
M. Messmer
,
Ricardo Tomé
,
Joaquim G. Pinto
, and
Ricardo M. Trigo

Abstract

Moisture transport over the northeastern Atlantic Ocean is an important process governing precipitation distribution and variability over western Europe. To assess its long-term variability, the vertically integrated horizontal water vapor transport (IVT) from a long-term climate simulation spanning the period 850–2100 CE was used. Results show a steady increase in moisture transport toward western Europe since the late-nineteenth century that is projected to expand during the twenty-first century under the RCP8.5 scenario. The projected IVT for 2070–99 significantly exceeds the range given by interannual–interdecadal variability of the last millennium. Changes in IVT are in line with significant increases in tropospheric moisture content, driven by the concurrent rise in surface temperatures associated with the anthropogenic climate trend. On regional scales, recent and projected precipitation changes over the British Isles follow the global positive IVT trend, whereas a robust precipitation decrease over Iberia is identified in the twenty-first century, particularly during autumn. This indicates a possible extension of stable and dry summer conditions and a decoupling between moisture availability and dynamical forcing. The investigation of circulation features reveals a mean poleward shift of moisture corridors and associated atmospheric rivers. In particular, in Iberia, a significant increase in the frequency of dry weather types is observed, accompanied by a decrease in the frequency of wet types. An opposite response is observed over the British Isles. These changes imply a stronger meridional north–south dipole in terms of pressure and precipitation distributions, enhancing the transport toward central Europe rather than to Iberia.

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Stephen A. Cohn
,
Shane D. Mayor
,
Christian J. Grund
,
Tammy M. Weckwerth
, and
Christoph Senff

The authors describe and present early results from the July–August 1996 Lidars in Flat Terrain (LIFT) experiment. LIFT was a boundary layer experiment that made use of recently developed Doppler, aerosol backscatter, and ozone lidars, along with radars and surface instrumentation, to study the structure and evolution of the convective boundary layer over the very flat terrain of central Illinois. Scientific goals include measurement of fluxes of heat, moisture, and momentum; vertical velocity statistics; study of entrainment and boundary layer height; and observation of organized coherent structures. The data collected will also be used to evaluate the performance of these new lidars and compare measurements of velocity and boundary layer height to those obtained from nearby radar wind profilers. LIFT was a companion to the Flatland96 experiment, described by Angevine et al.

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Elias M. Zubler
,
Ulrike Lohmann
,
Daniel Lüthi
,
Christoph Schär
, and
Andreas Muhlbauer

Abstract

Increasing the aerosol number in warm-phase clouds is thought to decrease the rain formation rate, whereas the physical processes taking place in mixed-phase clouds are more uncertain. Increasing number concentrations of soluble aerosols may reduce the riming efficiency and therefore also decrease precipitation. On the other hand, the glaciation of a cloud by heterogeneous freezing of cloud droplets may enhance the formation of graupel and snow. Using a numerical weather prediction model with coupled aerosol microphysics, it is found, in a statistical framework with 270 clean and polluted 2D simulations of mixed-phase precipitation over an Alpine transect, that the presence of the ice phase determines the magnitude and the sign of the effect of an increasing aerosol number concentration on orographic precipitation. Immersion/condensation freezing is the only ice-nucleating process considered here. It is shown that this indirect aerosol effect is much less pronounced in cold simulations compared to a warmer subset and that cloud glaciation tends to compensate the loss of rain in polluted situations. Comparing the clean and polluted cases, a reduction of rain by 52%, on average (std dev = 25%), over the transect in the polluted cases is found. For frozen precipitation a much broader range of differences is found (mean = +4%, std dev = 60%). Furthermore, this study shows that in comparison with the clean cases more precipitation spills over to the leeward side of the major ridge in the polluted cases (median = +14.6%).

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Emanuel Dutra
,
Gianpaolo Balsamo
,
Pedro Viterbo
,
Pedro M. A. Miranda
,
Anton Beljaars
,
Christoph Schär
, and
Kelly Elder

Abstract

A new snow scheme for the European Centre for Medium-Range Weather Forecasts (ECMWF) land surface model has been tested and validated. The scheme includes a new parameterization of snow density, incorporating a liquid water reservoir, and revised formulations for the subgrid snow cover fraction and snow albedo. Offline validation (covering a wide range of spatial and temporal scales) includes simulations for several observation sites from the Snow Models Intercomparison Project-2 (SnowMIP2) and global simulations driven by the meteorological forcing from the Global Soil Wetness Project-2 (GSWP2) and by ECMWF Re-Analysis ERA-Interim. The new scheme reduces the end of season ablation biases from 10 to 2 days in open areas and from 21 to 13 days in forest areas. Global GSWP2 results are compared against basin-scale runoff and terrestrial water storage. The new snow density parameterization increases the snow thermal insulation, reducing soil freezing and leading to an improved hydrological cycle. Simulated snow cover fraction is compared against NOAA/National Environmental Satellite, Data, and Information Service (NESDIS) with a reduction of the negative bias of snow-covered area of the original snow scheme. The original snow scheme had a systematic negative bias in surface albedo when compared against Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data. The new scheme reduces the albedo bias, consequently reducing the spatial- and time-averaged surface net shortwave radiation bias by 5.2 W m−2 in 14% of the Northern Hemisphere land. The new snow scheme described in this paper was introduced in the ECMWF operational forecast system in September 2009 (cycle 35R3).

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