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Mark D. Zelinka
,
Stephen A. Klein
, and
Dennis L. Hartmann

Abstract

Cloud radiative kernels and histograms of cloud fraction, both as functions of cloud-top pressure and optical depth, are used to quantify cloud amount, altitude, and optical depth feedbacks. The analysis is applied to doubled-CO2 simulations from 11 global climate models in the Cloud Feedback Model Intercomparison Project.

Global, annual, and ensemble mean longwave (LW) and shortwave (SW) cloud feedbacks are positive, with the latter nearly twice as large as the former. The robust increase in cloud-top altitude in both the tropics and extratropics is the dominant contributor to the positive LW cloud feedback. The negative impact of reductions in cloud amount offsets more than half of the positive impact of rising clouds on LW cloud feedback, but the magnitude of compensation varies considerably across the models. In contrast, robust reductions in cloud amount make a large and virtually unopposed positive contribution to SW cloud feedback, though the intermodel spread is greater than for any other individual feedback component. Overall reductions in cloud amount have twice as large an impact on SW fluxes as on LW fluxes, such that the net cloud amount feedback is moderately positive, with no models exhibiting a negative value. As a consequence of large but partially offsetting effects of cloud amount reductions on LW and SW feedbacks, both the mean and intermodel spread in net cloud amount feedback are smaller than those of the net cloud altitude feedback. Finally, the study finds that the large negative cloud feedback at high latitudes results from robust increases in cloud optical depth, not from increases in total cloud amount as is commonly assumed.

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Carlos R. Mechoso
,
Dennis L. Hartmann
, and
John D. Farrara

Abstract

The climatology and interannual variability of wave–mean flow interaction in the Southern Hemisphere (20–80°S, 0–55 km) is described for the winter months of June–September based on a sample of four years, 1979–82. The stratospheric jet stream shifts downward and poleward over the course of the winter in response to seasonal variations in thermal forcing. The shift occurs at different times in different years, however, so that the months of July and August show substantial interannual variability of monthly mean zonal winds. The poleward and downward shift of the jet axis in an individual year is usually abrupt and occurs in association with a burst of upwardly propagating planetary waves. The driving of the mean flow in the stratosphere generally has a dipolar structure with easterly accelerations near 40°S and westerly accelerations in polar latitudes. The structure of the wave driving is consistent with the structure of the observed mean flow accelerations.

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D. L. Hartmann
,
R. Buizza
, and
T. N. Palmer

Abstract

The scale dependence of rapidly growing perturbations is investigated by studying the dominant singular vectors of T21 and T42 versions of the ECMWF model, which show the most linear energy growth in a 3-day period. A spectral filter is applied to the optimization process to determine which spatial scales are most effective in promoting energy growth. When the initial perturbation is confined to the top half of the total spherical harmonic wavenumber spectrum (high wavenumber end), the growth rates and final structures of the disturbances are changed very little from the case in which all wavenumbers are included. These results indicate that synoptic waves that become fully developed in a period of three days can arise from initial perturbations that are entirely contained at subsynoptic scales. Rapid growth is associated with initial perturbations that consist of smaller spatial scales concentrated near the effective steering level. The linear evolution of these initial perturbations in a highly complex basic flow leads to disturbances of synoptic scale that extend through most of the depth of the troposphere. Growth rates are approximately doubled when the model resolution is increased from T21 to T42, which is consistent with greater growth being associated with smaller spatial scales. When the initial perturbation is confined to the lower half of the total wavenumber spectrum, which describes the larger horizontal scales, the growth rates are significantly reduced and the initial and final structures are very different from the case in which all wavenumbers are included. These low wavenumber perturbations tend to be more barotropic in structure and in growth characteristics. As expected from their linear growth rates, when the low-wavenumber perturbations are inserted in the T63 forecast model, they grow more slowly and result in less forecast dispersion than the high wavenumber perturbations.

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D. L. Hartmann
,
T. N. Palmer
, and
R. Buizza

Abstract

The linear structures that produce the most in situ energy growth in the lower stratosphere for realistic wintertime flows are investigated using T21 and T42 calculations with the ECMWF 19-level forecast model. Significant growth is found for relatively large scale structures that grow by propagating from the outer edges of the vortex into the strong jet features of the lower-stratospheric flow. The growth is greater when the polar vortex is more asymmetric and contains localized jet structures. If the linear structures are properly phased, they can induce strong nonlinear interactions with the polar vortex, both for Northern Hemisphere and Southern Hemisphere flow conditions, even when the initial amplitudes are small. Large extensions from the main polar vortex that are peeled off during wave-breaking events give rise to a separate class of rapidly growing disturbances that may hasten the mixing of these vortex extensions.

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Dennis L. Hartmann
,
Brittany D. Dygert
,
Peter N. Blossey
,
Qiang Fu
, and
Adam B. Sokol

Abstract

The vertical profile of clear-sky radiative cooling places important constraints on the vertical structure of convection and associated clouds. Simple theory using the cooling-to-space approximation is presented to indicate that the cooling rate in the upper troposphere should increase with surface temperature. The theory predicts how the cooling rate depends on lapse rate in an atmosphere where relative humidity remains approximately a fixed function of temperature. Radiative cooling rate is insensitive to relative humidity because of cancellation between the emission and transmission of radiation by water vapor. This theory is tested with one-dimensional radiative transfer calculations and radiative–convective equilibrium simulations. For climate simulations that produce an approximately moist adiabatic lapse rate, the radiative cooling profile becomes increasingly top-heavy with increasing surface temperature. If the temperature profile warms more slowly than a moist adiabatic profile in midtroposphere, then the cooling rate in the upper troposphere is reduced and that in the lower troposphere is increased. This has important implications for convection, clouds, and associated deep and shallow circulations.

Open access
W. G. Read
,
J. W. Waters
,
D. A. Flower
,
L. Froidevaux
,
R. F. Jarnot
,
D. L. Hartmann
,
R. S. Harwood
, and
R. B. Rood

Initial results of upper-tropospheric water vapor obtained from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are presented. MLS is less affected by clouds than infrared or visible techniques, and the UARS orbit provides daily humidity monitoring for approximately two-thirds of the earth. Best results are currently obtained when water vapor abundances are approximately 100–300 ppmv, corresponding to approximately 12-km height in the Tropics and 7 km at high latitudes. The observed latitude variation of water vapor at 215 hPa is in good agreement with the U.K. Universities's Global Atmospheric Modelling Project model. The ability to observe synoptic-scale features associated with tropopause height variations is clearly illustrated by comparison with the National Aeronautics and Space Administration Goddard Space Flight Center assimilation model. Humidity detrainment streams extending from tropical convective regions are also observed.

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M. Wendisch
,
M. Brückner
,
S. Crewell
,
A. Ehrlich
,
J. Notholt
,
C. Lüpkes
,
A. Macke
,
J. P. Burrows
,
A. Rinke
,
J. Quaas
,
M. Maturilli
,
V. Schemann
,
M. D. Shupe
,
E. F. Akansu
,
C. Barrientos-Velasco
,
K. Bärfuss
,
A.-M. Blechschmidt
,
K. Block
,
I. Bougoudis
,
H. Bozem
,
C. Böckmann
,
A. Bracher
,
H. Bresson
,
L. Bretschneider
,
M. Buschmann
,
D. G. Chechin
,
J. Chylik
,
S. Dahlke
,
H. Deneke
,
K. Dethloff
,
T. Donth
,
W. Dorn
,
R. Dupuy
,
K. Ebell
,
U. Egerer
,
R. Engelmann
,
O. Eppers
,
R. Gerdes
,
R. Gierens
,
I. V. Gorodetskaya
,
M. Gottschalk
,
H. Griesche
,
V. M. Gryanik
,
D. Handorf
,
B. Harm-Altstädter
,
J. Hartmann
,
M. Hartmann
,
B. Heinold
,
A. Herber
,
H. Herrmann
,
G. Heygster
,
I. Höschel
,
Z. Hofmann
,
J. Hölemann
,
A. Hünerbein
,
S. Jafariserajehlou
,
E. Jäkel
,
C. Jacobi
,
M. Janout
,
F. Jansen
,
O. Jourdan
,
Z. Jurányi
,
H. Kalesse-Los
,
T. Kanzow
,
R. Käthner
,
L. L. Kliesch
,
M. Klingebiel
,
E. M. Knudsen
,
T. Kovács
,
W. Körtke
,
D. Krampe
,
J. Kretzschmar
,
D. Kreyling
,
B. Kulla
,
D. Kunkel
,
A. Lampert
,
M. Lauer
,
L. Lelli
,
A. von Lerber
,
O. Linke
,
U. Löhnert
,
M. Lonardi
,
S. N. Losa
,
M. Losch
,
M. Maahn
,
M. Mech
,
L. Mei
,
S. Mertes
,
E. Metzner
,
D. Mewes
,
J. Michaelis
,
G. Mioche
,
M. Moser
,
K. Nakoudi
,
R. Neggers
,
R. Neuber
,
T. Nomokonova
,
J. Oelker
,
I. Papakonstantinou-Presvelou
,
F. Pätzold
,
V. Pefanis
,
C. Pohl
,
M. van Pinxteren
,
A. Radovan
,
M. Rhein
,
M. Rex
,
A. Richter
,
N. Risse
,
C. Ritter
,
P. Rostosky
,
V. V. Rozanov
,
E. Ruiz Donoso
,
P. Saavedra Garfias
,
M. Salzmann
,
J. Schacht
,
M. Schäfer
,
J. Schneider
,
N. Schnierstein
,
P. Seifert
,
S. Seo
,
H. Siebert
,
M. A. Soppa
,
G. Spreen
,
I. S. Stachlewska
,
J. Stapf
,
F. Stratmann
,
I. Tegen
,
C. Viceto
,
C. Voigt
,
M. Vountas
,
A. Walbröl
,
M. Walter
,
B. Wehner
,
H. Wex
,
S. Willmes
,
M. Zanatta
, and
S. Zeppenfeld

Abstract

Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the (AC)3 project was established in 2016 (www.ac3-tr.de/). It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, shipborne, and satellite data of physical, chemical, and meteorological properties of the Arctic atmosphere, cryosphere, and upper ocean that are available for the Arctic climate research community. Short-term changes and indications of long-term trends in Arctic climate parameters have been detected using existing and new data. For example, a distinct atmospheric moistening, an increase of regional storm activities, an amplified winter warming in the Svalbard and North Pole regions, and a decrease of sea ice thickness in the Fram Strait and of snow depth on sea ice have been identified. A positive trend of tropospheric bromine monoxide (BrO) column densities during polar spring was verified. Local marine/biogenic sources for cloud condensation nuclei and ice nucleating particles were found. Atmospheric–ocean and radiative transfer models were advanced by applying new parameterizations of surface albedo, cloud droplet activation, convective plumes and related processes over leads, and turbulent transfer coefficients for stable surface layers. Four modes of the surface radiative energy budget were explored and reproduced by simulations. To advance the future synthesis of the results, cross-cutting activities are being developed aiming to answer key questions in four focus areas: lapse rate feedback, surface processes, Arctic mixed-phase clouds, and airmass transport and transformation.

Open access