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W. D. Collins
and
A. K. Inamdar

Abstract

The existence and magnitude of a systematic bias in the clear-sky longwave fluxes from the Earth Radiation Budget Experiment (ERBE) is investigated. The bias is apparently introduced because the ERBE method for scene identification does not account for large zonal gradients in longwave absorption by water vapor. The ERBE fluxes are compared to fluxes calculated with a radiative transfer model from ship radiosonde measurements. The comparison is based upon an analysis of 5 yr of coincident satellite and radiosonde observations for equatorial ocean regions. The differences between the ERBE and model fluxes are examined as functions of sea surface temperature (SST) and relative humidity. The authors use height-mean relative humidity R̄H̄ as an index of atmospheric moisture. The average offset between the model and ERBE fluxes ranges between +2 and +6 W m−2 for SSTs above 295 K, and the gradients with respect to SST are nearly identical. However, the difference between the model and ERBE depends significantly on the tropospheric relative humidity. ERBE fluxes exceed model fluxes for R̄Hmacr; above 70%, and the maximum offset of +9 to +12 W m−2 is consistent with previous estimates. There are also indications that the clear-sky fluxes for R̄Hmacr; below 25% may be underestimated by about 10–15 W m−2. Since extreme values of height-mean humidity are relatively infrequent, the net bias introduced in the ERBE monthly mean clear-sky fluxes is generally less than the systematic error in estimates of the instantaneous fluxes. These findings support earlier work on the coupling between, SST and the atmospheric greenhouse effect, in particular the existence of a super greenhouse effect for oceans warmer than 300 K. Recent reports of much larger systematic differences are not supported by this analysis. The results indicate that comparison of GCM and ERBE clear-sky longwave fluxes will depend explicitly on atmospheric humidity.

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Sandrine Bony
,
William D. Collins
, and
David W. Fillmore

Abstract

While low-level clouds over the Pacific and Atlantic Oceans have been investigated extensively, low clouds over the Indian Ocean are not as well characterized. This study examines the occurrence of nonoverlapped low clouds over the Indian Ocean during the northeast monsoon using several sources of data. Climatologies derived from surface observations and from the International Satellite Cloud Climatology Project are reviewed. Another cloud climatology is developed using infrared and visible imagery from the Indian geostationary satellite. The new climatology has better spatial and temporal resolution than in situ observations. The three datasets are generally consistent and show several persistent features in the cloud distribution. During January–April, maxima in the occurrence of low clouds occur at subtropical latitudes over the Arabian Sea, the Bay of Bengal, the China Sea, and the southern Indian Ocean. The predominant types of low clouds differ in the northern and southern areas of the Indian Ocean region and China Sea. The Arabian Sea and the Bay of Bengal are covered mostly by cumulus clouds, while the southern Indian Ocean and the China Sea are covered mostly by large-scale stratiform clouds such as stratocumulus. These observations are consistent with atmospheric analyses of temperature, humidity, and stability over the Indian Ocean.

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Xiaoqing Wu
,
William D. Hall
,
Wojciech W. Grabowski
,
Mitchell W. Moncrieff
,
William D. Collins
, and
Jeffrey T. Kiehl

Abstract

A two-dimensional cloud-resolving model with a large domain is integrated for 39 days during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) to study the effects of ice phase processes on cloud properties and cloud radiative properties. The ice microphysical parameterization scheme is modified based on microphysical measurements from the Central Equatorial Pacific Experiment. A nonlocal boundary layer diffusion scheme is included to improve the simulation of the surface heat fluxes. The modified ice scheme produces fewer ice clouds during the 39-day simulation. The cloud radiative properties show significant improvement and compare well with various observations. Both the 39-day mean value (202 W m−2) and month-long evolution of outgoing longwave radiative flux from the model are comparable with satellite observations. The 39-day mean surface shortwave cloud forcing is −110 W m−2, consistent with other estimates obtained for TOGA COARE. The 39-day mean values of surface net longwave, shortwave, latent, and sensible fluxes are −46.2, 182.9, −109.9, and −7.8 W m −2, respectively, in line with the IMET buoy data (−54.6, 178.2, −102.7, and −10.6 W m−2).

The offline radiation calculations as well as the cloud-interactive radiation simulations demonstrate that a doubled effective radius of ice particles and enhanced shortwave cloud absorption strongly affect the radiative flux and cloud radiative forcing but have little impact on the cloud properties. The modeled albedo is sensitive to the effective radius of ice particles and/or the shortwave cloud absorption in the radiation scheme. More complete satellite observations and theoretical studies are required to fully understand the physical processes involved.

The results suggest that the ice microphysical parameterization plays an important role in the long-term simulation of cloud properties and cloud radiative properties. Future field observations should put more weight on the microphysical properties, cloud properties, and high-quality radiative properties in order to further improve the cloud-resolving modeling of cloud systems and the understanding of cloud–radiation interaction.

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Andrew Gettelman
,
William D. Collins
,
Eric J. Fetzer
,
Annmarie Eldering
,
Fredrick W. Irion
,
Phillip B. Duffy
, and
Govindasamy Bala

Abstract

Recently available satellite observations from the Atmospheric Infrared Sounder (AIRS) are used to calculate relative humidity in the troposphere. The observations illustrate many scales of variability in the atmosphere from the seasonal overturning Hadley–Walker circulation to high-frequency transient variability associated with baroclinic storms with high vertical resolution. The Asian monsoon circulation has a strong impact on upper-tropospheric humidity, with large humidity gradients to the west of the monsoon. The vertical structure of humidity is generally bimodal, with high humidity in the upper and lower troposphere, and a dry middle troposphere. The highest variances in humidity are seen around the midlatitude tropopause. AIRS data are compared to a simulation from a state-of-the-art climate model. The model does a good job of reproducing the mean humidity distribution but is slightly moister than the observations in the middle and upper troposphere. The model has difficultly reproducing many scales of observed variability, particularly in the Tropics. Differences in humidity imply global differences in the top of atmosphere fluxes of ∼1 W m−2.

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Masaru Yoshioka
,
Natalie M. Mahowald
,
Andrew J. Conley
,
William D. Collins
,
David W. Fillmore
,
Charles S. Zender
, and
Dani B. Coleman

Abstract

The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

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P. W. Webley
,
D. Atkinson
,
R. L. Collins
,
K. Dean
,
J. Fochesatto
,
K. Sassen
,
C. F. Cahill
,
A. Prata
,
C. J. Flynn
, and
K. Mizutani
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D.-Z. Sun
,
T. Zhang
,
C. Covey
,
S. A. Klein
,
W. D. Collins
,
J. J. Hack
,
J. T. Kiehl
,
G. A. Meehl
,
I. M. Held
, and
M. Suarez

Abstract

The equatorial Pacific is a region with strong negative feedbacks. Yet coupled general circulation models (GCMs) have exhibited a propensity to develop a significant SST bias in that region, suggesting an unrealistic sensitivity in the coupled models to small energy flux errors that inevitably occur in the individual model components. Could this “hypersensitivity” exhibited in a coupled model be due to an underestimate of the strength of the negative feedbacks in this region? With this suspicion, the feedbacks in the equatorial Pacific in nine atmospheric GCMs (AGCMs) have been quantified using the interannual variations in that region and compared with the corresponding calculations from the observations. The nine AGCMs are the NCAR Community Climate Model version 1 (CAM1), the NCAR Community Climate Model version 2 (CAM2), the NCAR Community Climate Model version 3 (CAM3), the NCAR CAM3 at T85 resolution, the NASA Seasonal-to-Interannual Prediction Project (NSIPP) Atmospheric Model, the Hadley Centre Atmospheric Model (HadAM3), the Institut Pierre Simon Laplace (IPSL) model (LMDZ4), the Geophysical Fluid Dynamics Laboratory (GFDL) AM2p10, and the GFDL AM2p12. All the corresponding coupled runs of these nine AGCMs have an excessive cold tongue in the equatorial Pacific.

The net atmospheric feedback over the equatorial Pacific in the two GFDL models is found to be comparable to the observed value. All other models are found to have a weaker negative net feedback from the atmosphere—a weaker regulating effect on the underlying SST than the real atmosphere. Except for the French (IPSL) model, a weaker negative feedback from the cloud albedo and a weaker negative feedback from the atmospheric transport are the two leading contributors to the weaker regulating effect from the atmosphere. The underestimate of the strength of the negative feedbacks by the models is apparently linked to an underestimate of the equatorial precipitation response. All models have a stronger water vapor feedback than that indicated in Earth Radiation Budget Experiment (ERBE) observations. These results confirm the suspicion that an underestimate of the regulatory effect from the atmosphere over the equatorial Pacific region is a prevalent problem. The results also suggest, however, that a weaker regulatory effect from the atmosphere is unlikely solely responsible for the hypersensitivity in all models. The need to validate the feedbacks from the ocean transport is therefore highlighted.

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James W. Hurrell
,
M. M. Holland
,
P. R. Gent
,
S. Ghan
,
Jennifer E. Kay
,
P. J. Kushner
,
J.-F. Lamarque
,
W. G. Large
,
D. Lawrence
,
K. Lindsay
,
W. H. Lipscomb
,
M. C. Long
,
N. Mahowald
,
D. R. Marsh
,
R. B. Neale
,
P. Rasch
,
S. Vavrus
,
M. Vertenstein
,
D. Bader
,
W. D. Collins
,
J. J. Hack
,
J. Kiehl
, and
S. Marshall

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad community of scientists. Additionally, the model source code and associated documentation are freely available to the scientific community to use for Earth system studies, making it a true community tool. This article describes this Earth system model and its various possible configurations, and highlights a number of its scientific capabilities.

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E. Kalnay
,
M. Kanamitsu
,
R. Kistler
,
W. Collins
,
D. Deaven
,
L. Gandin
,
M. Iredell
,
S. Saha
,
G. White
,
J. Woollen
,
Y. Zhu
,
M. Chelliah
,
W. Ebisuzaki
,
W. Higgins
,
J. Janowiak
,
K. C. Mo
,
C. Ropelewski
,
J. Wang
,
A. Leetmaa
,
R. Reynolds
,
Roy Jenne
, and
Dennis Joseph

The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system.

The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.

The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.

NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.

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C. O. Collins III
,
B. Blomquist
,
O. Persson
,
B. Lund
,
W. E. Rogers
,
J. Thomson
,
D. Wang
,
M. Smith
,
M. Doble
,
P. Wadhams
,
A. Kohout
,
C. Fairall
, and
H. C. Graber

Abstract

“Sea State and Boundary Layer Physics of the Emerging Arctic Ocean” is an ongoing Departmental Research Initiative sponsored by the Office of Naval Research (http://www.apl.washington.edu/project/project.php?id=arctic_sea_state). The field component took place in the fall of 2015 within the Beaufort and Chukchi Seas and involved the deployment of a number of wave instruments, including a downward-looking Riegl laser rangefinder mounted on the foremast of the R/V Sikuliaq. Although time series measurements on a stationary vessel are thought to be accurate, an underway vessel introduces a Doppler shift to the observed wave spectrum. This Doppler shift is a function of the wavenumber vector and the velocity vector of the vessel. Of all the possible relative angles between wave direction and vessel heading, there are two main scenarios: 1) vessel steaming into waves and 2) vessel steaming with waves. Previous studies have considered only a subset of cases, and all were in scenario 1. This was likely to avoid ambiguities, which arise when the vessel is steaming with waves. This study addresses the ambiguities and analyzes arbitrary cases. In addition, a practical method is provided that is useful in situations when the vessel is changing speed or heading. These methods improved the laser rangefinder estimates of spectral shapes and peak parameters when compared to nearby buoys and a spectral wave model.

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