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Abstract
This study uses information on Saharan aerosol from a dust transport model to calculate radiative forcing values. The transport model is driven by assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System. The model produces global three-dimensional dust spatial information for four different mineral aerosol sizes. These dust fields are input to an offline radiative transfer calculation to obtain the direct radiative forcing due to the dust fields. These estimates of the shortwave reduction of radiation at the top of the atmosphere (TOA) compare reasonably well with the TOA reductions derived from Earth Radiation Budget Experiment (ERBE) and Total Ozone Mapping Spectrometer (TOMS) satellite data. The longwave radiation also agrees with the observations; however, potential errors in the assimilated temperatures complicate the comparison. Depending on the assumptions used in the calculation and the dust loading, the summertime forcing ranges from 0 to −18 W m−2 over ocean and from 0 to +20 W m−2 over land.
Increments are terms in the assimilation general circulation model (GCM) equations that force the model toward observations. They are differences between the observed analyses and the GCM forecasts. Off west Africa the analysis temperature increments produced by the assimilation system show patterns that are consistent with the dust spatial distribution. It is not believed that radiative heating of dust is influencing the increments. Instead, it is suspected that dust is affecting the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) satellite temperature retrievals that provide the basis of the assimilated temperatures used by the model.
Abstract
This study uses information on Saharan aerosol from a dust transport model to calculate radiative forcing values. The transport model is driven by assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System. The model produces global three-dimensional dust spatial information for four different mineral aerosol sizes. These dust fields are input to an offline radiative transfer calculation to obtain the direct radiative forcing due to the dust fields. These estimates of the shortwave reduction of radiation at the top of the atmosphere (TOA) compare reasonably well with the TOA reductions derived from Earth Radiation Budget Experiment (ERBE) and Total Ozone Mapping Spectrometer (TOMS) satellite data. The longwave radiation also agrees with the observations; however, potential errors in the assimilated temperatures complicate the comparison. Depending on the assumptions used in the calculation and the dust loading, the summertime forcing ranges from 0 to −18 W m−2 over ocean and from 0 to +20 W m−2 over land.
Increments are terms in the assimilation general circulation model (GCM) equations that force the model toward observations. They are differences between the observed analyses and the GCM forecasts. Off west Africa the analysis temperature increments produced by the assimilation system show patterns that are consistent with the dust spatial distribution. It is not believed that radiative heating of dust is influencing the increments. Instead, it is suspected that dust is affecting the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) satellite temperature retrievals that provide the basis of the assimilated temperatures used by the model.
Abstract
The 4-day wave often dominates the large-scale wind, temperature, and constituent variability in the high-latitude Southern Hemisphere winter near the stratopause. This study examines the winter Southern Hemisphere vortex of 1998 using 4-times-daily output from a data assimilation system to focus on the polar 2-day, wavenumber-2 component of the 4-day wave. The data assimilation system products are from a test version of the finite volume data assimilation system (fvDAS) being developed at the Goddard Space Flight Center (GSFC) and include an ozone assimilation system. Results show that the polar 2-day wave in temperature and ozone dominates over other planetary-scale disturbances during July 1998 at 70°S. The period of the quasi-2-day wave is somewhat shorter than 2 days (about 1.7 days) during July 1998 with an average perturbation temperature amplitude for the month of over 2.5 K. The 2-day wave propagates more slowly than the zonal mean zonal wind, consistent with Rossby wave theory, and has Eliassen–Palm (EP) flux divergence regions associated with regions of negative horizontal potential vorticity gradients, as expected from linear instability theory. Results for the assimilation-produced ozone mixing ratio show that the 2-day wave represents a major source of ozone variation in this region. The ozone wave in the assimilation system is in good agreement with the wave seen in the Polar Ozone and Aerosol Measurement (POAM) ozone observations for the same time period. Some differences from linear instability theory are noted, as well as spectral peaks in the ozone field, not seen in the temperature field, that may be a consequence of advection.
Abstract
The 4-day wave often dominates the large-scale wind, temperature, and constituent variability in the high-latitude Southern Hemisphere winter near the stratopause. This study examines the winter Southern Hemisphere vortex of 1998 using 4-times-daily output from a data assimilation system to focus on the polar 2-day, wavenumber-2 component of the 4-day wave. The data assimilation system products are from a test version of the finite volume data assimilation system (fvDAS) being developed at the Goddard Space Flight Center (GSFC) and include an ozone assimilation system. Results show that the polar 2-day wave in temperature and ozone dominates over other planetary-scale disturbances during July 1998 at 70°S. The period of the quasi-2-day wave is somewhat shorter than 2 days (about 1.7 days) during July 1998 with an average perturbation temperature amplitude for the month of over 2.5 K. The 2-day wave propagates more slowly than the zonal mean zonal wind, consistent with Rossby wave theory, and has Eliassen–Palm (EP) flux divergence regions associated with regions of negative horizontal potential vorticity gradients, as expected from linear instability theory. Results for the assimilation-produced ozone mixing ratio show that the 2-day wave represents a major source of ozone variation in this region. The ozone wave in the assimilation system is in good agreement with the wave seen in the Polar Ozone and Aerosol Measurement (POAM) ozone observations for the same time period. Some differences from linear instability theory are noted, as well as spectral peaks in the ozone field, not seen in the temperature field, that may be a consequence of advection.
