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A new monthly volcanic forcing dataset is included in a coupled GCM for a more physically consistent treatment of the stratospheric sulfate aerosol history from explosive volcanism. The volcanic forcing is different from previous versions in that there is an individual evolution of the aerosol for each event. Thus the seasonal and latitudinal dependence of the volcanic aerosol can affect global climate in a more realistic way prior to the satellite period, compared to earlier volcanic forcing datasets. Negative radiative forcing from volcanic activity is greatest in the early 20th century prior to 1915 and in the late 20th century after 1960. The combination of volcanic and solar forcing contributes to an early-20th century warming, followed by relative cooling in late 20th century. Consequently, the addition of natural forcing factors to the anthropogenic GHG forcing in late 20th century is required to simulate the observed late 20th century warming.

Using two bias reduced forcing data sets we have simulated the initial soil moisture state for the North American continent (1985–1993). Differences between simulations were shown to persist over regions with the greatest soil memory, and are not strongly associated to patterns of difference in forcing fields. Therefore, processes that contribute to soil memory will also limit our ability to accurately estimate its initial state. Moreover, the regions associated with high soil memory may not be directly observable with present and near future sensors due to high vegetation water contents. Thus, research in these regions should be directed toward minimizing forcing variance and extension of observation networks and technology to increase the ability for monitoring soil moisture.

The AURA-MLS daily mean temperatures and zonal wind from NASA-MERRA reanalysis for latitudes between 60° N and 80° N are used to investigate the planetary wave (PW) characteristics in the stratosphere and lower mesosphere during sudden stratospheric warming (SSW) (November 2008 to March 2009). Here, we used a novel method called empirical mode decomposition (EMD) to extract the PWs from the temperature data. The EMD is an interesting approach to decompose signals into locally periodic components, the intrinsic mode functions (IMFs), and will easily identify the embedded structures, even those with small amplitudes. The spectral analysis reveals prevailing planetary wave periods of ~6-day, ~8-day, ~15-day, and ~21–23-day in IMFs 1, 2, 3, and 4, respectively. Clear upward propagation of these waves (20–30 days) is observed, suggesting that sources for these oscillations are in the troposphere.

To improve our understanding of the absorption of solar radiation in the atmosphere we have characterized the spectral absorption, the spatial and temporal abundance, and the radiative forcing of the oxygen collision pairs O2·O2 and O2·N2 (O2·X ≡ O2·O2 + O2·N2). The regional, vertical, seasonal, and annual patterns of O2·X abundance and radiative forcing are obtained from a general circulation model. We estimate the mean absorption by O2·X, heretofore neglected in large scale atmospheric models, is 0.75–1.2 W m−2, or 1–2% of total atmospheric solar absorption. O2·X absorption reduces surface insolation by 0.48–0.78 W m−2 and increases the net radiative flux at the tropopause by 0.32–0.52 W m−2. These ranges bracket the uncertainties due to spectral absorption cross sections, O2·N2 efficiency, O2·X abundance, and cloud distribution. Globally averaged, O2·X enhances absorption equally in clear and in cloudy skies. We create a global climatology of well-mixed collision complex abundances by scaling O2·X abundance to other complexes such as N2·N2 and O2·Ar. Collision complex abundance depends quadratically on the concentrations of the constituents. This dependence causes a 20% increase in O2·X abundance in the Arctic relative to the Tropics for the same sea level pressure. The variations in zonal mean O2·X abundance due to surface elevation, the annual mean meridional temperature gradient, and to seasonal temperature variations are 40, 15, and 10%, respectively. O2·X heating obeys the weakly absorbing, linear limit so it peaks at the surface in clear skies, but clouds shift this peak up by 200–300 mb on seasonal timescales. Surface albedo and clouds strongly modulate the solar forcing efficiency of O2·X (forcing per unit abundance) by altering the mean photon path length. These factors produce annual mean forcing maxima in the subtropics over bright deserts and regions of marine stratus. Inclusion of O2·X in models is likely to reduce cold biases in the summertime polar atmosphere, where O2·X contributes 2–4% of total solar heating.

Abstract Based on experimental observations with EISCAT VHF radar during the summer period, July 8–12, 2013, the observations of polar mesosphere summer echoes (PMSE) in the absence of particle precipitation indicate that particle precipitation is not a necessary condition for PMSE to exist. But, particle precipitation still affects PMSE when they both occur simultaneously. So in this paper, the relationship between PMSE and particle precipitation both represented by average electron density, occurring simultaneously for time interval of various lengths (t ≥ 2.56 min), is statistically analyzed using the Spearman rank and Pearson linear correlation coefficients. The new method by comparing the average electron density at altitude of 90 km (proxy of particle precipitation) and PMSE region at altitude of 80–90 km (proxy of PMSE) may compare the two phenomena directly and give some relationship between them. The percentage of events having positive values is dominant, which shows that the electron density variations due to the ionization produced by energetic particle precipitations might have some relationship with PMSE intensity. Moreover, the small percentage of negative correlation coefficient observed might be caused by the very strong precipitation at that time.

Observations of neutral winds from rocket release experiments within the premidnight and postmidnight substorm recovery phase aurora, show very large E-region neutral winds of several hundred m/s, where winds measured on the dusk side are even larger than those on the dawn side. These large winds are also associated with strong shears, and there is evidence that some of the regions below these shears may be unstable. The mechanisms which generate this strong vertical structure are not well understood. It is also not known whether the acceleration conditions in the pre and post midnight sectors of the aurora may produce significantly different neutral responses on the dawn and dusk sides. Simulations have been performed using a three-dimensional high resolution limited area thermosphere model to try to understand the neutral structure within the dawn and dusk side aurora. When simulations are performed using auroral forcing alone, for equivalent conditions within the dawn and dusk sectors, differences are found in the simulated response on each side. When measured values of auroral forcing parameters, and background winds and tides consistent with recent observations, are used as model inputs, some of the main features of the zonal and meridional wind observations are reproduced in the simulations, but the magnitude of the peak zonal wind around 140 km tends to be too small and the maximum meridional wind around 130 km is overestimated. The winds above 120 km altitude are found to be sensitive to changes in electric fields and ion densities, as was the case for the dawn side, but the effects of background winds and tides on the magnitudes of the winds above 120 km are found to be relatively small on the dusk side. The structure below 120 km appears to be related mainly to background winds and tides rather than auroral forcing, as was found in earlier studies on the dawn side, although the peak magnitudes of simulated wind variations in the 100 to 120 km altitude range are smaller than those observed. The source of the strong shears measured around I 10 km altitude on the dusk side is uncertain, but may be related to different kinds of oscillations, such as gravity waves, non migrating semidiurnal tides, or secondary oscillations produced by non linear interactions between waves.

Measurements of the neutral thermosphere within the postmidnight substorm recovery phase diffuse aurora show very large horizontal winds, and strong vertical structure. Rocket, satellite, and ground based observations during the ARIA (Atmospheric Response in Aurora) campaigns, and earlier dawn side rocket observations, indicate neutral winds of up to 200m/s, and a characteristic jet-like wind maximum around 110 to 120-km altitude, with strong shears above and below. The observed wind magnitudes are found to have a dependence on geomagnetic activity level, but recent modeling studies suggest that tides which propagate up from the troposphere and stratosphere may play an important role in generating the strong vertical variations in the neutral winds. The relative importance of auroral and tidal forcing in producing the measured wind structure is not known, however. Simulations have been performed using a three dimensional (3-D) high resolution limited area thermosphere model to understand the processes which generate the observed neutral structure within the postmidnight diffuse aurora. Parameters measured during the ARIA I observational campaign have been used to provide auroral forcing inputs for the model. Global background winds and tides have been provided by the CTIP (Coupled Thermosphere Ionosphere Plasmasphere) model. The sensitivity of the response of the neutral atmosphere to changes in different parameters has been examined. Variations in the amplitudes and phases of the propagating tides in the background winds are found to have significant effects on the neutral structure in the E region, and the wind structure below around I 10 km is found to be mainly produced by tidal forcing. Changes in the electric field and ion density affect the winds above around 120km, and the importance of auroral forcing is found to depend on background winds. Variations in the orientation of the aurora relative to the background field, which may be caused by changes in the interplanetary magnetic field, are also found to modify the wind structure. When both auroral forcing and propagating tides are included, many of the basic characteristics of the wind structure are displayed, although the great strength of the wind shears is not well reproduced. The strength of the shears may be related to a currently unmodeled process, or to different types of waves.

Evapotranspiration is integral to studies of the Earth system, yet it is difficult to measure on regional scales. One estimation technique is a terrestrial water budget, i.e., total precipitation minus the sum of evapotranspiration and net runoff equals the change in water storage. Gravity Recovery and Climate Experiment (GRACE) satellite gravity observations are now enabling closure of this equation by providing the terrestrial water storage change. Equations are presented here for estimating evapotranspiration using observation based information, taking into account the unique nature of GRACE observations. GRACE water storage changes are first substantiated by comparing with results from a land surface model and a combined atmospheric-terrestrial water budget approach. Evapotranspiration is then estimated for 14 time periods over the Mississippi River basin and compared with output from three modeling systems. The GRACE estimates generally lay in the middle of the models and may provide skill in evaluating modeled evapotranspiration.

The direct radiative forcing by aerosols over the Indian Ocean region is simulated for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during Spring 1999. The forcing is calculated for the top-of-atmosphere (TOA), surface, and atmosphere by differencing shortwave fluxes computed with and without aerosols. The calculation includes the effects of sea-salt, sulfate, carbonaceous, and soil-dust aerosols. The aerosol distributions are obtained from a global aerosol simulation including assimilation of satellite retrievals of aerosol optical thickness (AOT). The time-dependent, three-dimensional aerosol distributions are derived with a chemical transport model driven with meteorological analyses for this period. The surface albedos are obtained from a land-surface model forced with an identical meteorological analysis and satellite-derived rainfall and insolation. These calculations are consistent with in situ observations of the surface insolation over the central Indian Ocean and with satellite measurements of the reflected shortwave radiation. The calculations show that the surface insolation under clear skies is reduced by as much as 40 W/m2 over the Indian subcontinent by natural and anthropogenic aerosols. This reduction in insolation is accompanied by an increase in shortwave flux absorbed in the atmosphere by 25 W/m2. The inclusion of clouds in the calculations changes the direct effect by less than 2 W/m2 over the Indian subcontinent, although the reduction is much larger over China. The magnitude of the difference between all-sky and clear-sky forcing is quite sensitive to the three-dimensional spatial relationship between the aerosol and cloud fields, and other estimates of the difference for the INDOEX Intensive Field Phase are as large as 5 W/m2.