This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earthorbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dualfrequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research. Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5-7 km), temperature may be estimated directly from refractivity. This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5-7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude. Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

Comprehensive diagnostic comparisons and evaluations have been carried out with the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses of the vertically integrated atmospheric energy budgets. For 1979 to 1993 the focus is on the monthly means of the divergence of the atmospheric energy transports. For February 1985 to April 1989, when there are reliable top-of-the-atmosphere (TOA) radiation data from the Earth Radiation Budget Experiment (ERBE), the implied monthly mean surfacē uxes are derived and compared with those from the assimilating models and from the Comprehensive Ocean Atmosphere Data Set (COADS), both locally and zonally integrated, to deduce the implied ocean meridional heat transports.While broadscale aspects and some details of both the divergence of atmospheric energy and the surface¯ux climatological means are reproducible, especially in the zonal means, dierences are also readily apparent. Systematic dierences are typically $20 W m À2 . The evaluation highlights the poor results over land. Land imbalances indicate local errors in the divergence of the atmospheric energy transports for monthly means on scales of 500 km (T31) of 30 W m À2 in both reanalyses and $50 W m À2 in areas of high topography and over Antarctica for NCEP/NCAR. Over the oceans in the extratropics, the monthly mean anomaly time series of the vertically integrated total energy divergence from the two reanalyses correspond reasonably well, with correlations exceeding 0.7. A common monthly mean climate signal of about 40 W m À2 is inferred along with local errors of 25 to 30 W m À2 in most extratropical regions.Except for large scales, there is no useful common signal in the tropics, and reproducibility is especially poor in regions of active convection and where stratocumulus prevails. Although time series of monthly anomalies of surface bulk¯uxes from the two models and COADS agree very well over the northern extratropical oceans, the total ®elds all contain large systematic biases which make them unsuitable for determining ocean heat transports. TOA biases in absorbed shortwave, outgoing longwave and net radiation from both reanalysis models are substantial (>20 W m À2 in the tropics) and indicate that clouds are a primary source of problems in the model¯uxes, both at the surface and the TOA. Time series of monthly COADS surface¯uxes are shown to be unreliable south of about 20 N where there are fewer than 25 observations per 5 square per month. Only the derived surface¯uxes give reasonable implied meridional ocean heat transports.

The temperature record of the last 150 years is characterized by a long-term warming trend, with strong multidecadal variability superimposed. The multidecadal variability is also seen in other (societal important) parameters such as Sahel rainfall or Atlantic hurricane activity. The existence of the multidecadal variability makes climate change detection a challenge, since Global Warming evolves on a similar timescale. The ongoing discussion about a potential anthropogenic signal in the Atlantic hurricane activity is an example. A lot of work was devoted during the last years to understand the dynamics of the multidecadal variability, and external as well as internal mechanisms were proposed. This White Paper focuses on the internal mechanisms relevant to the Atlantic Multidecadal Oscillation/Variability (AMO/V) and the Pacific Decadal Oscillation/Variability (PDO/V). Specific attention is given to the role of the Meridional Overturning Circulation (MOC) in the Atlantic. The implications for decadal predictability and prediction are discussed.

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Atmospheric aerosols impact both human health and climate. PMX is the mass concentration of aerosol particles that have aerodynamic diameters less than X μm, PM₁₀ was initially selected to measure the environmental impact of aerosols. Recently, it was realized that fine particles are more hazardous than larger ones and should be measured. Consequently, observational data for PM_2.5 have been obtained but only for a much shorter period than that of PM₁₀. Optical extinction of aerosols, the inverse of meteorological visibility, is sensitive to particles less than 1.0 μm. These fine particles only account for a small part of total mass of aerosols although they are very efficient in light extinction. Comparisons are made between PM₁₀ and PM_2.5 over the period when the latter is available and with visibility data for a longer period. PM₁₀ has decreased by 44% in Europe from 1992 to 2009, 33% in the US from 1993 to 2010, 10% in Canada from 1994 to 2009, and 26% in China from 2000 to 2010. However, in contrast, aerosol optical extinction increased 7% in the US, 10% in Canada, and 18% in China during the above study periods. The reduction of optical extinction over Europe of 5% is also much less than the 44% reduction in PM₁₀. Over its short period of record PM_2.5 decreased less than PM₁₀. Hence, PM₁₀ is neither a good measure of changes in smaller particles or of their long-term trends, a result that has important implications for both climate impact and human health effects. The increased fraction of anthropogenic aerosol emission, such as vehicle exhaust, to total atmospheric aerosols partly explains this contrasting trend of optical and mass properties of aerosols