SYNOP

Abstract: The global analysis and forecast impact of observed humidity has been assessed by means of observing system experiments with the ECMWF 4D-Var data assimilation system. It is found that humidity data have a significant impact extending into the medium range (5–6 day forecasts), with a marked impact also on the wind and temperature fields. This contradicts some previous studies that have shown insignificant impact of humidity observations in general. The current, greater benefit of the humidity analysis may be due to improved model and data assimilation methods, and vastly increased availability of atmospheric moisture observations. The results show that each tested data type provides benefit to the analysis and forecast performance, which indicates that the humidity analysis is effective in extracting information from a wide variety of humidity observations. Data from the microwave sounding instruments (SSMI and AMSU-B) dominate the humidity analysis over the sea, whereas radiosondes, surface stations (SYNOP) and AMSU-B dominate over land. The infrared sounders (GOES, HIRS and AIRS) dominate in the upper troposphere, at 200–300 hPa. The lack of absolutely calibrated humidity data makes dealing with biases in observations and model one of the main issues for determining the global moisture distribution and a balanced hydrological cycle. In these experiments, SSMI adds water in the subtropical subsidence areas due to a bias with respect to the model. In several locations over land, radiosondes and SYNOP have opposite bias impacts in the boundary layer, resulting in local influence on precipitation when either dataset is withheld. The SYNOP data are biased wet and the radiosondes are biased dry with respect to the model. Copyright © 2007 Royal Meteorological Society

Abstract: . Climate change mitigation efforts require information on the current greenhouse gas atmospheric concentrations and their sources and sinks. Carbon dioxide ( CO2 ) is the most abundant anthropogenic greenhouse gas. Its variability in the atmosphere is modulated by the synergy between weather and CO2 surface fluxes, often referred to as CO2 weather. It is interpreted with the help of global or regional numerical transport models, with horizontal resolutions ranging from a few hundreds of kilometres to a few kilometres. Changes in the model horizontal resolution affect not only atmospheric transport but also the representation of topography and surface CO2 fluxes. This paper assesses the impact of horizontal resolution on the simulated atmospheric CO2 variability with a numerical weather prediction model. The simulations are performed using the Copernicus Atmosphere Monitoring Service (CAMS) CO2 forecasting system at different resolutions from 9 to 80 km and are evaluated using in situ atmospheric surface measurements and atmospheric column-mean observations of CO2 , as well as radiosonde and SYNOP observations of the winds. The results indicate that both diurnal and day-to-day variability of atmospheric CO2 are generally better represented at high resolution, as shown by a reduction in the errors in simulated wind and CO2 . Mountain stations display the largest improvements at high resolution as they directly benefit from the more realistic orography. In addition, the CO2 spatial gradients are generally improved with increasing resolution for both stations near the surface and those observing the total column, as the overall inter-station error is also reduced in magnitude. However, close to emission hotspots, the high resolution can also lead to a deterioration of the simulation skill, highlighting uncertainties in the high-resolution fluxes that are more diffuse at lower resolutions. We conclude that increasing horizontal resolution matters for modelling CO2 weather because it has the potential to bring together improvements in the surface representation of both winds and CO2 fluxes, as well as an expected reduction in numerical errors of transport. Modelling applications like atmospheric inversion systems to estimate surface fluxes will only be able to benefit fully from upgrades in horizontal resolution if the topography, winds and prior flux distribution are also upgraded accordingly. It is clear from the results that an additional increase in resolution might reduce errors even further. However, the horizontal resolution sensitivity tests indicate that the change in the CO2 and wind modelling error with resolution is not linear, making it difficult to quantify the improvement beyond the tested resolutions. Finally, we show that the high-resolution simulations are useful for the assessment of the small-scale variability of CO2 which cannot be represented in coarser-resolution models. These representativeness errors need to be considered when assimilating in situ data and high-resolution satellite data such as Greenhouse gases Observing Satellite (GOSAT), Orbiting Carbon Observatory-2 (OCO-2), the Chinese Carbon Dioxide Observation Satellite Mission (TanSat) and future missions such as the Geostationary Carbon Observatory (GeoCarb) and the Sentinel satellite constellation for CO2 . For these reasons, the high-resolution CO2 simulations provided by the CAMS in real time can be useful to estimate such small-scale variability in real time, as well as providing boundary conditions for regional modelling studies and supporting field experiments.

Abstract: The demand for verification of forecasting systems to ascertain their strengths and weaknesses is increasing dramatically as models evolve more rapidly. Precipitation forecasts have always been of great interest to forecasters because they influence daily life. The recent flooding over Europe has also shown how important it is to know how models can reproduce these events. The issue of precipitation verification is addressed here, starting from the assumption that model spatial scales have to be verified against data representing similar scales. Only in this way may the skill of forecasting system used herein be determined. The performance of the European Centre for Medium-Range Weather Forecasts model in predicting precipitation is discussed. The study concentrates on the period September to November 1999 during which high-density observations were available for the Alps. The high-resolution observing network over the Alpine region has been used to reconstruct a precipitation analysis that contains smoothed small-scale variability and represents with sufficient accuracy the average behavior of the observed field in the model grid box. The precipitation forecast is verified against both the precipitation analysis and the surface synoptic observations (SYNOP) available in real time via the Global Telecommunication System. Both verification approaches show that for the Alpine region, during autumn 1999, the model overestimates the precipitation amount. Overestimation is smaller when the forecast is compared with the precipitation analysis. It is also shown that verification against irregular and scattered observations (SYNOP data) is highly influenced by the variability of the precipitation in a grid box. A precipitation analysis is, therefore, important if model skill has to be defined.

Abstract: The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W modred observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime; small amplitude propagating and growing meanders characterized the variability there. The Central Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level streamfunctions, all with mesoscale resolution. Near 70°W the path envelope exhibited a relative node, confined within a 40-km band 55% of the time. Near 68°W the path envelope was over 3 times as wide, due to several elongated (“steep”) meander troughs and relatively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68°W, steepened, and persisted for one to several months. Two cases evolved into “S-shaped” paths and subsequently formed rings. Even the time-averaged fields showed a small trough in the mean path and thermocline structure. Whereas meanders of 20- to 60-day periods had similar spectral levels throughout 70°–67°W, meanders with long periods (>85 day) accounted for the local minimum in variance at 70°W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s−1) abyssal eddies; the time-averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough. The cross-stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form.