Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.

http://science.sciencemag.org/content/sci/269/5224/676.full.pdf

Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation

ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.

A number of results indicative of the overall performance of the data assimilation system, and implicitly of the observing system, are presented and discussed. The comparison of background (short-range) forecasts and analyses with observations, the consistency of the global mass budget, the magnitude of differences between analysis and background fields and the accuracy of medium-range forecasts run from the ERA-40 analyses are illustrated. Several results demonstrate the marked improvement that was made to the observing system for the southern hemisphere in the 1970s, particularly towards the end of the decade. In contrast, the synoptic quality of the analysis for the northern hemisphere is sufficient to provide forecasts that remain skilful well into the medium range for all years. Two particular problems are also examined: excessive precipitation over tropical oceans and a too strong Brewer-Dobson circulation, both of which are pronounced in later years. Several other aspects of the quality of the re-analyses revealed by monitoring and validation studies are summarized. Expectations that the ‘second-generation’ ERA-40 re-analysis would provide products that are better than those from the firstgeneration ERA-15 and NCEP/NCAR re-analyses are found to have been met in most cases. © Royal Meteorological Society, 2005. The contributions of N. A. Rayner and R. W. Saunders are Crown copyright.

The ERA‐40 re‐analysis

Investigating soil moisture-climate interactions in a changing climate: A review

From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

/pdf/the-changing-character-of-precipitation-5gto4ulnge.pdf

The Changing Character of Precipitation

There is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere. Hence, storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones, supplied with increased moisture, produce more intense precipitation events. Such events are observed to be widely occurring, even where total precipitation is decreasing: 'it never rains but it pours!' This increases the risk of flooding. The atmo- spheric and surface energy budget plays a critical role in the hydrological cycle, and also in the slower rate of change that occurs in total precipitation than total column water vapor. With modest changes in winds, patterns of precipitation do not change much, but result in dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in the mid- to high latitudes: the 'rich get richer and the poor get poorer'. This pattern is simulated by climate mod- els and is projected to continue into the future. Because, with warming, more precipitation occurs as rain instead of snow and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in summer, especially over continental areas. However, with more precipitation per unit of upward motion in the atmosphere, i.e. 'more bang for the buck', atmo- spheric circulation weakens, causing monsoons to falter. In the tropics and subtropics, precipitation patterns are dominated by shifts as sea surface temperatures change, with El Nino a good example. The volcanic eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and to widespread drought, as precipitation shifted from land to oceans and evaporation faltered, providing lessons for possible geoengineering. Most models simulate precipitation that occurs prematurely and too often, and with insufficient intensity, resulting in recycling that is too large and a lifetime of moisture in the atmosphere that is too short, which affects runoff and soil moisture.

/pdf/changes-in-precipitation-with-climate-change-3ofcyuiy76.pdf

Changes in precipitation with climate change

An evaluation is carried out of the moisture fields, the precipitation P and evaporation E, and the moisture transport and divergence in the atmosphere from the global atmospheric National Centers for Environmental Prediction (NCEP)–NCAR reanalyses produced with four-dimensional-data assimilation. The moisture fields are summarized by the precipitable water which is compared with analyzed fields from NVAP based primarily on Special Sensor Microwave Imager (SSM/I) over the oceans and rawinsonde measurements over land, plus TIROS Operational Vertical Sounder (TOVS). The moisture budgets are evaluated through computation of the freshwater flux at the surface E−P from the divergence of the total moisture transport, and this is compared with the reanalysis E−P that is based upon a 6-hour integration of the assimilating model and thus depends on the model parametrizations. The P field is evaluated using Xie– Arkin global precipitation estimates which, although containing considerable uncertainties, are believed to be reliable and good enough to show that there are substantial biases in the NCEP P. There are many fields of interest and which are improved over previous information available. On an annual mean basis the largest evaporation of over 6 mm/day is in the subtropical Indian Ocean. However, the NCEP moisture fields are shown to contain large and significant biases in the tropics. The tropical structures are less well defined and values are generally smaller where they should be high and higher where they should be low. In addition, the NCEP moisture fields contain less variability from year to year. The NCEP model P generally reveals a double intertropical convergence zone in the central Pacific and the location of the South Pacific Convergence Zone is not well captured. Rainfall amounts are lower than observed in the oceanic tropical convergence zones. The variability in the central tropical Pacific of P associated with El Nino-Southern Oscillation (ENSO) is underestimated in the NCEP reanalyses and, moreover, is not very well correlated with the Xie–Arkin product. A bias for too much rainfall in the model over the southeastern USA and southeast Asia is also present in northern summer. The comparison of E−P from the moisture budget with the model results reveal some strong systematic differences. In particular, remarkably, many island stations show up as bull’s-eyes in the difference field. These are identified as originating from small but systematic differences in vertical moisture profiles from those in the surrounding oceans, raising questions about the influence radius of rawinsonde moisture observations. Biases in E are inferred from the E−P differences in some places implying some spurious land moisture sources. While usually better, the residual method E−P estimates are inferior to those from the model parametrizations in some places. Both estimates are affected by biases in moisture, as analyzed, and the moisture divergence depends critically on the velocity divergence field. The model estimates also depend upon the parametrizations of subgrid scale processes, such as convection, that influence E and P. A discussion is given of sources of errors contributing to the moisture budgets.

/pdf/evaluation-of-the-atmospheric-moisture-and-hydrological-1m4oha0440.pdf

Evaluation of the atmospheric moisture and hydrological cycle in the NCEP/NCAR reanalyses

An analysis of the spring–summer 1988 drought and 1993 floods over North America reveals a reversal in the sign of anomalies in several fields. Large sea surface temperature anomalies of opposite signs existed in the tropical Pacific with strong La Nina conditions in 1988 and a mature El Nino in 1993. The distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered, implying a large-scale switch in the anomalous tropical heating and forcing of extratropical quasi-stationary waves in the atmosphere, influencing the subtropical jet stream over the North Pacific and across North America. In 1988 the jet stream and the closely related storm track of high-frequency disturbances in the upper troposphere were displaced into Canada well north of the normal location—the farthest north of any year from 1979 to 1993. In 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location a...

Physical Processes Involved in the 1988 Drought and 1993 Floods in North America

For the period 1987 to 1993, quantities central to the global moisture budget from the global analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Meteorological Center (NMC), and NASA/Goddard have been computed and compared. The precipitable water is computed and compared with satellite data from the Special Sensor Microwave Imager (SSM/I). Fluxes of moisture and their divergence have been used to estimate the vertically integrated moisture budget and thus evaporation minus precipitation (E − P) as residuals. Results of several test computations show that small biases exist in precipitable water as vertical resolution and methods of computing vertical integrals are changed, but the impact is small on the moisture budget. In the Tropics and subtropics the moisture budget is dominated by the divergence field rather than the moisture amounts, and consequently initialization of the analyses has an impact on the perceived moisture budget. The diurnal cycle is ...

Evaluation of the Global Atmospheric Moisture Budget as Seen from Analyses

Accurate but approximate formulae for determining the mass of the atmosphere in terms of the surface pressure ps are derived and applied to globally analyzed data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for 1985 through 1993. The formulae take into account effects of the shape of the Earth and variations in gravity with latitude and height. Variations in total mass occur because of changes in the water vapor loading of the atmosphere. Independent computations are made of the surface pressure due to water vapor pw, which is proportional to the precipitable water, using the ECMWF analyses of specific humidity. Spurious trends in both the mass of dry air and the atmospheric moisture are found to arise from changes in the analysis system at ECMWF, confounding attempts to seek real trends associated with climate change. For the recent 4-year period 1990 to 1993 the mean annual ps was 984.76 mbar with a maximum in July of 984.98 mbar and a minimum in December of 984.61 mbar which correspond to a total mean mass of the atmosphere of 5.1441×1018 kg with a range of 1.93×1015 kg throughout the year associated with changes in water vapor in the atmosphere. The global mean pw for 1985–1993 is 2.58 mbar, but values are 5 to 10% lower after mid-1992. Using the Special Sensor Microwave Imager data to make adjustments, the best estimate of the annual global pw is 2.4 mbar, corresponding to ∼2.5 cm of precipitable water. The total atmospheric moisture as given by pw varies with an annual cycle range of 0.36 mbar, a maximum in July, and a minimum in December. Thus the mean mass of water vapor is 1.25×1016 kg and the dry air mass is 5.132×1018 kg, corresponding to a mean surface pressure of 982.4 mbar. Overall uncertainties are ∼0.1 mbar or 0.5×1015 kg in total mass and about double those values for atmospheric moisture content. As well as the global means, hemispheric mean values and meridional profiles of ps and pw are presented for the mean annual cycle and as latitude-time series to show the interannual and longer-term variability.

The total mass of the atmosphere

An evaluation is carried out of the moisture fields, the precipitation P and evaporation E, and the moisture transport and divergence in the atmosphere from the global atmospheric National Centers for Environmental Prediction (NCEP) reanalyses produced with four-dimensional-data assimilation. The moisture fields are summarized by the precipitable water which is compared with analyzed fields from NVAP based primarily on Special Sensor Microwave Imager (SSM/I) over the oceans and rawinsonde measurements over land, plus TIROS Operational Vertical Sounder (TOVS). The moisture budgets are evaluated through computation of the freshwater flux at the surface E - P using residual techniques from the divergence of the total moisture transport, and this is compared with the reanalysis E - P that is based upon a 6-hour integration of the assimilating model and thus depends on the model parameterizations. The P field is evaluated using Xie-Arkin Global Precipitation Climatology Project (GPCP) estimates. An atlas presents climatological means for the period 1979 1995 (17 years) and standard deviations for annual, seasonal and monthly values, and the differences between estimates.

Christian J. Guillemot

Papers

Evaluation of the atmospheric moisture and hydrological cycle in the NCEP/NCAR reanalyses

Physical Processes Involved in the 1988 Drought and 1993 Floods in North America

Evaluation of the Global Atmospheric Moisture Budget as Seen from Analyses

The total mass of the atmosphere

Evaluation of the atmospheric moisture and hydrological cycle in the NCEP reanalyses