ECMWF Analyses Research Articles

The use of trajectory cluster analysis to interpret trace gas measurements at Mace Head, Ireland

A first step in interpreting the wide variation in trace gas concentrations measured over time at a given site is to classify the data according to the prevailing weather conditions. In order to classify measurements made during two intensive field campaigns at Mace Head, on the west coast of Ireland, an objective method of assigning data to different weather types has been developed. Air-mass back trajectories calculated using winds from ECMWF analyses, arriving at the site in 1995–1997, were allocated to clusters based on a statistical analysis of the latitude, longitude and pressure of the trajectory at 12 h intervals over 5 days. The robustness of the analysis was assessed by using an ensemble of back trajectories calculated for four points around Mace Head. Separate analyses were made for each of the 3 years, and for four 3-month periods. The use of these clusters in classifying ground-based ozone measurements at Mace Head is described, including the need to exclude data which have been influenced by local perturbations to the regional flow pattern, for example, by sea breezes. Even with a limited data set, based on 2 months of intensive field measurements in 1996 and 1997, there are statistically significant differences in ozone concentrations in air from the different clusters. The limitations of this type of analysis for classification and interpretation of ground-based chemistry measurements are discussed.

Persistence of stratospheric ozone layers in the troposphere

On 20th March 1994 at 12Z two dry layers with high ozone mixing ratio were encountered in the free troposphere by an ozonesonde ascent at Aberystwyth (52.4°N, 4.1°W). The layers resembled tropopause folds, but were not associated with any significant synoptic development, and did not have enhanced static stability. We use isentropic trajectory analysis, with winds taken from ECMWF analyses, to look for the origin of the layers. The analysis suggests that the two layers had different source regions. The lower layer spent 10 days over the Atlantic before reaching Northern Europe, and appears to have originated in the break-up of a stratospheric streamer between the 5th and 9th of March. The upper layer seems to have originated over the Western USA where the trajectories passed through a region of low Richardson number above the Rocky Mountains. In both cases, the low-water-vapour/high-ozone air mass had been advected with little mixing in the troposphere for at least 10 days. We argue that the air must be of stratospheric origin, since photo-chemistry cannot generate large amounts of ozone at this time of year and the trajectories do not, in any case, point to a boundary-layer origin. If, as the analysis suggests, the upper layer reached the troposphere by mixing across the jet then this may be evidence for stratosphere–troposphere exchange (STE) taking place other than by folding of the tropopause. The persistence of layers with anomalous chemical content in the troposphere for so long indicates that an accurate model of tropospheric transport and mixing is needed to assess the chemical impact of STE on tropospheric chemistry, in addition to a representation of the dynamical behaviour near to the tropopause.

Southern Extratropical Cyclone Behavior in ECMWF Analyses during the FROST Special Observing Periods

Abstract The data collected during the three special observing periods (SOPs) of the Antarctic First Regional Observing Study of the Troposphere project provide an excellent base upon which to study the behavior of cyclonic systems in winter, spring, and summer in the Southern Hemisphere. This paper provides a report on the behavior of these cyclonic systems during the three SOPs as revealed in the twice-daily ECMWF operational analyses. The study has been undertaken with an objective cyclone tracking algorithm applied to the digital analyses. The results revealed cyclone behavior generally in accord with long-term climatologies developed with this scheme. In the SOPs the authors observed many systems to be generated in the western part of the ocean basins and then to move east and, to a lesser extent, south. In the three periods they found a concentration of tracks just to the north of the Antarctic continent, being particularly noticeable in the Indian Ocean. At the same time, they found significant dif...

Evaluation of operational ECMWF surface freshwater fluxes over oceans during 1991–1997

The distribution of evaporation and precipitation over the ocean is one of the least understood elements of the climate system. Monthly-mean precipitation ( P) and evaporation ( E) fields from ECMWF operational analyses are studied for the period 1991 to 1997, and computed into the freshwater flux. Monthly-mean time-series of zonally averaged freshwater fluxes highlight several model changes affecting the analyses, in 1997 in particular. The monthly climatology built over the 1991–1996 period exhibits a coherent geophysical behaviour with regard to different sources of data. Global budgets of ECMWF precipitation and evaporation are 10% larger (110 mm/year higher) than climatologies from other sources, but the balance between evaporation and precipitation ( E− P) is preserved and is in line with the concept of the World Water Balance. In general, the zonal distribution of the E− P balance correlates qualitatively well with ocean surface salinity. Analysis of the geographical distribution of E and P and meridional transport of freshwater highlights fundamental discrepancies between ECMWF datasets and other sources of data. In particular, ECMWF analyses show significantly higher annual precipitations (700 mm/year more) in intertropical convergence zones, which contribute most to the differences observed in the meridional freshwater transport. For oceanographic applications, the haline and thermal density fluxes are compared. The thermal component generally dominates, except in equatorial regions where both components reach similar amplitudes. The salinity forcing is found to be especially strong in the western equatorial Pacific Ocean.

Assimilation of satellite altimeter data in a primitive-equation model of the Azores–Madeira region

Abstract The aim of this study is to implement satellite altimetric assimilation into a high-resolution primitive-equation ocean model and check the validity and sensitivity of the results. Beyond this paper, the remote objective is to get a dynamical tool capable of simulating the surface ocean processes linked to the air–sea interactions as well as to perform mesoscale ocean forecasting. For computational cost and practical reasons, this study takes place in a 1000 by 1000 sq km open domain of the Canary basin. The assimilation experiments are carried out with the combined TOPEX/POSEIDON and ERS-1 data sets between June 1993 and December 1993. The space–time domain overlaps with in situ data collected during the SEMAPHORE experiment and thus enables an objective validation of the results. A special boundary treatment is applied to the model by creating a surrounding recirculating area separated from the interior by a buffer zone. The altimetric assimilation is done by implementing a reduced-order optimal interpolation algorithm with a special vertical projection of the surface model/data misfits. We perform a first experiment with a vertical projection onto an isopycnal EOF representing the Azores Current vertical variability. An objective validation of the model's velocities with Lagrangian float data shows good results (the correlation is 0.715 at 150 dbar). The question of the sensitivity to the vertical projection is addressed by performing similar experiments using a method for lifting/lowering of the water column, and using an EOF in Z -coordinates. Some comparisons with in situ temperature data do not show any significant difference between the three projections, after five months of assimilation. However, in order to preserve the large-scale water characteristics, we felt that the isopycnal projection was a more physically consistent choice. Then, the complementary character of the two satellites is assessed with two additional experiments which use each altimeter data sets separately. There is an evidence of the benefit of combining the two data sets. Otherwise, an experiment assimilating long-wavelength bias-corrected CLS altimetric maps every 10 days exhibits the best correlation scores and emphasizes the importance of reducing the orbit error and biases in the altimetric data sets. The surface layers of the model are forced using realistic daily wind stress values computed from ECMWF analyses. Although we resolve small space and time scales, in our limited domain the wind stress does not significantly influence the quality of the results obtained with the altimetric assimilation. Finally, the relative effects of the data selection procedure and of the integration times (cycle lengths) is explored by performing data window experiments. A value of 10 days seems to be the most satisfactory cycle length.

Use and Impact of Satellite Atmospheric Motion Winds on ECMWF Analyses and Forecasts

Recent developments in the extraction technique of wind vectors from geostationary satellite images have led to a significant increase in the data coverage of this product. This is mainly due to the use of visible and water vapor imagery in addition to the infrared. To assess the impact of these additional data on the European Centre for Medium-Range Weather Forecasts analysis and forecast system, data assimilation experiments have been performed. Winds from the water vapor channel images of the two U.S. satellites, GOES-8 and -9, and from the high-resolution visible channel of the European satellite, Meteosat-5, were included in the assimilation process in addition to the other operational observations. The results showed consistent improvements in the quality of the model first-guess wind field and showed how the new winds have a significant positive impact on the short-range forecast of the circulation in the Tropics and Southern Hemisphere. The results of a global observing system experiment withholding all satellite winds from the observing system are also discussed. The relative impact of other data types and the question of forecast verification in data-sparse regions are addressed.

3D mapping of optical turbulence using an atmospheric numerical model

The first statistical results of simulations of optical turbulence over Cerro Paranal by an atmospheric non-hydrostatic model (Meso-Nh) are presented. Measurements from the whole PARSCA93 campaign are analyzed. Simulations are compared to optical measurements obtained by a Scidar and a DIMM working at the same time during 14 nights 13-26 May 1993 over Paranal (Chile). An orographic model with a horizontal resolution of 500 m is implemented in Meso-Nh to study its sensitivity to the orographic effects. The model is initialized with radiosoundings profiles provided by Antofagasta station (70.43 W, 23.43 S) and ECMWF analyses extracted from the nearest grid point (70.31 W, 23.62 S) to this meteorological station. A detailed quality study of radiosounding and analyses is presented. No radiosoundings were provided by Antofagasta station during 4 nights and numerical instabilies were generated by the model during the 13 and 26 May 1993 nights. So, only 8 nights were actually studied. Two statistical methods are presented: Method A has a high temporal resolution, Method B has a high vertical spatial resolution. Method A compares integrated values (seeing) provided by simulations and measurements, Method B compares the profiles. Different statistical estimators are computed for both the methods. We demonstrate that the model can reproduce well the spatial distribution of the optical turbulence in both the high and low atmospheric regions but the unreliability of Antofagasta radiosoundings hampers the statistical results of the numerical technique. To better test the performance of the model, a comparison between the numerical and the forecasting-by-persistence techniques is presented. With a poor statistical sample (only 8 nights), no conclusive statements can be made about the performances of the two techniques. The model ability to discriminate between the best (e = 0.38) and the worst (e = 1.38) nights of the campaign is promising for the future implementation of the numerical technique for flexible scheduling of telescopes.

Principal Modes of Climatological Seasonal and Intraseasonal Variations of the Asian Summer Monsoon

Abstract Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction produced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985–89. It is shown that the seasonal evolution of the Asian summer monsoon is adequately described by a few leading EOFs. These EOFs capture the variations of regional rainbands over the East Asian and Indian regions. The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cl...

The Soil–Precipitation Feedback: A Process Study with a Regional Climate Model

Month-long integrations with a regional climate model covering Europe and the Northern Atlantic are utilized to study the sensitivity of the summertime European precipitation climate with respect to the continental-scale soil moisture content. Experiments are conducted for July 1990 and 1993. For each of the two months, the control experiment with the initial soil water distribution derived from the operational ECMWF analysis is compared against two sensitivity experiments with dry and wet initial soil moisture distributions. The results demonstrate that summertime European precipitation climate in a belt ∼1000 km wide between the wet Atlantic and the dry Mediterranean climate heavily depends upon the soil moisture content. In this belt, changes in monthly mean precipitation amount to about half of the changes in mean evapotranspiration. Budget analysis of water substance over selected subdomains demonstrate that the simulated sensitivity cannot be interpreted with the classical recycling mechanism, that is, the surplus of precipitation that falls over wet (as compared to dry) soils does not directly derive from evapotranspiration. Rather, the surplus of precipitation primarily originates from water vapor extracted from the ambient atmospheric flow. Thus, the soil–precipitation feedback must rely on some indirect mechanism, whereby wet soils increase the efficiency of convective precipitation processes. In order to isolate the physical mechanisms underlying the soil–precipitation feedback, a detailed analysis including an investigation of the mean diurnal cycle throughout the integration period is performed. The key elements of the feedback are the following. First, wet soils (small Bowen ratios) imply the buildup of a comparatively shallow boundary layer. The surface fluxes of heat and moisture are thus concentrated into a comparatively small volume of air, leading to the buildup of high values of low-level moist entropy, thereby providing a source of convective instability. Second, the level of free convection is lowered in the wet experiment, thus facilitating the release of convective instability. Third, the net shortwave absorption at the soil decreases in the wet experiment (as a result of increased cloud cover), but this effect is overpowered by the decrease in net longwave emission (as a result of decreased emission, increased cloud backscatter, and increased water vapor greenhouse effect). Thus the net radiative energy flux is larger in the wet experiments, thereby increasing the moist entropy fluxes into the boundary layer. These three processes act in concert to increase the potential for convective activity.

The MSDOL Project: Assimilation of Gomos Ozone Data in a 3-D chemistry-transport model

In the frame of the preparation of the use of ENVISAT data, the EEC is supporting the MSDOL project: Monitoring of the Stratospheric Depletion of the Ozone Layer. The purpose is to assimilate the 400 vertical profiles of Ozone recorded each day by the experiment GOMOS (on the future ESA ENVISAT spacecraft ) in a 3-D chemistry-transport model, where the winds are the ECMWF analysis. The sequentially assimilated model will better represent the reality since all single measurements are extrapolated in space and time through chemistry and transport. The assimilated model will serve at least two purposes : the comparison with other data sets, and the estimate of the evolution of ozone as a function of time and space.The 3-D model is derived from the stratospheric chemistry-transport Rose model, in which the internally generated dynamics is replaced by the actual wind field.

Stratospheric water vapour and tropical tropopause temperatures in ECMWF analyses and multi-year simulations

Stratospheric water vapour and tropical tropopause temperatures in ECMWF analyses and multi-year simulations

Stratospheric water vapour and tropical tropopause temperatures in Ecmwf analyses and multi‐year simulations

AbstractStratospheric humidity analyses produced operationally by the European Centre for Medium‐Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near‐tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar‐night vortex. the lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Niño are illustrated.Analysed temperatures near the tropical tropopause are generally in good agreement with corresponding radiosonde measurements, with standard‐level biases of the order of 0.5 degC or less. the past two years are the coldest by about 1 degC in a series of tropical mean 100 hPa analyses extending back to 1979. A cooling trend of about 0.6 degC per decade is seen in the global means of the 100 hPa analyses.Moisture is spread zonally and upward from the tropical tropopause as the data assimilation proceeds, but the rate of upward transfer is much faster than observed. Substantial lateral mixing can occur within the stratosphere over the course of a season. Moistening at middle and high latitudes due to mixing with more humid tropospheric air is confined, realistically, to a shallow layer at the base of the stratosphere.The rate of upward transfer of tropical stratospheric moisture is much more realistic in a multi‐year simulation using a version of the model that has finer stratospheric resolution than the version used for the operational data assimilation. Temperatures at the tropical tropopause and in the Antarctic polar night are accurately simulated, apart from excessive persistence of cold south‐polar temperatures in late winter and early spring. the latter is conducive to drying the model stratosphere; lack of a parametrization of moistening due to methane oxidation is an obvious deficiency in this regard.

Storm tracks in regional climate simulations: verification and changes with doubled CO2

The ability of the CSIRO Atmospheric Research’s regional climate model (RCM) to simulatethe mean July storm track over Australia is investigated in this study. The observed storm trackis characterized by the time-averaged eddy kinetic energy in the ECMWF analyses for years1985–1992. RCM runs nested within these analyses are used to verify the ability of the modelto simulate the storm track with “perfect” boundary conditions. Comparison is also made with20 years of the CSIRO9 Mk2 global climate model (GCM) and the RCM nested within thisGCM, for equilibrium simulations with both current and doubled CO2 levels. Correlations ofthe meridional wind with a point within the storm track over Australia are used to characterizethe structure of the mean baroclinic wave within the storm track. The location of the stormtracks in both the RCM and the GCM are simulated very well. The main deficiency is the lackof intensity of the upper-level storm track, but the RCM is more realistic than the GCM, indicating that the intensity of the storm track is partly related to horizontal resolution. Themean wave structure in the GCM and the RCM simulations is very similar to observed. Underdoubled CO2 conditions, both the upper-level jetstream and storm track intensify, and moveequatorward, in response to the increasing upper-troposphere north–south temperature gradient.In the lower troposphere, the mean and eddy kinetic energy decrease to the southeast ofAustralia in response to the decrease in the north–south temperature gradient there. As aresponse to the generally decreasing mid-tropospheric north–south temperature gradient, themean baroclinic wave appears to become slightly more barotropic, as indicated by less tilt inthe zonal direction.

Evaluation of trajectories calculated from ecmwf data against constant volume balloon flights during etex

This paper validates trajectories calculated from ECMWF analyses against the tracks of constant volume balloons (CVBs) released during the European tracer experiment (ETEX). The altitudes of the calculated trajectories were adjusted to the altitudes of the respective balloons in short intervals to allow direct comparisons. The agreement between the calculated trajectories and the balloon tracks was very good for the first experiment (individual errors from 1 to 26%, average 15%), and excellent (errors from 2 to 11%, average 6%) for the second one. The agreement for the second experiment was probably partly better because the CVBs travelled above the planetary boundary layer, but the small errors also indicate that the ECMWF fields of the horizontal wind were of exceptionally good quality in the second experiment. This is in sharp contrast to the results of the dispersion models which all failed in the prediction of the perfluorocarbon tracer dispersion for the second experiment. A likely explanation for this is that vertical motions, possibly on small scales, were not correctly captured by the ECMWF analyses, but it is not possible to clarify this with the CVB data.

Calculating Monthly Radiative Fluxes and Heating Rates fromMonthly Cloud Observations

Abstract The radiative transfer model from NCAR’s general circulation model CCM3 is modified to calculate monthly radiative fluxes and heating rates from monthly observations of cloud properties from the International Satellite Cloud Climatology Project and temperature and humidity from ECMWF analysis. The calculation resolves the three-dimensional structure of monthly to interannual variations of radiative heating and is efficient enough to allow a wide range of sensitivity tests. Two modifications to the radiative transfer model improve the calculation of shortwave (SW) fluxes in a cloudy atmosphere. The first replaces an existing nonphysical parameterization of partially cloudy skies with a physically motivated one that increases substantially the accuracy of calculated SW fluxes while increasing the computational time of the calculation by only 10%. The second modification allows the specification of generalized cloud overlap properties. With these modifications, radiative fluxes are calculated from o...

Rainfall and the 6–9 day wave-like disturbance in West-Africa during summer 1989

Using ECMWF analyses and daily rain amounts of 569 stations in Western Africa for summer 1989, the study documents the composite structure of the 6–9 day oscillation and its influence on rain. Rain is modulated by vorticity as displayed in the wave composite. There are rainfall maxima coincident with cyclonic vorticity and rainfall minima coincident with anticyclonic vorticity at the 700hPa level, at 17.5°N and 7.5°N.

Transport of near‐tropopause air into the lower midlatitude stratosphere

AbstractDuring the last week of January 1992 ozonesonde observations over Europe revealed a layer of very low ozone concentrations in the stratosphere—below 100 parts per billion (109) by volume in the potential‐temperature range 360–380 K. A coincident lidar observation revealed that the air was virtually free of volcanic aerosol, which filled the lower stratosphere at that time. The layer corresponded well with low potential vorticity (PV) in ECMWF analysis fields. Trajectory calculations confirmed a subtropical origin for the layer, and PV fields suggest that it was formed in a streamer of low‐PV air drawn from the troposphere over North America as the sub‐tropical jet stream turned northwards on 25 January. Ozone profiles on the equatorward flank of the subtropical jet stream contain similar mixing ratios to those seen in the layer over Europe in the same potential‐temperature range. The mass of the layer at its maximum extent is estimated as 6 × 1015 kg. Most of the air in the layer eventually returned to the subtropics after mixing with ambient midlatitude air.

Basin-Scale, High-Wavenumber Sea Surface Wind Fields from a Multiresolution Analysis of Scatterometer Data

Abstract A numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields. To capture the prevailing wavenumber characteristics (power-law dependence) of sea surface wind vector components, wavelet coefficients are computed from the scatterometer measurements along the satellite tracks. The statistics of the wavelet coefficients are then used to simulate high-resolution wind components over the off-track regions where scatterometer data are not available. Using this technique, daily wind fields with controlled spectral features have been produced by combining the low-wavenumber wind fields from ECMWF analyses with the high-wavenumber measurements from the ERS-1 scatterometer. The resulting surface wind fields thus reflect nearly all available measurements affecting surface wind, including the synoptic surface pressure. The new surface wind forces a b...

Northern Hemisphere atmospheric blocking as simulated by 15 atmospheric general circulation models in the period 1979-1988

As a part of the Atmospheric Model Intercomparison Project (AMIP), the behaviour of 15 general circulation models has been analysed in order to diagnose and compare the ability of the different models in simulating Northern Hemisphere midlatitude atmospheric blocking. In accordance with the established AMIP procedure, the 10-year model integrations were performed using prescribed, time-evolving monthly mean observed SSTs spanning the period January 1979–December 1988. Atmospheric observational data (ECMWF analyses) over the same period have been also used to verify the models results. The models involved in this comparison represent a wide spectrum of model complexity, with different horizontal and vertical resolution, numerical techniques and physical parametrizations, and exhibit large differences in blocking behaviour. Nevertheless, a few common features can be found, such as the general tendency to underestimate both blocking frequency and the average duration of blocks. The problem of the possible relationship between model blocking and model systematic errors has also been assessed, although without resorting to ad-hoc numerical experimentation it is impossible to relate with certainty particular model deficiencies in representing blocking to precise parts of the model formulation.

Climatological Ertel's potential-vorticity flux and mean meridional circulation in the extratropical troposphere − lower stratosphere

Abstract. We investigated to what extent the isentropic, non-geostrophic formulation of zonally averaged circulation derived for stratospheric conditions is applicable to climatological transport in the extratropical troposphere and lower stratosphere. The study is based on 10 years of daily data of ECMWF analysis and on the ECHAM3 climate model of the German Climate Computing Centre. The main result is a scalar isentropic mixing coefficient, Kyy, and a mean meridional transport circulation consistently derived from the same data base. For both data sources, isentropic mean meridional circulation is derived from horizontal mass flow rate for 4 representative months. Alternatively, a mean meridional circulation is calculated from total diabatic heating rates of the ECHAM3 model. It is shown that only the latter is in good agreement with the ECMWF mean meridional circulation. Isentropic analysis also comprises the seasonal cycle of the climatological meridional gradient and flux of Ertel's potential vorticity (PV). Application of Tung's flux-gradient relation yields that for all seasons Kyy is positive in height-latitude regions where statistical significance is reached. Large Kyy values, marking regions of more efficient mixing, have been found in the subtropical vertical band of weak westerly wind and in mid-latitudes in regions of upward-propagating baroclinic wave activity in the middle and upper troposphere. Based on the ECMWF data and results of baroclinic-wave behaviour, strong indications are presented that positive zonally averaged PV flux polewards of the jet core in the NH is strengthened by stationary waves and nonlinear effects. Reduced eddy transport is apparent in winter and spring slightly below the subtropical tropopause jet. The seasonal cycle of Kyy from ECHAM3 data is to a great extent in agreement with the result based on ECMWF analysis. In the model, reduced interannual variability enlarges the height-latitude range where sign of Kyy is significant.Key words. Meteorology and atmospheric dynamics · Climatology · General circulation · Middle atmosphere is significant.