ECMWF Data Research Articles

On the homogeneity of radiosonde wind time series

Radiosonde observations of temperature and wind have been an important source of upper air data during the past few decades. To be useful for climate studies, the quality and temporal homogeneity of this dataset is essential. Large efforts have been put into quality control and homogenization of radiosonde temperature data. Much less emphasis has been put on the bias correction of wind data derived from radiosonde ascents. This paper takes a first look into the temporal homogeneity of the global radiosonde wind data set as it has been compiled in the 40-year ECMWF reanalysis (ERA-40) and operational ECMWF analysis databases as well as in the Integrated Global Radiosonde Archive (IGRA). More than 1000 breaks have been detected with a preliminary detection and adjustment method based on the analysis of time series of background departures from the ERA-40 and the operational ECMWF data assimilation systems. Most breaks can be attributed to significant inhomogeneities (median size 2 m/s) in the observed radiosonde wind data. Especially errors in the north alignment could be detected quite well. During some periods, especially in the mid-1970s, the homogeneity of the ERA-40 background wind is questionable, however. An additional estimation of the break sizes using neighboring stations as reference is therefore recommended. As far as could be seen, the homogenization has little systematic impact on global mean observed wind trends.

Error analysis for CO and CH<sub>4</sub> total column retrievals from SCIAMACHY 2.3 μm spectra

Abstract. A detailed sensitivity analysis of the Iterative Maximum Likelihood Method (IMLM) algorithm and its application to the SCIAMACHY 2.3 μm spectra is presented. The sensitivity analysis includes a detailed assessment of the impact of aerosols in the 2.3 μm range. Results show that near strong aerosol sources mineral dust and biomass aerosols can have an effect of ~7–10% on the CH4 total columns retrieved from this wavelength range if aerosol scattering is neglected in the retrieval algorithm. Similar but somewhat larger effects are found for CO, but due to the larger variability of CO these errors are less important. Away from strong sources much smaller effects of a few percent are found. Using CH4 as a proxy for CO and/or including aerosol information in the retrieval algorithm significantly reduces these errors for both CO and CH4. Spectroscopic uncertainties are mostly negligible except for uncertainties in the CH4 intrinsic line intensities, which can be important. Application of the IMLM algorithm to the SCIAMACHY 2.3 μm spectra shows that the quality of the retrieved CO and CH4 total columns is good, except for a bias for large instrument-noise errors which is partly due to remaining calibration issues. Polarization sensitivity of the SCIAMACHY instrument has a negligible effect on the retrieved CO and CH4 total columns. The H2O total columns, which have to be retrieved simultaneously with CO and CH4 due to overlapping absorption lines, agree well with H2O total columns from ECMWF data. This ensures that the fit to the H2O absorptions is of sufficient quality not to hamper the retrieved CO and CH4 total columns from SCIAMACHY spectra.

Impact of SSM/I Observations Related to Moisture, Clouds, and Precipitation on Global NWP Forecast Skill

Abstract This paper presents the results from the Observing System Experiments (OSEs) with the current ECMWF data assimilation and modeling system for quantifying the impact on both analysis and forecast quality of Special Sensor Microwave Imager (SSM/I) observations sensitive to moisture and clouds as well as precipitation. SSM/I radiances have been assimilated operationally in clear-sky areas for 8 yr and in cloud- and rain-affected areas since June 2005. This paper examines experiments set up such that clear-sky and rain-affected observations were either added to a baseline with a restricted observing system configuration or withdrawn from the full system. The experiment duration was 10 weeks of which the first 14 days were excluded from the evaluation to allow the system to lose the memory of the initial conditions at day −1. It is shown that both clear-sky and rain-affected observations account for the bulk correction of moisture in the ECMWF analysis. SSM/I data adds 1 day of forecast skill over the first 48 h when evaluated in addition to a baseline-observing system. In the tropics, the rain-affected data contributes more skill to the moisture forecast than the clear-sky data at 700 hPa and above. In the Northern and Southern Hemispheres, the effect is generally weaker and slightly in favor of clear-sky observations. A similar performance can be seen with respect to the wind vector forecast skill, which reflects the connection between the analysis of moisture and dynamics.

MIPAS: an instrument for atmospheric and climate research

Abstract. MIPAS, the Michelson Interferometer for Passive Atmospheric Sounding, is a mid-infrared emission spectrometer which is part of the core payload of ENVISAT. It is a limb sounder, i.e. it scans across the horizon detecting atmospheric spectral radiances which are inverted to vertical temperature, trace species and cloud distributions. These data can be used for scientific investigations in various research fields including dynamics and chemistry in the altitude region between upper troposphere and lower thermosphere. The instrument is a well calibrated and characterized Fourier transform spectrometer which is able to detect many trace constituents simultaneously. The different concepts of retrieval methods are described including multi-target and two-dimensional retrievals. Operationally generated data sets consist of temperature, H2O, O3, CH4, N2O, HNO3, and NO2 profiles. Measurement errors are investigated in detail and random and systematic errors are specified. The results are validated by independent instrumentation which has been operated at ground stations or aboard balloon gondolas and aircraft. Intercomparisons of MIPAS measurements with other satellite data have been carried out, too. As a result, it has been proven that the MIPAS data are of good quality. MIPAS can be operated in different measurement modes in order to optimize the scientific output. Due to the wealth of information in the MIPAS spectra, many scientific results have already been published. They include intercomparisons of temperature distributions with ECMWF data, the derivation of the whole NOy family, the study of atmospheric processes during the Antarctic vortex split in September~2002, the determination of properties of Polar Stratospheric Clouds, the downward transport of NOx in the middle atmosphere, the stratosphere-troposphere exchange, the influence of solar variability on the middle atmosphere, and the observation of Non-LTE effects in the mesosphere.

Numerical modeling of the central Black Sea ecosystem functioning during the eutrophication phase

A one-dimensional coupled physical–biogeochemical model has been developed to simulate the ecosystem of the central Black Sea at the end of the 1980s when eutrophication and invasion by gelatinous organisms seriously affected the stability and dynamics of the system. The physical model is the General Ocean Turbulence Model (GOTM) and the biogeochemical model describes the foodweb from bacteria to gelatinous carnivores through 24 state variables including three groups of phytoplankton: diatoms, small phototrophic flagellates and dinoflagellates, two zooplankton groups: micro- and mesozooplankton, two groups of gelatinous zooplankton: the omnivorous and carnivorous forms, an explicit representation of the bacterial loop: bacteria, labile and semi-labile dissolved organic matter, particulate organic matter. The model simulates oxygen, nitrogen, silicate and carbon cycling. In addition, an innovation of this model is that it explicitly represents processes in the anoxic layer. Biogeochemical processes in anaerobic conditions have been represented using an approach similar to that used in the modeling of diagenetic processes in the sediments lumping together all the reduced substances in one state variable [Soetaert, K., Herman, P., 1996. A model of early diagenetic processes from the shelf to abyssal depths. Geochimica et Cosmochimica Acta 60 (6) 1019–1040]. In this way, processes in the upper oxygenated layer are fully coupled with anaerobic processes in the deep waters, allowing to perform longterm simulations. The mathematical modeling of phytoplankton and zooplankton dynamics, detritus and the microbial loop is based on the model developed by Van den Meersche et al. [Van den Meersche, K., Middelburg, J., Soetaert, K., van Rijswijk P.H.B., Heip, C., 2004. Carbon–nitrogen coupling and algal–bacterial interactions during an experimental bloom: Modeling a 13c tracer experiment. Limnology and Oceanography 49 (3), 862–878] and tested in the modeling of mesocosm experiments and of the Ligurian sea ecosystem [Raick, C., Delhez, E., Soetaert, K., Gregoire, M., 2005. Study of the seasonal cycle of the biogeochemical processes in the Ligurian sea using an 1D interdisciplinary model. Journal of Marine Systems 55 (3–4) 177–203]. This model has been extended to simulate the development of top predators, the aggregation of detritus as well as the degradation and chemical processes in suboxic/anoxic conditions (e.g. denitrification, anoxic remineralization, redox reactions). The coupled model extends down to the sediments ( ≃ 2000 m depth) and is forced at the air–sea interface by the 6 hourly ERA-40 reanalysis of ECMWF data. The model has been calibrated and validated using a large set of data available in the Black Sea TU Ocean Base. The biogeochemical model involves some hundred parameters which are first calibrated by hand using published values. Then, an identifiability analysis has been performed in order to determine a subset of identifiable parameters (i.e. ensemble of parameters that can be together estimated from the amount of data we have at our disposal, see later in the text). Also a subset of 10 identifiable parameters was isolated and an automatic calibration subroutine (Levenberg Marquart) has been used to fine tune these parameters. Additionally, in order to assess the sensitivity of model results to the parameterization of the two gelatinous groups, Monte Carlo simulations were performed perturbing all the parameters governing their dynamics. In order to calibrate the particle dynamics and export, the chemical model was run off-line with the particle and microbial loop model in order to check its capacity of simulating anoxic waters. After a 10 4 year run, the model simulated NH 4 and H 2 S profiles similar to observations but steady state was not reached suggesting that the Black Sea deep waters are not at steady state. The fully coupled model was then used to simulate the period 1988–1992 of the Black Sea ecosystem. The model solution exhibits a complex dynamics with several years of transient adjustment. This complexity is imparted by the explicit modeling of top predators. The integrated chlorophyll and phytoplankton biomasses, the maximum concentration and depth of maximum, mesozooplankton biomass, depth of oxycline, primary production, bacterial production, surface concentrations of nutrients and plankton simulated by the model and obtained from available data analysis were compared and showed a satisfactory agreement. Also, as in the data, the model shows a continuous development of phytoplankton throughout the year, with an intense spring bloom dominated by diatoms and a fall bloom dominated by dinoflagellates. Dinoflagellates dominate from summer until late fall while small phototrophic flagellates are never dominant in terms of biomass, but are present almost throughout the year except in winter. The model simulates an intense silicate removal associated to increased diatoms blooms which were promoted by increased nutrient conditions, and by the presence of gelatinous zooplankton. This silicate pumping leads to silicate limitation of diatoms development in summer allowing the development of dinoflagellates.

Observation and analysis of a large amplitude mountain wave event over the Antarctic peninsula

We use measurements from the Atmospheric Infrared Sounder (AIRS) on the AQUA satellite to observe the 3‐dimensional structure of a gravity wave event over the Antarctic peninsula, and determine the horizontal and vertical wavelengths, propagation direction, and temperature amplitude, and from these we estimate wave momentum flux. Using theoretical knowledge of the weighting functions and radiative transfer for AIRS radiance measurements at temperature sensitive channels in the infrared, we derive a method of estimating wave temperature amplitude directly from the radiance measurements. Comparison of the radiance‐based temperature amplitudes to the temperature amplitude in AIRS retrieved temperature fields shows close agreement. Because the radiances have 3‐times better horizontal resolution than the retrievals, our analysis suggests we can routinely observe important geophysical properties of waves with horizontal wavelengths as short as 80 km using AIRS radiances. We further analyze a nearly identical wave event appearing in the European Centre for Medium Range Forecasts (ECMWF) temperature and wind fields from both assimilation and forecast data. Analysis of the ECMWF data and nearby radiosonde wind profiles allows the interpretation as a mountain wave event forced by flow over the topography of the Antarctic peninsula.

Assessing consistency between EOS MLS and ECMWF analyzed and forecast estimates of cloud ice

Cloud ice water content (IWC) from MLS retrievals and ECMWF analyses and forecasts are compared for August 2004 to July 2005. ECMWF data are sampled along MLS tracks and filtered according to MLS sensitivity. At 147 hPa, there is good spatial agreement with the analyses biased high by 10%. Over landmasses, the analyses are biased low up to 50%. This underestimation grows in the forecasts, with a 40% reduction by day 10. At 215 hPa, the analyses are biased low by 10–60%. However, at this level the forecast IWC undergoes little change. These biases, in conjunction with those in precipitation and top of the atmosphere radiative fluxes, along with consideration of the changes in vertical velocity, cumulus cloud mass flux and cloud top detrainment, indicates a systematic reduction of the modeled deep convection over the warm pool in conjunction with a weakened large‐scale circulation and enhanced upper‐level vertical stratification.

Profiling the Lower Troposphere over the Ocean with Infrared Hyperspectral Measurements of the Marine-Atmosphere Emitted Radiance Interferometer

Abstract Measurements of the spectra of infrared emission from the atmosphere were taken by a Marine-Atmospheric Emitted Radiance Interferometer (M-AERI) deployed on the NOAA ship Ronald H. Brown during the Aerosol and Ocean Science Expedition (AEROSE) in the tropical Atlantic Ocean from 29 February to 26 March 2004. The spectra are used to retrieve profiles of temperature and humidity in the lower troposphere up to a height of 3000 m. The M-AERI retrievals of the atmospheric structure require an initial guess profile. In this work, retrievals obtained from four separate initializations are compared, using 1) radiosondes launched from the Ronald H. Brown, 2) NOAA/NWS/NCEP model reanalyses, 3) ECMWF model analyses, and 4) ECMWF model forecasts. The performance of the M-AERI retrievals for all four first-guess sources is then evaluated against the radiosonde measurements. The M-AERI retrievals initialized using radiosondes reproduce the radiosonde profiles quite well and capture much of the observed vertical structure as should be expected. Of the retrievals initialized with model fields, those obtained using the ECMWF data yielded results closest to the radiosonde observations and enabled detection of the Saharan air layer (SAL) evident during AEROSE. However, the NCEP reanalysis, as well as the corresponding retrievals, failed to detect the SAL. These results demonstrate the ability of the M-AERI profile retrievals to identify the anomalous humidity distributions in the lower troposphere, but underscore the need for suitable vertical resolution in the first-guess profile used in the retrievals under such conditions.

A new concept for high resolution temperature analysis over complex terrain

Summary The analysis of high resolution temperature data over complex topography is often problematic due to the specific influence of orography and thus, requires a special methodology. The new concept of Low Level Temperature (LLT) is defined, and can be obtained when potential temperature observations are reduced to the height of the so-called Minimum Topography, a special low level topography that accentuates basins and valleys but smoothes out single summits and scarped slopes. The Vienna Enhanced Resolution Analysis (VERA) is used to produce a comprehensive set of LLT analyses over the Alpine region by evaluating three-hourly synoptic ECMWF data for the period 1980–2001. LLT fields are then evaluated climatologically in order to gain two-dimensional representations and single grid point time series. Mean LLT fields for different months and times of the day provide highly resolved spatial and temporal information on temperature fields over basins and valleys i.e., the main settlement areas in mountainous terrain.

Choosing Dome C, Antarctic Plateau as Future Astronomical Observatory

In this paper we present year-round climatological analysis made from radio-sounding measurements and ECMWF Data. And we end up with a generalized conclusion (scientific aspect) from statistical results of site-testing at Concordia concerning the feasibility of the future astronomical exploitation of Dome C.

Comparison of the Hadley cells calculated from two reanalysis data sets

The mass stream function of mean meridional circulation is calculated from the ECMWF and NCEP/NCAR reanalysis data sets using a superposition computation scheme. The comparison of results shows that the common ascending leg of the Hadley cell calculated from the ECMWF data is strong and narrow, and averagely lies more north of the equator in comparison with its counterpart from the NCEP/NCAR data, and furthermore the Hadley cell from the ECMWF data shows an obvious double-layer structure. Therefore, there are obvious differences between Hadley cells displayed by the two objective analysis data sets.

Technical note: analytical estimation of the optimal parameters for the EOF retrievals of the IASI Level 2 Product Processing Facility and its application using AIRS and ECMWF data

Abstract. The Empirical Orthogonal Function (EOF) retrieval technique consists of calculating the eigenvectors of the spectra to later perform a linear regression between these and the atmospheric states, this first step is known as training. At a later stage, known as performing the retrievals, atmospheric profiles are derived from measured atmospheric radiances. When EOF retrievals are trained with a statistically different data set than the one used for retrievals two basic problems arise: significant biases appear in the retrievals and differences between the covariances of the training data set and the measured data set degrade them. The retrieved profiles will show a bias with respect to the real profiles which comes from the combined effect of the mean difference between the training and the real spectra projected into the atmospheric state space and the mean difference between the training and the atmospheric profiles. The standard deviations of the difference between the retrieved profiles and the real ones show different behavior depending on whether the covariance of the training spectra is bigger, equal or smaller than the covariance of the measured spectra with which the retrievals are performed. The procedure to correct for these effects is shown both analytically and with a measured example. It consists of first calculating the average and standard deviation of the difference between real observed spectra and the calculated spectra obtained from the real atmospheric state and the radiative transfer model used to create the training spectra. In a later step, measured spectra must be bias corrected with this average before performing the retrievals and the linear regression of the training must be performed adding noise to the spectra corresponding to the aforementioned calculated standard deviation. This procedure is optimal in the sense that to improve the retrievals one must resort to using a different training data set or a different algorithm.

Zonally symmetric normal modes associated with the AO/NAM under a seasonally varying background climatology

This paper describes a numerical study of the role of zonally symmetric normal modes in the dynamics of the Arctic Oscillation (AO) or Northern Hemisphere Annular Mode (NAM)with seasonally varying climatological fields. Ageneral theory of the mechanism for normal mode selection involving a fully developed non-linear interaction between different modes is proposed, and this explains not only the prominence of the AO/NAM in the Northern Hemisphere, but also the existence of the Antarctic Oscillation (AAO) in the Southern Hemisphere. Following this normal mode selection mechanism, zonally symmetric normal mode with spatial structures resembling those of the AO and AAO is extracted by use of ECMWF data and the linear operator of a two-dimensional model with a seasonally varying basic state. The interannual variability of this mode is also investigated in the light of this theory.

Interannual variability of surface heat fluxes in the Adriatic Sea in the period 1998–2001 and comparison with observations

Surface heat fluxes of the Adriatic Sea are estimated for the period 1998–2001 through bulk formulae with the goal to assess the uncertainties related to their estimations and to describe their interannual variability. In addition a comparison to observations is conducted. We computed the components of the sea surface heat budget by using two different operational meteorological data sets as inputs: the ECMWF operational analysis and the regional limited area model LAMBO operational forecast. Both results are consistent with previous long-term climatology and short-term analyses present in the literature. In both cases we obtained that the Adriatic Sea loses 26 W/m 2 on average, that is consistent with the assessments found in the literature. Then we conducted a comparison with observations of the radiative components of the heat budget collected on offshore platforms and one coastal station. In the case of shortwave radiation, results show a little overestimation on the annual basis. Values obtained in this case are 172 W/m 2 when using ECMWF data and 169 W/m 2 when using LAMBO data. The use of either Schiano's or Gilman's and Garrett's corrections help to get even closer values. More difficult is to assess the comparison in the case of longwave radiation, with relative errors of an order of 10–20%.

An objective classification system of air mass types for Szeged, Hungary, with special interest in air pollution levels

This paper determines the characteristic air mass types over the Carpathian Basin for the winter (December, January, and February) and summer (June, July and August) months dependant on levels of the main air pollutants. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns with the level of air pollutants in Szeged. The data comprise daily values of twelve meteorological and eight pollutant parameters for the period 1997–2001. Objective definition of the characteristic air mass types is achieved by using the methods of Factor Analysis and Cluster Analysis. According to the results, during the winter months five air mass types (clusters) were detected based on higher concentrations of primary pollutants that occur with high irradiance and low wind speed. This is the case when an anticyclone is found over the Carpathian Basin and over the region south of Hungary, influencing the weather of the country. Low levels of pollutants occur when zonal currents exert influence over Hungary. During the summer months anticyclones and anticyclone ridge situations are found over the Carpathian Basin. (During the prevalence of anticyclone ridge situations, the Carpathian Basin is found at the edge of a high pressure centre.) As a result of high irradiance and very low NO levels, secondary pollutants are highly enriched.

Validation of the operational IASI level 2 processor using AIRS and ECMWF data

The algorithms being implemented in EUMETSAT’s IASI Level 2 Product Processing Facility are validated with real case situations using AIRS data and comparing the retrieved atmospheric states with ECMWF analyses. The tests have been performed for clear-sky ocean scenes during daytime. The Empirical Orthogonal Function (EOF) retrievals show very good performance, with retrieved atmospheric states standard deviations between 1 and 2 K in temperature and 10% and 20% in relative humidity when compared with ECMWF analysis in the troposphere. The EOF retrievals show relatively smooth profiles. Results from an iterative retrieval show a standard deviation between 2 and 3 K in temperature and 10% and 30% in relative humidity when compared with ECMWF analyses in the troposphere. They tend to show meteorologically reasonable discontinuities in both temperature and relative humidity. This seems to be the reason why they do not compare as well with ECMWF analyses as the EOF retrievals do. Whether they are closer to reality or not will have to be tested with co-located radiosondes or similar more accurate data, which generally do not exhibit such smooth vertical profiles as ECMWF analyses do.

Sensitivity of Coastal Currents near Point Conception to Forcing by Three Different Winds: ECMWF, COAMPS, and Blended SSM/I–ECMWF–Buoy Winds

Abstract Previous observational and modeling studies have indicated the importance of finescale winds in determining the circulation near Point Conception in the Santa Maria Basin (SMB) and the Santa Barbara Channel (SBC), California. There has not been a systematic attempt, however, to analyze and quantify the sensitivity of the near-surface circulation to different wind data. Here, a regional circulation model of the SMB and SBC is driven using three wind datasets: the European Centre for Medium-Range Weather Forecasts (ECMWF; ≈ 110 km × 110 km horizontal grid), the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS; 9 km × 9 km horizontal grid), and a blended wind product that combines Special Sensor Microwave Imager (SSM/I), ECMWF, and buoy and coastal wind data and that is referred to as SEB. A springtime period (March–May 1999) in which equatorward wind dominates and wind stress curls are strong is chosen for the study. Two groups of experiments are conducted: with and without assimilating moored temperature observations. The focus is on long time scales of greater than weeks and on mean currents. Comparisons between these experiments and between model and observation show that the circulation driven by the ECMWF wind is much weaker than that by the other two winds. On the other hand, the SEB dataset shows locally intensified wind stress curls behind capes and coastal bends, whereas these wind stress curls are weak in COAMPS. It is found that these small-scale variations in the wind field force alongshore inhomogeneous pressure gradients that in turn can significantly affect the near-coast currents. The result is that modeled currents forced by SEB agree better with observations than do those produced by COAMPS. Empirical orthogonal function analyses were conducted on the near-surface currents, sea level, wind, and wind stress curl. The mode-1 current (≈40%) is unidirectional (i.e., generally equatorward or poleward) and correlates well with the mode-1 wind (≈90%). The mode-2 current (≈20%) is cyclonic in the SBC and poleward inshore and equatorward offshore in the SMB; it correlates well with mode-1 sea level (≈70%), which suggests that mode-2 currents are driven by the pressure gradient. It is significant that neither mode-2 current nor mode-1 sea level correlates well with mode-1 wind stress curl (≈70%); rather, they correlate well with the time integral of the mode-1 wind stress curl. These conclusions support a previous theoretical idea that cyclonic circulation in the SBC and the inshore currents of the SMB are both driven by alongshore pressure setup induced by the time integral of the wind stress curl, rather than by the wind stress curl itself. This idea of a pressure setup is consistent with the differences found between the currents driven by COAMPS and SEB winds.

ECMWF Analyses and Forecasts of Stratospheric Winter Polar Vortex Breakup: September 2002 in the Southern Hemisphere and Related Events

Abstract Breakup of the polar stratospheric vortex in the Northern Hemisphere is an event that is known to be predictable for up to a week or so ahead. This is illustrated using data from the 45-yr ECMWF Re-Analysis (ERA-40) for the sudden warmings of January 1958 and February 1979 and operational ECMWF data for February 2003. It is then shown that a similar level of skill was achieved in operational forecasts for the split of the southern stratospheric vortex in late September 2002. The highly unusual flow conditions nevertheless exposed a computational instability of the forecast model. Analyses and forecasts from reruns using improved versions of the forecasting system are presented. Isentropic maps of potential vorticity and specific humidity provide striking pictures of the advective processes at work. Forecasts as well as analyses are shown to be in good agreement with radiosonde measurements of the temperature changes associated with vortex movement, distortion, and breakup during August and September. Forecasts from 17 September onward capture the remarkable temperature rise of about 60°C recorded at 20 hPa by the Halley radiosonde station as the vortex split. Objective forecast verification and data denial experiments are used to characterize the performance of the observing and data assimilation systems and to infer overall forecast, analysis, and observation accuracy. The observations and analyses from 1957 onward in the ERA-40 archive confirm the extreme nature of the 2002 event. Secondary vortex development by barotropic instability is also discussed; in analyses for early October 2002, the process is active in the breakup of the weaker of the two vortices formed by the late-September split.

Cut off and Weakening Processes of an Upper Cold Low

Near the tropopause over the North Pacific in summer an isolated low pressure system (Upper Cold Low, UCL) is often generated by the deepening and cutting off of a trough in the mid-latitude westerlies. The tracks and structures of these UCLs have been investigated in previous studies, but understanding of the cut off and weakening processes remains poor. In this paper, the tracks of UCLs generated in the 1999 summer are analyzed using ECMWF data. The physical processes occurring in one ofthese systems are investigated in detail using the ECMWF data, and the meso-scale model MM5. We focus particularly on cut off and weakening processes, and on the structure of the vertical velocity in the UCL.The summer of 1999 was hot over Japan, and part of the Tibetan high pressure, around 200 hPa, was shifted northward. This allowed some UCLs to approach Japan in July and August. A UCL on August 19th is selected for detailed analysis, and was generated in the following process. Positive vorticity in a westerly wave at 200 hPa was extended by a northeast wind in the upper layer only. The positive vorticity was cut off by non-liner effects and upper level divergence, associated with convective clouds, generating the isolated UCL. The structure of the cyclonic circulation and the warm and cold cores were similar to those in previous studies. The structure of the vertical motion of the moving UCL was explained by dry dynamics and there was upward motion on the front side of the UCL, in the direction of movement. Upper level clouds in the UCL strengtJhened this upward motion. Convective clouds were seen in the system. The latent heat of these convective clouds played an important role in weakening the cold core of the UCL.

Middle atmosphere summer duration as an indicator of long-term circulation changes

Summer duration (SD) is defined here as the time interval between spring and autumn turn around of zonal winds in the stratosphere. SD long-term trends are obtained from analysis of middle stratosphere NCEP and ECMWF data. They are found to be dependent on latitude and altitude. Wind data are available since 1948. The corresponding analysis suggests a breakpoint in the trend at around 1980: SD increases before 1980, and decreases afterwards. Corresponding changes of stratospheric wave activity are analyzed and found to be a major contribution to the SD trends. Long-term computer runs of the Whole Atmosphere Community Climate Model (WACCM 1b) are consistent with these results. Vegetation data on the ground indicate similar trends with a break.