UK Meteorological Office Research Articles

Technical note: An investigation of possible improvements in accuracy of regressions between diffuse and global solar irradiation

Horizontal hourly and sub-hourly diffuse and beam irradiance are required for the estimation of global irradiance on a given tilted surface. This information is used by engineers and architects for various solar energy applications and also to calculate solar heat gain in buildings. Although horizontal global irradiance is a commonly measured parameter for many sites, horizontal diffuse irradiance is not so readily obtainable. For such sites that measure global irradiation alone a simple but reasonably accurate method is required to estimate diffuse irradiance from its global counterpart. The aim of this study is to evaluate the accuracies of estimation of diffuse irradiance by developing annual, seasonal and monthly regressions between sub-hourly diffuse-ratio and clearness-index. An error analysis has been undertaken to compare these regression models. Practicalapplications: Diffuse and beam components of global irradiation are required to accurately simulate solar heat gain in buildings. The UK Meteorological office keeps records of diffuse and global irradiances for 9 stations, but there are a further 84 stations that solely record global irradiance. Hence there is a need to estimate diffuse or beam irradiance for these sites. This paper presents an investigation into the uses of annual, seasonal and monthly regressions between diffuse and global horizontal sub-hourly solar irradiation. These regressions have proven to be a simple but reasonably accurate method of estimating diffuse irradiance from its global counterpart.

Quantization analysis of weather radar data with synthetic rainfall

Quantization is a process by where continuous signals are transformed into discrete values. It is an important part of the signal processing involved in using weather radar. Technological advances have made it easier to increase the number of quantization levels, as witnessed by the replacement of a 3 bit system by an 8 bit system by the UK Meteorological Office. Research has been conducted in the past demonstrating the error statistics of quantized rainfall, although these studies have used real radar data. The novelty of this study is in using synthetic rain, generated with a Poisson cluster model to represent hourly rainfall, and subsequently disaggregated using a fractal cascade to a fine 5 min time scale. The advantage of this approach is the length of time series that can be generated far outweighs the limited duration of historical rainfall series, especially at such fine time scales. This provides sufficient rainfall data, especially high intensity rainfall, to say something statistically significant about the error statistics. The models are parameterised for different months and also for a non-seasonal set. Rainfall is then generated for a summer case, a winter case, and for the non-seasonal case. It is discovered that the error distribution varies significantly as the parameters change for 3 bit rainfall. This error distribution is relatively constant for 8 bit data, within its working range (up to 126 mm/h). At a fine time scale, such high intensity events are not uncommon. This knowledge is useful when investigating historical radar data at lower quantization levels, for the purpose of flood frequency analysis, and remains relevant, especially, if as some studies have shown, the occurrence of high intensity storms is likely to increase.

Acoustic analysis of shock production by very high-altitude meteors—I: infrasonic observations, dynamics and luminosity

Four very high-velocity and high-altitude meteors (a Leonid, two Perseids and a high-speed sporadic fireball) have been unambiguously detected at the ground both optically using precision all-sky cameras and acoustically via infrasound and seismic signals. Infrasound arriving from altitudes of over 100 km is not very common, but has been previously observed for re-entering spacecraft. This, however, is the first reported detection of such high-altitude infrasound unambiguously from meteors to our knowledge. These fragile meteoroids were found to generate acoustic waves at source heights ranging from 80 to 110 km, with most acoustic energy being generated near the lowest heights. Time residuals between observed acoustic onset and model predictions based on ray-tracing points along the photographically determined trajectories indicate that the upper winds given by the UK meteorological office (UKMO) model systematically produce lower residuals for first arrivals than those from the Naval Research Laboratory Horizontal Wind Model (HWM). Average source energies for three of the four events from acoustic data alone are found to be in the range of 2×10 8−9 J. One event, EN010803, had unusually favorable geometry for acoustic detection at the ground and therefore has the smallest photometric source energy (10 −5 kt; 6×10 7 J) of any meteor detected infrasonically. When compared to the total optical radiation recorded by film, the results for the three events produce equivalent integral panchromatic luminous efficiencies of 3–7%, within a factor of two of the values proposed by Ceplecha and McCrosky [1976. Fireball end heights—a diagnostic for the structure of meteoric material. Journal of Geophysical Research 81, 6257–6275] for the velocity range (55–70 km s −1) appropriate to our events. Application of these findings to meteor showers in general suggest that the Geminid shower should be the most prolific producer of infrasound detectable meteors at the ground of all the major showers, with one Geminid fireball producing detectable infrasound from a given location every ∼400 h of observation.

Combined analysis of rainfall and lightning data produced by mesoscale systems in the central and eastern Mediterranean

In the frame of this study, the rain and lightning activity produced by rainfall events in the central and eastern Mediterranean has been studied. The analysis focuses on the study of the rainfall events that occurred during a period of 6 months spanning from October 2003 up to March 2004. The data used consist of 6-h accumulated precipitation amounts reported by an average number of 220 rain gauges of the surface network and the lightning strokes detected by the ATD system operated by the UK Meteorological Office. It was found that for almost 50% of the cases when rain was recorded but in absence of lightnings, the 6-h accumulated precipitation was smaller or equal to 1 mm. This percentage drops to about 15% for the cases in presence of lightnings. The distribution of 6-h accumulated precipitation without lightnings presents a clear maximum at 1 mm while the distribution in presence of lightnings presents a clear maximum at 10 mm and an important swift towards larger precipitation amounts.

Resampling of regional climate model output for the simulation of extreme river flows

The objective of this paper is to investigate whether resampling of the output from a regional climate model (RCM) can provide realistic long-duration sequences of precipitation and temperature for the simulation of extreme river flows. This is important to assess the impact of climate change on river flooding. Daily streamflows of the river Meuse in western Europe are considered. Resampling is performed with a nearest-neighbour technique that was already successfully applied to the observed daily precipitation and temperature in the river basin. Streamflows are simulated with the semi-distributed HBV rainfall-runoff model. Two simulations of the KNMI regional climate model RACMO are considered. One of these simulations is driven by the global atmospheric model HadAM3H of the UK Meteorological Office for the period 1961-1990 and the other by ERA40 re-analysis data. Much attention is given to the bias correction of RACMO precipitation. It was found that a relatively simple nonlinear correction adjusting both the biases in the mean and variability led to a better reproduction of observed extreme daily and multi-day precipitation amounts than the commonly used linear scaling correction. This also resulted in more realistic discharge extremes, suggesting that a correct representation of the variability of precipitation is important for the simulation of extreme flood quantiles. For the Meuse basin it is further shown that it is advantageous to correct for the variability of the 10-day precipitation amounts rather than that of the daily amounts. Despite the remaining biases in the RCM data, the simulated extreme flood quantiles correspond quite well with those obtained using observed precipitation and temperature.

Ambient noise levels in north central Italy

The characteristics of background seismic noise in north central Italy have been investigated by means of velocity power spectral analysis within the frequency range 0.1–15 Hz. The method proposed by McNamara and Buland (2004) has been applied to estimate the probability density function (PDF) of power spectra computed for ten different stations. Since the target region is the most industrialized area of Italy, a large variability among the power spectra for different sites is observed in the frequency range 1–15 Hz, with the noise levels at two stations exceeding the New High Noise Model (NHNM) of Peterson (1993). The 95th percentile of the PDF varies from −165 to −125 dB (relative to (m/s)2/Hz). This variability could significantly affect the detection capabilities of a network installed for recording the small to moderate size seismicity occurring in north central Italy. We also observed that the dispersion of the powers, estimated at each site as the difference between the 95th and the 5th percentiles, shows a positive trend with frequency that can be ascribed to the diurnal variation of the background noise levels. In the frequency range 0.1–1 Hz, the dominant feature is the double frequency (DF) peak of microseisms generated by oceanic storms. At one of the considered stations, the seasonal variability of the maximum amplitude of the DF peak has been observed in the period from April 2004 to December 2005. Considering the barometric maps provided by the UK Meteorological Office, we observed that the strongest powers in the range 0.10–0.25 Hz occur when intense storms are present over the North Atlantic Ocean, whereas the measurements of the height, frequency, and azimuth of the sea waves at two buoys of the Rete Ondametrica Italiana deployed in the Adriatic and Tyrrhenian seas suggest that the DF microseisms in the frequency range 0.25–0.50 Hz are generated by storms over the Mediterranean Sea. Finally, the analyzed region is characterized by two large‐scale geologic features, namely, the Po Plain and the Alps. The impedance contrast between these two units causes a noise reduction of about 10 dB in the frequency range 0.2–0.6 Hz.

Vertically integrated moisture flux convergence as a predictor of thunderstorms

Vertically Integrated Moisture Flux Convergence (VIMFC) alone and in combination with the lifted stability index of the most unstable layer (SMUL) is evaluated as a thunderstorm predictor. By using six-hourly standard pressure weather analysis data from the European Centre for Medium-range Weather Forecasts (ECMWF) during 30 days in the summers of 1992 and 1994 containing several severe weather events along with quiescent events in northwestern Europe 17,206 events are obtained. The location and time of a lightning discharge are obtained from the Arrival Time Difference (ATD) sferics lightning location system from the UK Meteorological Office. Using the Heidke Skill Score (HEIDKE) to determine the best threshold we conclude that VIMFC alone, does not perform well as a dichotomous thunderstorm predictor compared to the stability index. However, the Thundery Case Probability (TCP) tested as function of VIMFC results in a high correlation with thunderstorms. By combining SMUL and VIMFC the surplus value as a thunderstorm predictor of VIMFC was established. TCP percentages up to 95% were found in an unstable environment with high positive values of VIMFC. In a marginally unstable environment with a high positive VIMFC the thunderstorm probability is higher than in a very unstable environment with no or negative VIMFC. These results are illustrated with a study of the case of the disastrous flash flood at Vaison-La-Romaine (southeastern France) on September 22, 1992. Although latent instability was present in a large area surrounding Vaison-La-Romaine, nearly all and especially the most severe thunderstorm activity occurred within the smaller area with positive VIMFC and latent instability.

The influence of the atmospheric boundary layer on nocturnal layers of noctuids and other moths migrating over southern Britain

Insects migrating at high altitude over southern Britain have been continuously monitored by automatically operating, vertical-looking radars over a period of several years. During some occasions in the summer months, the migrants were observed to form well-defined layer concentrations, typically at heights of 200-400 m, in the stable night-time atmosphere. Under these conditions, insects are likely to have control over their vertical movements and are selecting flight heights that are favourable for long-range migration. We therefore investigated the factors influencing the formation of these insect layers by comparing radar measurements of the vertical distribution of insect density with meteorological profiles generated by the UK Meteorological Office's (UKMO) Unified Model (UM). Radar-derived measurements of mass and displacement speed, along with data from Rothamsted Insect Survey light traps, provided information on the identity of the migrants. We present here three case studies where noctuid and pyralid moths contributed substantially to the observed layers. The major meteorological factors influencing the layer concentrations appeared to be: (a) the altitude of the warmest air, (b) heights corresponding to temperature preferences or thresholds for sustained migration and (c) on nights when air temperatures are relatively high, wind-speed maxima associated with the nocturnal jet. Back-trajectories indicated that layer duration may have been determined by the distance to the coast. Overall, the unique combination of meteorological data from the UM and insect data from entomological radar described here show considerable promise for systematic studies of high-altitude insect layering.

Twentieth century simulation of the southern hemisphere climate in coupled models. Part 1: large scale circulation variability

The ability of five, global coupled climate models to simulate important atmospheric circulation characteristics in the Southern Hemisphere for the period 1960–1999 is assessed. The circulation features examined are the Southern Hemisphere annular mode (SAM), the semi-annual oscillation (SAO) and the quasi-stationary zonal wave 3 (ZW3). The models assessed are the National Center for Atmospheric Research Community Climate System Model Version 3 (CCSM3), the Commonwealth Scientific and Industrial Research Organisation Mark 3, the Geophysical Fluid Dynamics Laboratory Model, the Goddard Institute for Space Studies Model ER (GISS-ER) and the UK Meteorological Office Hadley Center Coupled Model Version 3. The simulations were compared to the NCAR–NCEP reanalyses. The models simulate a SAO which differs spatially from the observed over the Pacific and Indian oceans. The amplitudes are too high over the southern ocean and too low over the midlatitudes. These differences are attributed to a circumpolar trough which is too deep and extends too far north, and to the inability of the models to simulate the middle to high latitude temperature gradient. The SAM is well-represented spatially by most models but there are important differences which may influence the flow over the Pacific and in the region extending from the Ross to Weddell Seas. The observed trend towards positive polarity in the SAM is apparent in the ensemble averages of the GISS-ER and CCSM3 simulations, suggesting that the trend is due to external forcing by changes in the concentration of ozone and greenhouse gases. ZW3 is well-represented by the models but the observed trend towards positive phases of ZW3 is not apparent in the simulations suggesting that the observed trend may be due to natural variability, not external forcing.

Vertical structure of extreme currents in the Faroe-Bank Channel

Abstract. Extreme currents are studied with the aim of understanding their vertical and spatial structures in the Faroe-Bank Channel. Acoustic Doppler Current Profiler time series recorded in 3 deployments in this channel were investigated. To understand the main features of extreme events, the measurements were separated into their components through filtering and tidal analysis before applying the extreme value theory to the surge component. The Generalized Extreme Value (GEV) distribution and the Generalized Pareto Distribution (GPD) were used to study the variation of surge extremes from near-surface to deep waters. It was found that this component alone is not able to explain the extremes measured in total currents, particularly below 500 m. Here the mean residual flow enhanced by tidal rectification was found to be the component feature dominating extremes. Therefore, it must be taken into consideration when applying the extreme value theory, not to underestimate the return level for total currents. Return value speeds up to 250 cm s–1 for 50/250 years return period were found for deep waters, where the flow is constrained by the topography at bearings near 300/330° It is also found that the UK Meteorological Office FOAM model is unable to reproduce either the magnitude or the form for the extremes, perhaps due to its coarse vertical and horizontal resolution, and is thus not suitable to model extremes on a regional scale. Keywords. Oceanography: Physical (Currents; General circulation; General or miscellaneous)

The 11-year solar-cycle effects on the temperature in the upper-stratosphere and mesosphere: Part I—Assessment of observations

Three independent temperature datasets have been analyzed for quantifying the influence of the 11-year solar cycle modulation of the UV radiation. The datasets used include: US rocketsondes, the OHP lidar, and the global temperature database made by the successive SSU on the NOAA satellites, adjusted and provided by the UK Meteorological Office. These measurements cover the upper stratosphere and the mesosphere, where the direct photochemical effect is expected. The improvement of the analysis compared to previous ones was possible because the overall quality and the continuity of many data series have been checked more carefully during the last decade in order to look for anthropogenic fingerprints and the one used here have been recognized as the best series according to their temporal continuity. The analysis of the different data set is based on the same regression linear model. The 11-year solar temperature response observed presents a variable behavior, depending on the location. However, an overall adequate agreement among the results has been obtained, and thus the global picture of the solar impact in the upper stratosphere and lower mesosphere has been obtained and is presented here. In the tropics, a 1–2 K positive response in the mid and upper stratophere has been found, in agreement with photochemical theory and previous analyses. On the opposite, at mid-latitudes, negative responses of several Kelvin have been observed, during winters, in the analyses of the datasets analyzed here. In the mesosphere, at sub-tropic and mid-latitude regions, we observe a positive response all the year round increasing by a factor of two during winter.

Dangerous climate change

The UK government and meteorological office are together hosting a conference to discuss "dangerous" climate change. Nicola Jones reports back from the met office headquarters in Exeter.

Seasonal cycles of mixing ratio and 13C in atmospheric methane at Suva, Fiji

A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08′S, 178°26′E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid‐2002, have been analyzed for methane mixing ratio and δ13C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Niña conditions. A modeling study was carried out using a modified version of the UK Meteorological Office's Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its δ13C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Niña conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in δ13C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.

A probabilistic description of the wind over Liverpool Bay with application to oil spill simulations

Surface winds from the UK Meteorological Office mesoscale (12 km grid) atmospheric model have been used to define the wind at a location in Liverpool Bay during 1997–2001. Winds from the SW (centred on 240°) with a speed of about 10 m/s (20 knots) were the most frequent, although weaker winds from the SE were also common. The wind spectra were red in character and showed no evidence for a peak at the synoptic (2–5 day) time scale; however, a zero-up-crossing analysis suggested a dominant periodicity at 3.1 days, and at this time scale the winds were spatially coherent over a distance of 300 km. A wind direction transition matrix was derived to quantify the probability with which the wind changed between two specified directions. This information was then used with an estimate of the mean duration of a wind event to compute a stochastic wind time series that contained a similar energy level, periodicity, and direction variability to the archived wind data. The archived and stochastic winds were then used in 1000 oil spill contingency simulations during which estimates of the mean and minimum times taken for oil to reach the coastline, and the percentage of the oil impacting selected sites were computed. The stochastic winds provided more realistic results, when compared against those derived using the wind archive, than those obtained using a wind rose representation of the winds. The derivation and use of a stochastic wind time series has application to a range of modelling studies.

Refinements in the use of equivalent latitude for assimilating sporadic inhomogeneous stratospheric tracer observations, 1: Detecting transport of Pinatubo aerosol across a strong vortex edge

Abstract. The use of PV equivalent latitude for assimilating stratospheric tracer observations is discussed - with particular regard to the errors in the equivalent latitude coordinate, and to the assimilation of sparse data. Some example measurements are assimilated: they sample the stratosphere sporadically and inhomogeneously. The aim was to obtain precise information about the isentropic tracer distribution and evolution as a function of equivalent latitude. Precision is important, if transport across barriers like the vortex edge are to be detected directly. The main challenges addressed are the errors in modelled equivalent latitude, and the non-ideal observational sampling. The methods presented allow first some assessment of equivalent latitude errors and a picture of how good or poor the observational coverage is. This information determines choices in the approach for estimating as precisely as possible the true equivalent latitude distribution of the tracer, in periods of good and poor observational coverage. This is in practice an optimisation process, since better understanding of the equivalent latitude distribution of the tracer feeds back into a clearer picture of the errors in the modelled equivalent latitude coordinate. Error estimates constrain the reliability of using equivalent latitude to make statements like "this observation samples air poleward of the vortex edge" or that of more general model-measurement comparisons. The approach is demonstrated for ground-based lidar soundings of the Mount Pinatubo aerosol cloud, focusing on the 1991-92 arctic vortex edge between 475-520K. Equivalent latitude is estimated at the observation times and locations from Eulerian model tracers initialised with PV and forced by UK Meteorological Office analyses. With the model formulation chosen, it is shown that tracer transport of a few days resulted in an error distribution that was much closer to Gaussian form, although the mean error was not significantly affected. The analysis of the observations revealed a small amount of irreversible transport of aerosol across the vortex edge during late January 1992, coincident with a strongly disturbed vortex.

A new general circulation model of Jupiter's atmosphere based on the UKMO Unified Model: Three-dimensional evolution of isolated vortices and zonal jets in mid-latitudes

We have been developing a new three-dimensional general circulation model for the stratosphere and troposphere of Jupiter based on the dynamical core of a portable version of the Unified Model of the UK Meteorological Office. Being one of the leading terrestrial GCMs, employed for operational weather forecasting and climate research, the Unified Model has been thoroughly tested and performance tuned for both vector and parallel computers. It is formulated as a generalized form of the standard primitive equations to handle a thick atmosphere, using a scaled pressure as the vertical coordinate. It is able to accurately simulate the dynamics of a three-dimensional fully compressible atmosphere on the whole or a part of a spherical shell at high spatial resolution in all three directions. Using the current version of the GCM, we examine the characteristics of the Jovian winds in idealized configurations based on the observed vertical structure of temperature. Our initial focus is on the evolution of isolated eddies in the mid-latitudes. Following a brief theoretical investigation of the vertical structure of the atmosphere, limited-area cyclic channel domains are used to numerically investigate the nonlinear evolution of the mid-latitude winds. First, the evolution of deep and shallow cyclones and anticyclones are tested in the atmosphere at rest to identify a preferred horizontal and vertical structure of the vortices. Then, the dependency of the migration characteristics of the vortices are investigated against modelling parameters to find that it is most sensitive to the horizontal diffusion. We also examine the hydrodynamical stability of observed subtropical jets in both northern and southern hemispheres in the three-dimensional nonlinear model as initial value problems. In both cases, it was found that the prominent jets are unstable at various scales and that vorteces of various sizes are generated including those comparable to the White Ovals and the Great Red Spot.

Solar forcing of climate: model results

The role of stratospheric ozone changes in determining the climate response to solar forcing is investigated using a version of the Unified Model from the UK Meteorological Office which includes a mixed-layer ocean of constant depth (HadSM3). Two experiments have been performed, both of which include a wavelength-dependent reduction in total solar irradiance (TSI) of 7.5 W m −2 (0.55%). The second experiment includes, in addition, an estimate of the accompanying stratospheric ozone changes. A large change in TSI is used (approximately a factor of two greater than the `best-guess' change between present mean levels and the Maunder Minimum) to demonstrate the sensitivity of the climate system to this forcing. Results show that in the annual mean, the temperature response of the model is enhanced by the inclusion of the ozone changes, by approximately 15–20%. We compare results from our TSI and ozone experiment to those of Shindell et al. [Science 294 (2001) 2149] who performed a similar study with the GISS GCM. Temperature changes are greater in our simulation, as expected from the larger magnitude forcing, however the circulation response is very different: our results do not resemble the Arctic Oscillation, whilst those of Shindell et al. [loc. cit.] project strongly onto this leading mode of variability. The lack of a fully resolved stratosphere in our model is a potential reason for this distinction. To test this possibility, we repeated our combined irradiance and ozone experiment using identical model formulations, but with different vertical extents: the first extends to a height of 5 hPa, the second to 0.01 hPa. Both simulations produce a relatively weak surface pressure response to solar forcing that does not strongly resemble the Arctic Oscillation.

Prediction of nearshore wave energy distribution by analysis of numerical wave model output, East Sussex coastline, UK

Abstract Prediction of wave energy distribution in coastal areas is necessary if an assessment of the likelihood of cliff collapse is to be undertaken. Use has been made of numerical modelling to predict relative wave heights along the chalk cliff coastline of East Sussex between Brighton and Eastbourne, UK. In this study, wave modelling has been undertaken using the University of Delaware REFDIF-1 software with a 100m mesh size to predict nearshore wave heights for boundary unit wave height conditions from a range of different incident directions. The results from this wave modelling have been combined with the frequency distribution of incident waves obtained from analysis of time series of 12 years of hindcast wave data in the English Channel, obtained from the UK Meteorological Office. The resulting distribution of nearshore wave heights is presented as a surrogate for the distribution of wave energy over the 12-year period. Some concern exists about the quality of the output data, in particular of the effect of the relatively coarse bathymetry grid used for the model. Some wave focusing is evident from the model output, caused by the presence of local shoals in the model grid, leading to a ‘banding’ effect in the model output. Some suggestions are made for the improvement of the modelling scheme, including the use of finer mesh size, bathymetric smoothing and the use of a spectral model such as REFDIF-S.

Analysis of the Algerian severe weather event in November 2001 and its impact on ozone and nitrogen dioxide distributions

An analysis of the severe weather event in November 2001 over the western Mediterranean is presented focusing on satellite-based trace gas measurements from the Global Ozone Monitoring Experiment (GOME) on board the European Remote Sensing Satellite (ERS-2). This study is supplemented by a synoptic analysis and simulations of the three-dimensional stratospheric chemical transport model ROSE. Arctic air masses moved rapidly from Scandinavia to the Iberian peninsula and were mixed with subtropical air over the still warm Mediterranean Sea. This caused severe thunderstorms and extreme rainfall along the coasts of Morocco and Algeria and later on the Balearic Islands. Associated with the meridional transport an intrusion of stratospheric air below 3 km above sea level was observed. The maximum potential vorticity (PV) derived from UK Meteorological Office analysis data was about 9.3 potential vorticity units (pvu) at 330 K at the equatorward position of 35°N. These very high values went along with remarkably enhanced total ozone levels obtained from GOME backscatter measurements of collocated GOME/ERS-2 overpasses. Further investigation of GOME data showed unusually high levels of nitrogen dioxide (NO 2 ) above the western Mediterranean. We present a new method to derive the tropospheric content of nitrogen dioxide (NO 2 ) from a combination of satellite measurements and results of a chemical transport model. We show that about two-third of the total atmospheric content of nitrogen dioxide in the observed plume is found in the troposphere, due to lightning activity, advection and vertical transport in the thunderstorms from the planetary boundary layer (PBL) to atmospheric levels above clouds. DOI: 10.1034/j.1600-0889.2003.00078.x

An improved description of soil hydraulic and thermal properties of arctic peatland for use in a GCM

AbstractThe UK Meteorological Office Surface Exchange Scheme (MOSES), which is currently implemented within Version 3 of the Hadley Centre GCM, was tested for an arctic peatland site in northern Finland (Kevo). This implementation of MOSES incorporated a new depth‐dependent parameterization of the thermal and hydraulic properties of peat with parameter values derived from measurements reported in the literature. The effect of increasing the number of model soil layers from four to 13 shallower layers was also investigated. Driving data were used that were collected during June, when the peat was still frozen below about 80 mm, to September 1997.Best model performance was given by the 13‐layer, depth‐dependent parameter description for both surface heat fluxes and soil temperatures. The simulated heat fluxes compared well with measurements, but simulated surface temperatures were too high. In preliminary runs the simulated distribution of unfrozen water in the soil was also unrealistic. In particular the model was unable to predict the rapid transition to above‐freezing conditions that occurred throughout the soil profile about the second week in July. Adjusting a parameter (k) of the soil‐freezing curve, which for peat can be used as a fitting parameter, produced a big improvement in the soil temperature profiles. A more accurate simulation of the freezing and thawing behaviour of organic soils requires that the processes that are hidden in the modified value of k are explicitly represented. Copyright © 2003 John Wiley & Sons, Ltd.