bluuebunny/crossref_metadata_2025 huggingface dataset:
A broad‐scale circulation index for the interannual variability of the Indian summer monsoon
Authors: Goswami B. N. , Krishnamurthy V. , Annmalai H.
Published: February 2007

A broad‐scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain‐gauge data with satellite estimates of precipitation for the period 1979‐96, it is shown that the variability of precipitation on seasonal to interannual time‐scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E–110°E, 10°N–30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June‐to‐September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north‐western Pacific region (110°E–160°E, 10°N–30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E–110°E, 10°S–5°N).

It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. an index of the monsoon Hadley (MH) circulation is defined as the meridional wind‐shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR, Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium‐Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi‐biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Niño and Southern Oscillation during the latest eight years.
Slow and ultraslow equatorial kelvin waves: The UARS‐CLAES view
Authors: Canziani Pablo O.
Published: February 2007

Temperature retrievals from the Cryogenic Limb Array Etalon Spectrophotometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) are used to determine the behaviour of the slower Kelvin‐wave modes in the lower and middle stratosphere. the CLAES dataset permits their study during the easterly phase of the quasi‐biennial oscillation and the onset and descent of the westerly phase.

Given the sampling and operational characteristics of the CLAES, which result in breaks in the time series, it was necessary to implement unequally spaced data series spectral methods, as an intermediate step in the application of the Hayashi method for bi‐dimensional spectral analysis. Comparisons with results for fast and ultrafast Kelvin waves obtained from retrievals by other sensors on board the UARS are very good, confirming the validity of this approach, which does not require interpolations of any kind to fill in the missing data before the use of the spectral methods.

The wide‐band reconstruction of the time series allowed the observation of slow Kelvin waves (approximately 15‐day periods, zonal wave numbers 1 and 2) and ‘ultraslow’ Kelvin waves (approximately 25 to 30‐day periods, zonal wave number 1) in the lower stratosphere, for the period January 1992‐May 1993. Slow wave‐number‐2 events occur at the same time as ‘ultraslow’ wave events, in a way similar to that previously observed between fast and ultrafast Kelvin waves in the middle and upper stratosphere. It was possible to determine the behaviour of ‘ultraslow’ Kelvin waves in the CLAES observation period. These waves had only been previously reported in radiosonde data.

Results for the slow Kelvin waves are in good agreement with previous observations, mostly obtained from radiosonde data. the effect of the shears, both vertical and meridional, upon the vertical propagation of the observed Kelvin waves is discussed.
Severe flooding over central and southern greece associated with pre‐cold frontal orographic lifting
Authors: Kotroni V. , Lagouvardos K. , Kallos G. , Ziakopoulos D.
Published: February 2007

An extreme rainfall event, associated with a cold front, which occurred on 11‐12 January 1997 over Greece is investigated through observational analysis and numerical simulations. the analysed storm system produced large amounts of rainfall, with devastating impacts over central and southern Greece, where at some sites 24‐hour accumulated precipitation exceeded 300 mm. the model analysis was performed with the three‐dimensional non‐hydrostatic Colorado State University Regional Atmospheric Modeling System. the model results caught the larger‐scale features present in the observational analyses while developing more realistic finer‐scale features. Sensitivity tests are also performed in order to investigate the role of the topography of the Greek peninsula on the enhancement of convection. Indeed, the orientation of the flow was optimal for advection of moist air with a maximized fetch over the sea, and at an angle of impact optimum for orographic lifting ahead of the main topographic axes of continental Greece.
Observations of oblate hail using dual polarization radar and implications for hail‐detection schemes
Authors: Smyth T. J. , Blackman T. M. , Illingworth A. J.
Published: February 2007

Coincidental radar measurements and in situ ground‐truth observations of large, oblate hail has allowed new insights into the fall mode of oblate hailstones. the effect of the fall mode upon hail‐detection algorithms using multi‐parameter radar is investigated and discussed, as is the vexed problem of extracting realistic rainfall rates in regions where hail and rain coexist. Observations presented here show that oblate hail can be associated with nonzero values of differential reflectivity, Zdr, which, together with the coincidental values of the linear depolarization ratio, suggest some degree of alignment. Although large hail can dominate backscatter through the diameter‐to‐the‐sixth‐power dependence, even if it is aligned it makes a negligible contribution to specific differential phase, Kdp. Rainfall rates extracted using Kdp in the presence of hail are more robust than those using the contaminated reflectivity in the horizontal polarization, Zh, and Zdr measurements, but proposed hail‐detection algorithms which compare the observed values of Kdp and Zh do seem error prone. the hailstones measured in this study were of sufficient dimensions to allow Mie scattering to occur at S‐band (wavelength λ = 9.75 cm). This served to complicate the interpretation of the differential‐phase measurements. A new hail‐detection algorithm has been formulated which compares the differential phase predicted from Zh and Zdr with that actually observed. This avoids the problem of estimating Kdp from the derivative of a noisy differential‐phase profile.
The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK meteorological office unified model
Authors: Martin G. M.
Published: February 2007

The quality of the simulation of the Asian summer monsoon in the climate version of the UK Meteorological Office's Unified Model, and the impact upon this of increased horizontal resolution, is investigated using two atmosphere‐only model runs forced with observed sea surface temperatures (SSTs) and sea ice extents. the runs each cover the period from 1979 to 1988, but have different horizontal resolutions, with one at climate resolution (2.5° latitude by 3.75° longitude, about 300 km at midlatitudes) and the other at global forecast model resolution (0.833° latitude by 1.25° longitude, approximately 100 km at midlatitudes).

The characteristic monsoon circulation and the spatial distribution of precipitation are in good agreement with observations. However, the model has a tendency to overestimate the strength of the monsoon, and also exhibits an early monsoon onset. the large‐scale interannual variations in circulation appear to be simulated reasonably well (as far as can be determined using this short dataset), although the magnitude of the interannual variability of precipitation is overestimated. However, the regional circulation and precipitation changes between El Niño and La Niña years show some significant differences between the model and the observations. the dominant mode of intraseasonal variability seen in both model simulations is, in agreement with observations, associated with the active/break cycle of the monsoon (although this only explains about 10% of the total variance in both simulations). There is some evidence that the SST changes associated with El Niño may produce a coherent forcing of the secondary (east‐west) mode of intraseasonal variability during the onset phase of the monsoon in the model. However, comparison with observations suggests that this may not be representative of what occurs in the real atmosphere. There is no evidence that the SST variations are causing the system to prefer either the active or the break monsoon phase, as was suggested by Palmer.

With increased model horizontal resolution, extra detail is provided in the precipitation distribution, but the mean monsoon simulation is scarcely altered and the systematic errors remain. the interannual variations in circulation and precipitation appear not to be greatly altered, and the overall pattern of intraseasonal variability is also unaffected. This study suggests that the systematic errors in the monsoon simulation are not a result of poor horizontal resolution, but may be due to problems with the model physics.
Soil‐moisture nudging experiments with a single‐column version of the ECMWF model
Authors: Hu Yuchun , Gao Xiaogang , Shuttleworth W. James , Gupta Hoshin , Mahfouf Jean‐Francois , Viterbo Pedro
Published: February 2007

The soil‐moisture nudging technique suggests using model forecast errors in near‐surface air temperature and relative humidity to re‐initialize (update) soil moisture in atmospheric models. This study investigates the application of soil‐moisture nudging using a single‐column version of the European Centre for Medium‐Range Weather Forecasts (ECMWF) model. the model was applied at 16 sites selected to sample a range of climates and land covers across the globe, with atmospheric forcing taken from the ECMWF operational analysis for 15 June 1994 and 15 December 1994. When observation errors are set to zero, the Optimal Interpolation technique for deriving estimates of nudging coefficients shows computational instability because strong correlation between near‐surface temperature and near‐surface relative humidity forecast errors makes the coefficient matrix of the linear equations ill‐posed. Therefore, Principal Component Analysis (PCA) is used as a pre‐processor to identify the independent and dependent components of temperature and relative humidity errors. With PCA, the soilmoisture nudging coefficients appropriate to the dominant principal component become stable and effective for correcting soil moisture within days. Such coefficients, when derived for the northern hemisphere summer over the 16 sites, are sufficiently consistent to propose using their all‐site, daily‐average values in a globally applicable soil‐moisture analysis scheme. Tests of this method at the First ISLSCP (International Satellite Land Surface Climatology Program) Field Experiment (FIFE) site confirm that soil‐moisture nudging can provide good estimates of near‐surface weather variables and surface fluxes in a numerical weather prediction (NWP) model which gives poor simulation of precipitation (thereby poor soil moisture). However, the PCA method is unable to give an accurate determination of the location of the soil moisture within soil layers that are accessible to the atmosphere. Further, these tests show that, if the NWP model has good simulation of precipitation but poor simulation of surface radiation, soil‐moisture nudging could wrongly vary the soil moisture so as to provide good estimates of the surface sensible‐heat flux, but poor estimates of surface latent‐heat flux. an approach to distinguish error sources from surface radiation and soil moisture is necessary for further improvement.
A similarity hypothesis for shallow‐cumulus transports
Authors: Grant A. L. M. , Brown A. R.
Published: February 2007

A similarity theory for cumulus convection is proposed, and applied to the problem of cumulus transports, using data from several large‐eddy simulations to test the theory. the parameters in the similarity hypothesis include the cloud‐base mass flux, the buoyancy of a parcel undergoing reversible ascent, through the cloud layer and the depth of the cloud layer. Using arguments based on the turbulence kinetic‐energy budget a velocity‐scale is derived, in addition to the conventional Convective Available Potential Energy scale. This new scale is analogous to the convective velocity‐scale used in boundary‐layer theories, but incorporates the effects of latent‐heat release. the ratio between the cloud‐base mass flux and the velocity‐scale is found to be a key parameter in describing cumulus convection.

It is shown that the similarity hypothesis can be applied to mass‐flux schemes to determine the fractional entrainment rate. For this the entrainment rate is assumed to be related to the rate of generation of turbulent kinetic energy in the cloud ensemble. the resulting scaling is tested against fractional entrainment rates diagnosed from the large‐eddy simulations.
Momentum budgets over idealized orography with a non‐hydrostatic anelastic model. I: Two‐dimensional flows
Authors: Héreil Philippe , Stein Joël
Published: February 2007

Numerical simulations of two‐dimensional (2D) mountain waves are carried out with a non‐hydrostatic anelastic model to investigate the interaction between 2D nonlinear orographic effects and the synoptic flow. the problem is idealized to isolate the essential features of real orographic flows. For each simulation, the complete momentum equations of the model are summed in different domains of variable size representative of a grid mesh of a general‐circulation model. the resulting budgets of momentum, instantaneous or averaged in time, are used to test the hypothesis of current drag parametrizations in three nonlinear regimes of 2D orographic flows. For mountain waves breaking in the troposphere, the mean flow is decelerated in the vertical between the ground and the theoretical breaking level. In the horizontal, the deceleration of the large‐scale flow is uniformly distributed by non‐resolved acoustic waves. When non‐Boussinesq effects lead mountain waves to break in the lower stratosphere, the large‐scale flow is decelerated on a vertical wavelength centred on the breaking level. In the case of non‐breaking trapped lee waves, the deceleration of the mean flow is strongly dependent of the non‐hydrostatic character of the primary propagating wave. Off‐line tests of two drag parametrizations show that some adaptations are necessary to improve the prediction of the impact of subgrid‐scale orographic effects on the large‐scale flow for the set of investigated idealized orographic events.
A dynamic chemical model of bi‐directional ammonia exchange between semi‐natural vegetation and the atmosphere
Authors: Flechard C. R. , Fowler D. , Sutton M. A. , Cape J. N.
Published: February 2007

A mechanistic, dynamic compensation point model to simulate the vegetation/atmosphere exchange of ammonia (NH3) is described. the model is applied to long‐term micrometeorological measurements of NH3 exchange obtained over moorland in southern Scotland (1995‐96). the model describes the gaseous bi‐directional exchange between the atmosphere, leaf surface water films, plant stomata and apoplast. A simple chemistry module is included to simulate the exchange of water‐soluble atmospheric pollutants at the air‐water interface on wet plant surfaces. Initialization of the chemistry module is achieved during rain events using measured rain chemical composition. the exchange of NH3 with stomata is based on the concept of a stomatal compensation point parametrized by means of the apoplast ammonium/hydronium I ratio. the trans‐cuticular transfer of ammonium may also constitute a sink for dissolved ammonia on plant cuticular water films, and is parametrized using a trans‐cuticular resistance and the concentration difference between leaf surface water and the apoplast. the leaching of base cations from the inside of the plant to foliar surfaces is simulated in a similar fashion to ammonium transfer, and the heterogeneous oxidation of sulphur dioxide (SO2) in thin water films is also treated. Numerical iterative procedures at each time‐step allow the calculation of pH and dissolved ion concentrations.

Modelled NH3 fluxes were compared with 3259 half‐hourly micrometeorological measurements over moorland, and with two existing modelling approaches, the static canopy compensation point model and the canopy resistance model. the dynamic and static canopy compensation point models both gave long‐term estimates of the NH3 dry deposition flux to moorland within 10% of actual measurements, while the canopy resistance approach overestimated deposition by about 30%. the dynamic model performed best during wet conditions for which it was designed, and performed reasonably well during dry conditions using a more empirical resistance approach.

The model was also capable of simulating SO2 dry deposition fluxes to within 20% of measured fluxes. the model provides a tool that may also be used to simulate scenarios, whereby NH3/SO2 concentration ratios in the atmosphere vary, and examine ‘co‐deposition’ interactions of these two species.
A case‐study of pronounced perturbations to cloud properties and boundary‐layer dynamics due to aerosol emissions
Authors: Taylor J. P. , Ackerman A. S.
Published: February 2007

During June 1994 the UK Meteorological Office C‐130 aircraft took part in the Monterey Area Ship Tracks experiment. On 13 June 1994 a merchant vessel, the Sanko Peace, was observed steaming below a layer of broken stratus clouds in a shallow (300 m deep) boundary layer. These clean maritime clouds were significantly modified by the aerosol emissions from the ship. Droplet concentrations increased from around 10 cm−3 in the background cloud to a peak of 120 cm−3 in the ship track. This resulted in a roughly 50% reduction in average droplet size and drizzle flux. Less than an hour downwind of the Sanko Peace, the cloud top in the ship track was observed to be elevated by 100 m above the surrounding cloud tops.

A one‐dimensional (1‐D) model with size‐resolved cloud microphysics, a detailed radiation code, and a turbulence closure scheme is used to simulate the observed ship track. the model predicts the observed microphysical changes reasonably well and also predicts a deepening of the boundary layer, though the simulated deepening is considerably slower than was observed. the driving force in the modelled response is an increased buoyancy flux due to enhanced radiative cooling in the cloud layer. the modelled boundary layer deepens in response to an increase in cloud water, which results from suppression of the drizzle flux. Differences between the modelled and observed profiles of cloud water in the ship track are difficult to reconcile with the modelled mechanism for the deepening, and are attributed to shortcoming in the 1‐D model representation.
Cloud bands induced by isolated mountains
Authors: Sang J. G.
Published: February 2007

The wave equation is solved analytically to obtain the solutions of diverging types of trapped waves, caused by an isolated hill, at the top of a convective boundary layer with an overlying stable layer above. Wind veering in the upper stable layer favours the wave trains to be to the left of the mean wind direction. the elevated lifting latent heat may make the crest line of the diverging wave a diabatic heating band, if the water vapour supply is sufficient.

Analytical solution is also used to analyse the effects of a heating band, which makes a small angle with the mean wind direction. the trapped components of the induced waves may spread horizontally and excite wave trains running parallel to the original heating band. the untrapped waves hold an in‐phase relation with the heating. the resultants of trapped and untrapped waves possess multiscale structure and properties similar to those of cloud streets. These analyses may provide an explanation for the formation and propagation of cloud streets.
Development and quality control of the FASTEX data archive
Authors: Jaubert Geneviève , Piriou Catherine , Loehrer Scot M. , Petitpa Alain , Moore James A.
Published: February 2007

The Fronts and Atlantic Storm Track EXperiment (FASTEX) was conducted over the North Atlantic during January and February 1997. the FASTEX Data Archive is a combination of the datasets collected as a part of the two‐month intensive field season and many supplementary datasets and products retrieved from the participating nations and cooperating agencies. the archive is composed of the FASTEX Central Archive (FCA) and a group of specialized databases.

The technical design of the FCA allowed rigorous management of the data and made the checking of the data easy to accomplish. This rigorous management was accomplished using customized quality control evaluation techniques as determined by the individual datasets received at the FCA.

An important aspect of FASTEX data archival activities was the collection, processing and production of a high‐resolution rawinsonde and dropsonde dataset. These data are of importance to model‐verification studies and mesoscale analysis in FASTEX. Data from rawinsondes released from land sites and operational and research ships as well as dropsondes from five aircraft are included. the activities of two groups involved in the preparation of this dataset are described.

The humidity measurements by the rawinsondes and dropsondes were examined in some detail using two methods. First, the three primary types of sondes used over the North Atlantic Ocean (Vaisala rawinsondes and the new and old generation Vaisala dropsondes) were intercompared. Some initial evidence was found of biases in the dropsonde data depending on the type of expendable used and the temperature regime. Second, to further clarify these biases a comparison of the precipitable water content calculated from the rawinsondes and dropsondes with that derived from SSM/I measurements was conducted. the results from this verified many of the conclusions of the initial intercomparison and brought additional insight.

The FASTEX dataset is one of the most rich and varied datasets focusing on the North Atlantic. the data are readily accessible to the scientific community at the FCA via the World Wide Web at: http://www.cnrm.meteo.fr/fastex/.
Impact of the additional FASTEX radiosonde observations on the high‐resolution limited‐area model (HIRLAM) data‐assimilation and forecasting system
Authors: Amstrup Bjarne , Huang Xiang‐Yu
Published: February 2007

During the Fronts and Atlantic Storm‐Track EXperiment (FASTEX) the radiosonde network was considerably enhanced over the North Atlantic area, making this an ideal period for assessing the impact of additional radiosonde observations, in data‐sparse regions, on the atmospheric data‐assimilation and numerical weather‐forecast systems.

We have carried out an observing‐system experiment (OSE) with a limited‐area data‐assimilation system. Two parallel experiments have been run, one with and one without the extra radiosonde data.

We have used both observation verification and field (or analysis) verification to compare the analyses and forecasts from the parallel experiments. On a daily basis, the extra data can have both positive and negative impacts on the different meteorological variables considered here. However, averaged over the whole period the impact is definitely positive, especially on upper model levels. Our results strongly support the inclusion of extra radiosonde data, like those provided by FASTEX. They could be of significant benefit in reducing the errors in operational numerical weather‐prediction products if they were available routinely.
Bimodal ice spectra in frontal clouds
Authors: Field Paul R.
Published: February 2007

Aircraft observations from five Lagrangian descents in frontal clouds from around the British Isles are presented. A common bimodal ice size spectral evolution is observed using two‐dimensional optical array probes, and it is demonstrated that depositional growth alone is not responsible for the observed evolution. It is suggested that the process of aggregation is chiefly responsible for the large‐particle mode of bimodal ice size spectra. Synthetic bimodal spectra that are a function of depth below cloud top and ice‐water content are presented.
Observations of microphysics pertaining to the development of drizzle in warm, shallow cumulus clouds
Authors: French Jeffrey R. , Vali Gabor , Kelly Robert D.
Published: February 2007

The evolution of drizzle in small cumulus clouds in a subtropical environment was studied. Observations were made using three instrumented aircraft, a ground‐based radar, and a 95 GHz airborne Doppler radar during the Small Cumulus Microphysics Study in east central Florida in summer 1995. Data from six clouds, from two days, are examined in this paper. Both sets of clouds were less than 2 km in depth at observation times; in‐cloud temperatures were > 10 °C.

Droplet spectra observed in these clouds were generally bi‐modal. The sizes of the droplets within the large mode are consistent with growth through condensation. It remains unclear how droplets within the small mode formed, but their existence appears to be tied to entrapment and mixing.

Drizzle drops (drops with diameters > 50 μm) were found at all levels in the clouds. The penetration‐averaged concentrations of drizzle drops were less than 15 L−1, except in the uppermost regions of clouds on one of the days where drizzle drop concentrations exceeded 100L−1. In general, the presence of drizzle drops in the upper regions of the clouds is consistent with models of droplet growth through condensation and stochastic collection. The existence of drizzle at low and mid‐levels may be due to collection initiated by ultra‐giant aerosols, or to the redistribution of drizzle drops from neighbouring or earlier cloud elements.
Observations and simulations of a non‐stationary coastal atmospheric boundary layer
Authors: Sunuararajan Ragothaman , Tjernström Michael
Published: February 2007

This paper studies the turbulent and mesoscale structures of a coastal marine atmospheric boundary layer in the southern Baltic Sea. Data from a field experiment are examined for a case during autumn, and this case is subsequently modelled with a mesoscale numerical model.

During this day, the marine atmospheric boundary layer develops from a well‐mixed cloud‐capped state into a stably stratified, sheared boundary layer. This development is due to the dissipation of the cloud layer by radiation processes and by synoptic‐scale subsidence. The boundary layer becomes non‐stationary, and spectral analysis shows that the turbulence structure deviates from the ideal steady‐state homogeneous boundary‐layer turbulence, which forms the basis for most turbulence closures in numerical models. It is shown that the subsiding boundary‐layer top and the increasing stability alter the spatial scale of the turbulent eddies; they become vertically distorted and smaller than those expected during steady‐state conditions. In spite of these deviations, the turbulence statistics are sufficiently well behaved for simplified higher‐order closure modelling to be applicable.

When the mixed layer collapses due to the dissipating clouds, the transition triggers an inertial oscillation and a non‐classical low‐level jet forms. At the same time, the changes in depth and stability of the marine boundary layer are perturbed by the mesoscale interaction between the flow and the coastal terrain. Thus, the upstream terrain increasingly blocks the flow, so that the inertia) jet forms only for the part of the flow that overcomes the terrain obstacle. A numerical simulation of the case reveals the primary mechanisms, and several sensitivity simulations, wherein the terrain and the time‐evolution of the forcing are manipulated, confirm the hypotheses proposed as causing the observed (low structure.
COLA AGCM simulation of the effect of anomalous spring snow over Eurasia on the Indian summer monsoon
Authors: Bamzai A. S. , Marx L.
Published: February 2007

The ability of the Center for Ocean‐Land‐Atmosphere Studies Atmospheric General Circulation Model to capture the inverse snow‐monsoon relation observed in satellite‐derived data is tested. The effect of initial enhanced and reduced spring Eurasian snow mass on the evolution of the subsequent Indian summer monsoon is examined through 11‐member ensemble integrations from 1 March to 1 October. The results of the sensitivity experiment indicate that remote responses from the snow‐mass anomalies include significant changes in the sensible‐heat flux, latent‐heat flux and sea level pressure, leading to changes in summer precipitation over the land mass of the Indian subcontinent. For the pre‐monsoon season, the ensemble average of the temperature of the atmospheric column for enhanced snow mass is anomalously colder over large‐scale regions of the Eurasian continent compared with the ensemble average for the reduced snow mass. Low‐level south‐westerlies associated with monsoonal circulation are increased. The results indicate the importance of land‐surface processes in the evolution of the monsoon circulation and lend hope to the use of Eurasian spring snow mass towards improved forecasting of the summer monsoon.
Northern hemisphere winter stratospheric variability in The Met. Office Unified Model
Authors: Lahoz W. A.
Published: February 2007

This paper uses the ensemble approach to study the simulated northern winter stratospheric variability in a tropospheric‐stratosphere version of The Met. Office Unified Model (UM). The runs are for the December‐March periods from 1979/80 to 1997/98; the ensemble for each winter has nine members. We use observed sea surface temperatures (SSTs), a fixed ozone climatology and fixed greenhouse gases. This paper concerns itself with the following questions: (1) Does the UM reproduce observed temperature trends with only SST variations? and (2) Can any significant trend be attributed to SST variations? The results of this paper suggest that the answer to question (1) is that the UM reproduces the observed temperature trend with only SST variations. In particular, northern winter in the simulated 1990s (1989/90 to 1997/98) is colder than northern winter in the simulated 1980s (1979/80 to 1988/89) at both 10 hPa and 50 hPa, with the model cooling trend (0.5‐1.5 K between the decades for 30°N‐70°N) comparable to that from observations (0.5‐1 K decade−1 for 30°N‐70°N). The temperature differences between the simulated 1990s and 1980s generally are not significant at 10 hPa, but are generally significant at 50 hPa at the 95% significance level. The results in this paper provide a first step toward answering question (2) by suggesting that SST variations alone may explain the cooling trend at 50 hPa. Because the model results show that there is no significant cooling trend at 10 hPa outside a natural variability, one cannot rule this out as an explanation. A complete answer to question (2) requires the inclusion of all likely influences (in particular ozone and water vapour trends) and is beyond the scope of this paper.
A hybrid method for wave propagation from a localized source, with application to mountain waves
Authors: Broutman Dave , Rottman James W. , Eckermann Stephen D.
Published: February 2007

Wave propagation from a localized source into a non‐uniform medium is analysed using a hybrid method. This consists of an integral representation in the near‐field and a numerical ray tracing in the far‐field. The ray tracing includes the calculation of wave‐action density and is initialized from the stationary‐phase approximation to the near‐field integral. The method is tested on the problem of mountain waves in a turning wind, as studied by Shutts. A ray calculation with rotation added to the Shutts model is also presented and is interpreted with a simple analytical model in which the horizontal divergence of rays balances the vertical convergence of rays. In the example presented here, the inclusion of rotation speeds up the approach to wave‐breaking amplitudes.
Southern hemisphere winter ozone fluctuations
Authors: Vigliarolo P. K. , Vera C. S. , Diaz S. B.
Published: February 2007

In this paper the relationship between ozone and atmospheric variability is explored over the southern hemisphere during the austral winter season, with special emphasis on synoptic transient fluctuations.

The analysis of ozone tracks (or high‐frequency ozone variability) shows that they have a significant correspondence with storm tracks at middle and high latitudes. Moreover, ozone tracks maximize over the Indian Ocean slightly downstream of the storm‐track maximum, while over the Pacific region both ozone and storm tracks show decreased amplitudes.

In particular, over southern South America (a region of climatological winter ozone minima and moderate to high ozone variability) the leading winter synoptic‐scale variability mode was identified through a rotated extended empirical orthogonal function analysis applied to the meridional‐wind perturbation at 300 hPa. The resulting mode is characterized by a baroclinic wave travelling eastward along subpolar latitudes, which maximizes near the tropopause level. Composite ozone fields based on this mode confirm, from a statistical point of view, the classical relationship between ridges (troughs) and minimum (maximum) ozone content. Furthermore, it is shown that dynamical processes in the upper troposphere and lower stratosphere associated with subpolar waves are responsible for the observed ozone distribution. This happens due to the barotropic equivalent vertical structure of the wave, together with the fact that ozone partial pressure maximizes near the level where the waves attain maximum amplitudes.
Coriolis effects on orographic and mesoscale flows
Authors: Hunt J. C. R. , Olafsson H. , Bougeault P.
Published: February 2007

A perturbation theory is developed for the effects of rotation on stably stratified flow over mountains at low Froude number, F = U0/(NH) where N is the buoyancy frequency, U0 is the wind speed and H is the mountain height. The Rossby number, R0 = U0/(f D) where f is the Coriolis parameter, and D the along‐wind length of the mountain, is assumed to be a large number. The mountain width, βD, is assumed to be larger than D. Typically R0 is found to lie in the range 3–10. The results are compared with the recent numerical simulations. It is found that as the flow impacts on the mountain, in the northern hemisphere it turns to the left (with your back to the wind); also wave activity over the top of the mountain is greatest on the left side but the pressure drop is greatest on the right in the northern hemisphere. Over the Rossby deformation distance, LR, of the order of HN/f, e.g. 150 km for the Pyrenees, a new wake structure develops that can extend downwind over 1000 km (or a spin‐down distance). There is a momentum defect within the wake but the wind speed increases either side of the wake. Coriolis forces induce a deflection upwards of the isopycnals (and hence more precipitation) on the left, and downwards on the right: this is consistent with some of the differences in mesoscale weather and climate phenomena that are observed on the different flanks of elongated mountains and between the different side of wide valleys, and also in the wakes downwind of mesoscale convection cells. The large perturbation pressure change predicted by the theory is of the order of pU20/F (where p is the density), which is consistent with the magnitude of the terms introduced into the recent European Centre for Medium‐Range Weather Forecasts orographic parametrizations, but it should be noted that these large Rossby‐Froude orographic effects on drag and wave flux are asymmetric with respect to the mountain's centre line. The theory shows how ‘lift’ forces on the mountain are caused, and how these are related to circulation in horizontal planes around the mountains.
Tropical singular vectors computed with linearized diabatic physics
Authors: Barkmeijer J. , Buizza R. , Palmer T. N. , Puri K. , Mahfouf J.‐F.
Published: February 2007

With the introduction of diabatic processes in the forward and adjoint tangent models of the European Centre for Medium‐Range Weather Forecasts's model, it is possible to determine singular vectors (SVs) for situations where diabatic physics may be important in producing perturbation growth. In this paper, the linear physical parametrizations are used to compute SVs for the tropical region, or subsets thereof, with an optimization time of 48 h. Perturbation growth is measured in terms of the so‐called total energy norm, augmented with a term for specific humidity.

Difficulties that may arise in computing tropical SVs, such as associated with spurious upper‐tropospheric perturbation growth, are described. Also, the impact on the SV structure by including a specific‐humidity term in the defining norm is discussed. Using a term for specific humidity based on background‐error statistics in the norm at initial time yields SVs with a more realistic specific‐humidity vertical profile.

SVs are determined in various configurations for two tropical cyclones. Results show that targeting in the vicinity of the cyclone is required to obtain SVs associated with the cyclone dynamics. The dominant targeted SVs for tropical cyclones show resemblance to fast‐growing structures found for idealized vortices.
A two‐layer canopy compensation point model for describing bi‐directional biosphere‐atmosphere exchange of ammonia
Authors: Nemitz Eiko , Milford Celia , Sutton Mark A.
Published: February 2007

A new resistance model is described to interpret the biosphere‐atmosphere exchange fluxes of ammonia (NH3) with vegetation, and compared with previous modelling approaches for NH3. The new model constitutes an extension of an existing one‐layer canopy compensation point model: in addition to bi‐directional foliar stomatal exchange and deposition to leaf cuticles, the model treats NH3 emission from a ground layer. This may originate from fertilizer evaporation, the soil or decomposing plant parts. The emission potentials of the foliage and ground surface are given by the NH3 gas concentrations at equilibrium with the ammonium (NH+4) concentration in the apoplastic fluid or soil solution. From these concentrations, as well as the transfer resistances of the different exchange pathways, the net compensation point of the canopy (Xc) may be derived. The net flux is determined by the relative magnitude of Xc and the NH3 air concentration above the vegetation. The two‐layer canopy compensation point is applied to: (i) an oilseed rape canopy, in which NH3 emission from decomposing leaf litter at the ground surface presents a second major source; and (ii) a wheat stubble field, in which emission from the soil contributes significantly to the net exchange. For both canopies, the model performance is contrasted with the single‐layer Xc model, which is not able to reproduce the temporal patterns of exchange. The two‐layer model is proposed as the optimum compromise between simplicity and accuracy, capable of describing bi‐directional NH3 exchange in atmospheric transport models over a very wide range of vegetation types.
Relationship between the Indian monsoon rainfall and the tropospheric temperature over the Eurasian continent
Authors: Liu Xiaodong , Yanai Michio
Published: February 2007

Based on climatological data such as the all‐India rainfall (AIR) for 1949–98, air temperatures in the troposphere and at the land surface for 1949–98 as well as precipitation and outgoing long‐wave radiation (OLR) for 1979–98, the statistical relationship between the Indian monsoon rainfall and tropospheric temperature (TT) over the Eurasian continent is examined by using the correlation analysis and singular value decomposition (SVD) analysis methods. The main results are summarized as follows. (1) The June‐September (JJAS) AIR has significant positive correlations with the 200–700 hPa TT averaged for Eurasia in JJAS. The area with the highest simultaneous correlation of upper TT with AIR for JJAS is located in western Eurasia. There is also a region with positive simultaneous correlations in the southern subtropics covering southern Africa. The JJAS AIR also shows significant positive correlations with TT over the Eurasian continent in March‐May (MAM) and with the TT averaged over the Tibetan Plateau in September and October. Although some precursory springtime temperature signals for the anomalous summer monsoon are found in the upper TT over Eurasia, the variation in the upper TT appears mostly independent from that of the land surface temperature. (2) The anomalies of upper TT in JJAS are closely coupled with the variations in OLR related to the monsoon rainfall over tropical Asia and Africa. The spatial structure of the first SVD mode for OLR anomalies shows the coherent variations in convective activity from northern tropical Africa to India, while the pattern of the first SVD mode for 300 hPa temperature anomalies shows a distribution which is nearly symmetric with respect to the equator, with variations of the same sign over subtropical areas in both hemispheres and variations of the opposite sign around equatorial Africa. This pair of coupled patterns suggests that when the monsoon rains from tropical Africa to India become more intense, the upper troposphere from northern Africa to western Asia and over southern Africa becomes warmer. (3) The observed temperature variation in the upper troposphere over the Eurasian continent (especially western Eurasia) during the summer monsoon season is considered to be a result of the anomalous subsidence and thermal advection, dynamically induced by anomalous tropical heating associated with the enhanced monsoon rainfall. This study suggests that the tropical convective activity linked to the Indian monsoon rainfall has an impact on the atmospheric variability on a global scale.
A comparison of extreme European daily precipitation simulated by a global and a regional climate model for present and future climates
Authors: Durman C. F. , Gregory J. M. , Hassell D. C. , Jones R. G. , Murphy J. M.
Published: February 2007

The intensity and distribution of daily precipitation is predicted to change under scenarios of increased greenhouse gases (GHGs). In this paper, we analyse the ability of HadCM2, a general circulation model (GCM), and a high‐resolution regional climate model (RCM), both developed at the Met Office's Hadley Centre, to simulate extreme daily precipitation by reference to observations. A detailed analysis of daily precipitation is made at two UK grid boxes, where probabilities of reaching daily thresholds in the GCM and RCM are compared with observations. We find that the RCM generally overpredicts probabilities of extreme daily precipitation but that, when the GCM and RCM simulated values are scaled to have the same mean as the observations, the RCM captures the upper‐tail distribution more realistically. To compare regional changes in daily precipitation in the GHG‐forced period 2080–2100 in the GCM and the RCM, we develop two methods. The first considers the fractional changes in probability of local daily precipitation reaching or exceeding a fixed 15 mm threshold in the anomaly climate compared with the control. The second method uses the upper one‐percentile of the control at each point as the threshold. Agreement between the models is better in both seasons with the latter method, which we suggest may be more useful when considering larger scale spatial changes. On average, the probability of precipitation exceeding the 1% threshold increases by a factor of 2.5 (GCM and RCM) in winter and by 1.7 (GCM) or 1.3 (RCM) in summer.
Simple adjoint method for three‐dimensional wind retrievals from single‐Doppler radar
Authors: Xu Q. , Gu H. , Yang S.
Published: February 2007

A new simple adjoint (SA) method is developed for retrieving three‐dimensional wind from single‐Doppler observations. The method uses the full momentum equations and the mass continuity equation as weak constraints, in addition to the strong constraints of the radial‐wind and reflectivity equations. With these weak constraints, the unknown source term in the previous two‐dimensional SA method is now explicitly estimated in terms of perturbation pressure and buoyancy. This improves the wind retrievals, although the retrieved perturbation pressure and buoyancy may not be accurate. Bi‐spline basis functions are used to express the retrieved fields on coarse finite‐element meshes. The bi‐spline representation filters short‐wave noise and effectively reduces the number of unknowns that need to be retrieved, so the retrievals are further improved. The method is tested with Doppler‐radar data collected during the Phoenix II field experiment. The retrieved winds are very close to the dual‐Doppler observed winds and the difference is about 1.0 m s−1. The retrievals can be further improved by adding smoothness constraints in the cost function, especially when the bi‐spline representation is not used. The robustness of the method is examined in terms of sensitivities to the weights specified for the weak constraints in the cost function.
Evaluation of the mass‐flux approach to parametrizing deep convection
Authors: Swann Hugh
Published: February 2007

Cloud‐resolving model (CRM) simulations of deep convection in a variety of environmental conditions are used to analyse assumptions in bulk‐updraught convection parametrization schemes, which are commonly employed in climate and weather‐prediction models. It is shown that the mass‐flux approach can faithfully represent the effect of convection on the large‐scale fields, provided that the mass flux and other bulk properties of the updraughts (and downdraughts) are accurately predicted. Entrainment and detrainment (which govern the shape of the mass‐flux profile and the dilution of the bulk updraught with environment air) are calculated from CRM simulations. Limitations in the basic premiss of the classical entraining/detraining plume model are revealed and an alternative method of predicting bulk‐updraught temperature is suggested.