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doi: 10.1002/qj.49712454502pmid: N/A
Examples are used to provide an appreciation of the most recent developments in real‐time experimental and operational seasonal forecasting and also of some of the wide issues surrounding the provision and use of such forecasts. The stimulus provided by the study of the El Niño Southern Oscillation phenomenon is discussed, especially with respect to operational seasonal forecasting in the US National Weather Service. The Meteorological Office's seasonal‐forecasting activities and objectives are then reviewed briefly, leading to a description of, and some tentative results from, a major European collaborative project on Prediction Of Climate Variations On Seasonal to Interannual Time‐scales. A wider international perspective is given by considering the relevant projects of the Word Climate Programme and also the proposal for an international research institute for seasonal‐to‐international climate prediction. In conclusion, some further general issues are raised for consideration.
doi: 10.1002/qj.49712454503pmid: N/A
The role of the extratropics in determining the distribution of tropical convection and its inherent time‐scales has been investigated, using results from a perpetual January integration of the UK Universties Global Atmospheric Modelling Programme (UGAMP) General Circulation Model. This study has concentrated on the influence of upper tropospheric extratropical troughs in the Pacific waveguide, and of the east Asian cold surge events. The mechanisms through which the extratropics influence the model's tropical convection have been identified, and the validity of the model's behaviour has been assessed by comparing the results with previous observational studies.
Yuter, Sandra E.; Houze, Robert A.
doi: 10.1002/qj.49712454504pmid: N/A
The natural variability of precipitating cloud systems over the western Pacific ‘warm pool’ is investigated by analysing aircraft C‐band radar data collected on 24 aircraft missions during the Coupled Ocean‐Atmosphere Response Experiment of the Tropical Ocean and Global Atmosphere programme (TOGA COARE) in relation to the infrared (IR) temperature patterns measured contemporaneously by geosynchronous satellite. the data are analysed at fine (24 km × 24 km) and coarse (240 km diameter) horizontal resolutions, which correspond to typical resolutions of mesoscale and general‐ciruclation models, repectively. The analysis is statistical and imposes no a priori conceptual model or subjectively decided‐upon structure categories. The mean IR temperature of cloud tops, and the sizes of rain areas mapped by radar (and objectively subdivided into convective and stratiform subareas), are tallied and relatted to each other to obtain statistics respresentative of the four‐month period of the TOGA COARE.
doi: 10.1002/qj.49712454505pmid: N/A
The TOGA‐COARE (Tropical Ocean/Global Atmosphare Coupled Ocean‐Atomspahere Response Experiment) was in an international observational campaign designed to study the processes that occur in the Pacific warm pool region. Data from the Meteorologiacal Research Flight C130 aircraft are presented to illustrate some of the characteristics of the surface energy balance in this region. The dominant terms in the balance are the turbluent latent‐heat flux and the solar flux. The C130 moisture flux data are compared with a parametrization based on data from the RV Moana Wave, one of the ships that took part in the COARE. It is found that there is no evidence for a systematic difference between the aircraft and ship flux estimates. In addition to the comparison with the COARE parametrization the effect of gustiness due to convective clouds on the surface latent‐heat flux is estimated for three flights, and the thermal balance of the mixed layer in undisturbed conditions described. The data suggest that direct solar absorption is an important part of the thermal balance, and the implications for diurnal variations in the mixed‐layer temperature are considered.
Considine, Geoffrey; Curry, Judith A.
doi: 10.1002/qj.49712454506pmid: N/A
A new cloud model has been developed based upon a reconsideration of cloud‐top entrainment mechanisms. A finite‐thickness entrainment layer at cloud top allows the cloudy air to lose liquid water to dry entrained air by sedimentation. The model use a steady‐state ensemble of one‐dimensional updraughts and downdraughts, combined with a simplified traetment of droplet sedimentation, autoconversion, and collection of cloud drops by drizzle, to examine vertical profiles of cloud liquid‐water content and droplet concentration. The model is demonstrated both for idealized cases and in comparison with a nocturnal stratocumulus case from the ASTEX. The sedimentation flux divergence in the entrainment layer at cloud top is shown to have a large impact on the cloud microphysics.
Boers, R.; Jensen, J. B.; Krummel, P. B.
doi: 10.1002/qj.49712454507pmid: N/A
Six case‐studies are presented of stratocumulus clouds observed during the summer phase of the Southern Ocean Cloud EXperiment (SOCEX). The experiment was conceived to investigate the microphysical properties of clouds in the unpolluted background region over the Southern Ocean. The case‐studies show evidence of decoupled stratocumulus layers, i.e. situations where cumulus clouds were covered by a uniform sheet of stratocumulus cloud. Optical depths of the clouds ranged from 2 to 23. Cloud droplet concentrations varied from 50 to 180cm−3, and mostly decreased with altitude, indicating the importance of droplet coalescence in reducing the number of large cloud droplets. Drizzle was observed on four of the six flights.
Jackson, D. R.; Driscoll, S. J.; Highwood, E. J.; Harries, J. E.; Russell, J. M.
doi: 10.1002/qj.49712454508pmid: N/A
In this paper we present observations of water vapour (version 18 retrievals) in the equatorial tropopause region made by the Halogen Occulatation Experiment (HALOE) instrument, which files on the Upper Atmosphere Research Satellite. These data make an important new contribution to the observationl knowledge of this region, since water vapour here has largely been observed to date using in situ instruments, and the only global longitudinally‐resolving satellite observations to be reported are those from the Stratospheric Aerosol and Gas Experiment II (SAGE II). Firstly, the HALOE water vapour climatology is presented. Comparison of HALOE and SAGE II climatologies shows considerable agreement. An exception is over Asia in June, July and August (JJA), where the HALOE fields show a distinct region of moist air (associated with convection due to the Asian summer monsoon), whereas no such feature was reported in the SAGE II observations. This suggests that HALOE is sensitive to water vapour variability near the tropopause over regions of strong convection. Secondly, the troposphere to stratosphere transport of air at low latitudes is investigated. Pressure/longitude sections near the equator show that dry air with water vapour mixing ratios of less than 2.4 parts per million by volume (p.p.m.v.) enters the stratosphere in December, January and February, then travels upwards and polewards in the subsequent seasons. A more detailed examination of pressure/longitude sections indicates episodes when dry air also enters the equatorial stratosphere in March, April and May and JJA at times of strong convenction. There is also evidence of entry of air with higher water vapour mixing ratios, of between 3.6 and 4.0 p.p.m.v, into the stratosphere in September, October and November, chiefly at 10°N and 20°N. Further examination of the region of moist air over Asia in JJA (mentioned above) shows that there is clear cross‐isentropic upward transport in the 375–410 K region of unsaturated air. The HALOE water vapour data set reported here provides an important new insight into low latitude processes near the equatorial tropopause.
Rogers, H. L.; Norton, W. A.; Lambert, A.; Grainger, R. G.
doi: 10.1002/qj.49712454509pmid: N/A
The isentropic transport of stratospheric air from the tropics into nothern hemisphere mid‐latitudes is examined using Mt. Pinatubo aerosol observed by the Improved Stratospheric and Mesospheric Sounder (ISAMS). The technique of contour advection is used to produce high‐resolution aerosol fields which are used to help diagnose transport processes. By sampling the high‐resolution contour‐advection fields at the satellite measurement points, we show that the small‐scale information present in the contour‐advection calculation is consistent with the ISAMS measurement. Result of this study indicate that transport from the tropics into mid‐latitudes is strongly dependent on altitude. In the lowest regions of the stratosphere, the upward influence of the subtropical jet and the synoptic‐scale waves that form on the storm tracks are important in transporting air from the tropics and in the distribution of aerosol in mid‐latitudes. Synodpitc‐scale spirals form in the aerosol field on the 500 K isentrope at the end of storm tracks associated with LC1 baroclinic life‐cycle behaviour. At altitudes near the 600 K isentrope, direct tropospheric effects diminish and intrusions of air from the tropics to mid‐latitudes weaken. At altitudes approaching the middle stratosphere, planetary‐wave activity becomes stronger and the associated displacements of the polar vortex to low latitudes produce trasport from the tropics, particularly during mid‐winter warming events.
Prunet, Pascal; Thépaut, Jean‐Noël; Cassé, Vincent
doi: 10.1002/qj.49712454510pmid: N/A
The estimation‐error covariance matrix associated with the analysis of the atmospheric state is used in one particular meteorological situation to examine the potential benefit of radiance data for numerical weather prediction. The gain of information content obtained from simulated Infrared Atmospheric Sounding Interferometer (IASI) data is studied and compared with the current information present in the TIROS Operational Vertical Sounder (TOVS) radiances.
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