摘要:

AbstractThe results of a diagnostic study using radar and satellite imagery, together with surface reports and output from regional and mesoscale models, are presented for an occasion of a rapidly deepening cyclone that crossed the British Isles. The study shows how air descended from near‐tropopause level in the form of mesoscale dry intrusions which appeared to overrun parts of the warm conveyor belt ahead of a surface cold front. One of the dry intrusions is analysed in detail, because it is thought to have led to the large changes that were observed in the character of the wide frontal rain band: in some places the precipitation was entirely suppressed, whilst in other places the precipitation became convective, with a tornadic squall line developing. Although the effect of the dry intrusion was seen most clearly in the precipitation distribution as determined by radar, the approach of the dry intrusion was also clearly detected in the satellite water‐vapour imagery, and foreshadowed in the model forecasts.The mesoscale model, despite some limitations, reproduced many of the observed mesoscale features. It also provided insight into the detailed behaviour of the dry intrusion associated with the tornadic squall line. It showed that the dry intrusion was characterized by a mesoscale filament of high potential vorticity (PV) which was extruded from the lower part of a large region of high PV near a low tropopause. The extruded PV anomaly plunged to within a kilometre of the surface where it overran part of the warm conveyor belt. The dry intrusion was also characterized by relatively low wet‐bulb potential temperature, θw, and it led to potential instability where it overran the warm air. The warm conveyor belt itself was characterized not only by a high θwbut also by a separate strip of high PV generated locally, mainly by condensation. The quality of the mesoscale‐model simulation gives grounds for optimism that such models are capable in principle of predicting severe mesoscale weather events when, as in this case, they are orchestrated by resolved larger‐sca

ISSN:0035-9009    

DOI:10.1002/qj.49712152302

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA new dynamic microphysical model of the melting layer is presented, developed from a previous model. This new model is applied to obtain vertical reflectivity profiles (VRPs) and vertical attenuation profiles (VAPs) through the melting layer which both allow for and neglect propagation effects. The model profiles are compared with measured profiles at a range of different frequencies; in particular, a detailed comparison was made for frequencies in the X‐band. The VRPs are used to generate relations betweenbright‐bandpeak‐intensity and surface rainfall‐rate, R0, at frequencies in the S‐, C‐ and X‐bands. Relations between other model bright‐band parameters and R0are found to be more complicated functions of model‐input parameters. The model profiles are also applied to the problem of correcting for the radar bright‐band in the Meteorological Office's radar data. Early results, from three detailed case studies, seem promising; however, the technique has yet to be tested in an oper

ISSN:0035-9009    

DOI:10.1002/qj.49712152303

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA year of wind data measured by a 50 MHz wind profiler at Darwin, Australia are analysed. The behaviour of vertical motions in the precipitation‐free boundary layer is shown to be different from the free troposphere. However, these differences are not reflected in the spectral characteristics of the horizontal wind fields. There are no significant differences between the horizontal‐velocity spectra above and within the mixed layer. The power spectra of the horizontal winds show little seasonal differences for the monsoon, dry and transition seasons as well as for the data set overall. The bispectral characteristics are also similar, with the strongest interactions between the lowest frequencies, and a weak self‐interaction peak at low frequencies. A cross‐peak in the zonal wind above 5 km is also present for frequency sums corresponding to a period of around 10 days. There are no detectable nonlinear interactions at high frequencies. The wind field is not strongly polarized except at the very lowest frequencies (i.e. seasonal changes). Thus there appears to be universality of the spectral characteristics observed despite the expected variations in the forcing in different seasons and stability regimes. These observations have interesting implications for internal‐wave models and two‐dimensional turbulence models of mesoscale

ISSN:0035-9009    

DOI:10.1002/qj.49712152304

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA theoretical and experimental analysis of the humidity and temperature spatial differentials (δ/δxj,) in an urban area is presented. The local isotropy form of the spectra of the streamwise and vertical differentials of a scalar field is derived. The experimental results for the humidity and temperature fields in the lowest part of the urban boundary layer are in satisfactory agreement (at least in form under stable conditions) with the local isotropy model indicating that these fields can be locally isotropic. A strong anisotropy was observed at small wavenumbers of the vertical differentials spectra and it is probably connected with surface‐layer plumes. The analysis of the humidity and temperature differentials is extended to their co‐spectra and the spectra of the vertical differentials are compared to the streamwise ones. The comparison of the structure parameters of the temperature or the humidity field in the streamwise and the vertical direction, estimated using a spectral and a variance (structure function) method, showed no systematic departure from the isotropic prediction of equ

ISSN:0035-9009    

DOI:10.1002/qj.49712152305

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAn aqueous‐phase cloud‐chemistry model is used to investigate the conversion of ammonia gas to ammonium aerosol in an air stream as it passes through a hill cap cloud. The sensitivity of the conversion to the scavenging of soluble gases such as SO2and HNO3, and to the sulphur chemistry occurring in the cloud, is investigated. The subsequent effect of conversion on the pattern of deposition of reduced nitrogen (NHx) to the surface before, during and after processing by the cap cloud is also considered.The fraction of ammonia present in the inflowing air stream converted to ammonium depends critically on the ratio of ammonia to the concentration of all other species present in solution with which it is capable of forming stable ammonium salts. If sulphate, generated by the oxidation of S(IV) in solution by ozone (and hydrogen peroxide when available), is the only such species, ammonium production is directly linked to sulphate production. When aqueous ammonia is in deficit to acidic species (e.g. HSO 3−), at least 85% of the input ammonia is converted to ammonium. When ammonia is in excess, the fraction of ammonia converted to ammonium depends upon the degree of this excess.It is calculated that conversion of ammonia to ammonium changes patterns of deposition of reduced nitrogen to the surface significantly. Within the cap cloud the efficient deposition of ammonium in cloud droplets (compared with the less efficient deposition of submicron aerosol) dominates over the dry deposition of ammonia gas, most of which is absorbed into the aqueous phase of the cloud. This generates a total reduced‐nitrogen flux within the cloud that is typically several times larger than the pre‐cloud flux. To the lee of the cap cloud, deposition of reduced nitrogen is also predicted to be much lower than pre‐cloud values. This lowered deposition results firstly from the depletion of gas‐phase ammonia, and secondly from the inefficient deposition of aerosol particles which individually carry an increased loading of ammonium after passage through the cap cloud (and so collectively constitute a larger proportion of the total loading of reduced nitrogen). This conversion enables the NHxloading of processed air streams to be transported over much longer distances before being deposited. The depleted concentrations of ammonia may enable further emissions of ammonia into the air stream as it passes over recently fertilized terrain, thus replenishing some of the NH, lost over clou

ISSN:0035-9009    

DOI:10.1002/qj.49712152306

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractSome issues, concerning uniform, stratified flow over a three‐dimensional mountain, that have not been fully explored are studied using primitive‐equation models. When the Froude number (Fr) is small (e.g.Fr<0.5), we find that the Coriolis force cannot be neglected for flow over small‐scale mountains (characteristic lengthL<50 km) even though the Rossby number (Ro) is large. When the Coriolis force is neglected, a pair of symmetric vortices is induced on the lee side, irrespective of the scale of the mountain, as long asFrlies roughly between 0.1 and 0.5. The major effect of the Coriolis force is to modify or diminish the lee vortices due to the leftward deflection on the upstream side of the mountain. The Coriolis deflection also reduces the upstream propagation. WhenFris ≥ 1, most of the flow goes over the mountain and the Coriolis effect is small for the sameRo. Therefore, the importance of the Coriolis effect is determined by bothRoandFr.As the scale of the mountain increases from small‐scale to mesoscale (100 km

ISSN:0035-9009    

DOI:10.1002/qj.49712152307

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper gives an account of the global soundings system (GLOSS), which is the new method introduced at the Meteorological Office at Bracknell for the processing of global TOVS radiance data for assimilation into numerical weather prediction (NWP) models. The assimilating NWP models themselves provide the prior information necessary to infer temperature and humidity information from radiances. After a brief summary of the historical background, the GLOSS processing is described, noting in particular the differences from similar work elsewhere. Results are then presented from NWP impact studies of the assimilation of temperature profiles derived from satellite soundings. An advantage is demonstrated for the GLOSS temperature retrievals relative to the retrievals distributed by NESDIS and produced from the same radiance data. The advantage of GLOSS is clear and consistent in the extratropical regions of the northern hemisphere and, especially, of the southern hemisphere. In the tropics the results are more mixed. The paper concludes with a note of additional work required before operational implementation of GLOSS, and with an outline of other expected future developments.

ISSN:0035-9009    

DOI:10.1002/qj.49712152308

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractSome simulation experiments relevant to the problem of determining the minimum data requirements for the accurate reconstruction of tropical‐cyclone‐scale vortices from given sparse data distributions are described. Also, the accuracy with which the vortex track can be predicted using these data is investigated. The experiments are based on a 96‐hour numerical integration of the barotropic vorticity equation on a beta‐plane, starting with a symmetric tropical‐cyclone‐scale vortex embedded in zonal shear flow. After 48 hours, when asymmetries in the vortex circulation have developed, the model fields in this ‘control’ calculation are used as a ‘perfect’ data set to assess the analysis strategy described. The 20 km horizontal resolution of these new initial data is progressively degraded to test the ability of the analysis procedure to reconstruct the perfect analysis. The analyses of the degraded data sets are used as an initial condition for a further 48‐hour integration of the model, and the resulting vortex tracks are compared with that in the second half of the control run. The influence of the analysed vortex asymmetry on the subsequent vortex track is also studied.As the resolution of the simulated initial data is degraded, so is the information that these data provide on the symmetric circulation and the vortex‐induced asymmetries, and ultimately it becomes necessary to introduce ‘synthetic’ data in the analysis. The calculations indicate some of the problems that need to be overcome to introduce such data properly.The analysis method provides a general way to partition the flow into four components: the large‐scale environment, the symmetric vortex, the vortex asymmetry, and the small‐scale environment. The large‐scale environment is characterized by the lowest few wave numbers in a two‐dimensional Fourier analysis. The residual field is subjected to an azimuthal Fourier analysis about the cyclone centre, and the last three components of the partition are defined by the symmetric component, the wave‐number‐one component and the sum of all higher wave‐number components of this analysis, respectively. An attractive feature of such a partition is that the total flow across the centre of the symmetric vortex, which governs the vortex motion in a barotropic model, is contained in only two components: the large

ISSN:0035-9009    

DOI:10.1002/qj.49712152309

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMeasurements of the transmission of solar infrared radiation through the earth's atmosphere, by the HALOE experiment on the UARS spacecraft, were made at high southern latitudes during October 1991. These observations are direct measurements of atmospheric transmission, and do not need to be passed through a composition or temperature retrieval process; they are, therefore, amenable to direct interpretation. During October 1991 the profiles of transmittance versus height in the atmosphere show clear evidence for the arrival (from more northerly latitudes) of layers of some material which absorbs infrared radiation, at heights of up to 28 km. The spatial structure of the absorbing material shows considerable variability with longitude at a given latitude. The spectral properties of the detected absorption, measured at the various HALOE wavelengths, are consistent with absorption by sulphate aerosol, and clearly implicate the volcanic eruptions from Mts Pinatubo and Hudson during 1991. These results provide direct and detailed evidence for the arrival of layers of what appears to be sulphate aerosol during the 1991 southern spring, at latitudes as high as 80 degrees south.

ISSN:0035-9009    

DOI:10.1002/qj.49712152310

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe nature and causes of seasonal (July to September (JAS)) rainfall variability over tropical north Africa are investigated using a combination of empirical and modelling approaches. Concentration is focused on three regions: the Sahel, Soudan and Guinea Coast.The variability of seasonal rainfall through the twentieth century is analysed for each region. The well known dipole of anomalies between the Sahel and Guinea Coast is evident, and new analysis reveals that this fluctuates primarily on time‐scales of five years or less. Attention is then focussed on the causes of rainfall variability in each region, by examining the relationships with global sea‐surface‐temperature (SST) patterns; a partitioning of the data into low‐ and high‐frequency components is found to be particularly useful.So as to attribute more convincingly the primary cause of seasonal rainfall variability to global SST forcing, a suite of general circulation model experiments are performed, aimed at simulating JAS rainfall anomalies for ten past years between 1949 and 1990. Each is forced by the observed SST patterns for the appropriate year. In almost every case, the model quite skilfully simulates the magnitude and pattern of JAS rainfall anomalies across tropical north Africa, thus strengthening the idea that global SST variations are indeed responsible for most of the variability of seasonal rainfall. The relative impact of two additional sources of variability is also investigated. First the role of internal atmospheric variability is quantified (using an ‘analysis of variance’ technique), and found to be small in all three regions. Second, and perhaps more controversially, the possible effects of land‐surface‐moisture feedback are explored. This is done by replacing the normal interactive soil‐moisture scheme with a model‐derived climatology; results suggest that in some years moisture evaporated from the land surface may play a key role, but that in general SST forcing still dominates. Finally, an assessment of the model's skill at sub‐seasonal time‐scales reveals that fluctuations of monthly rainfall about each year's seasonal mean (intraseasonal variations) are insensitive to SST forcing, in part due to a larger influence of intern

ISSN:0035-9009    

DOI:10.1002/qj.49712152311

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY