摘要:

AbstractObservations of boundary‐layer‐humidity probability‐distributions made with the Meteorological Office tethered‐balloon facility have been derived from nine representative days between 1996 and 1999. Measurements include those made at several heights simultaneously. Four types of distribution have been defined, including skewed and multimodal types. These have been analysed on scales relevant to sub‐grid parametrizations. Four distribution functions have been fitted to the observations and their accuracy in predicting cloud fraction assessed. It was found that a standard beta‐type distribution gave the best representation of the observed humidity‐distribution. In addition, it was found that for grid scales greater than about 5 km, advection plays a significant role in determining the form of the humidity distribution, and that this must be accounted for in any parametrizations. The cloud‐fraction error arising from errors in the mean total specific‐humidity and saturation specific‐humidity is also discussed. That from the former is found to

ISSN:0035-9009    

DOI:10.1002/qj.49712757302

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA new method of parametrizing the effects of form drag due to unresolved orography is proposed in which an explicit orographic‐stress profile is specified. This approach is evaluated using the results of numerical model simulations and the performance is found to be at least as good as that obtained with the traditional effective‐roughness‐length parametrization for cases of neutral flow normal to ridges. The new method is more readily extended to represent the changes in form drag as a function of wind direction for flow over anisotropic orography, and also simplifies the problem of parametrizing the scalar transfer. It is also suggested that this more explicit approach will provide a more convenient framework for parametrizing form drag in stable conditions and for understanding and improving the interactions with gravity‐wave‐drag parametrizatio

ISSN:0035-9009    

DOI:10.1002/qj.49712757303

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe Hadley Centre climate model version HadAM2 is used to study the sensitivity of modelled Antarctic climate to the parametrization of surface and boundary‐layer heat fluxes under stable conditions. Specifically, the impact of changing the dependence of surface exchange coefficients and eddy diffusivities on the Richardson number is investigated. Three alternative parametrizations are implemented; in all of these the exchange coefficients decrease more rapidly with increasing stability than they do in the standard parametrization used in the model. When only the surface flux scheme is replaced by one of these alternatives, cooling is largely restricted to the surface, with some compensating warming occurring at the lowest atmospheric levels, and little change is seen in the low‐level wind field over Antarctica. If alternative schemes are implemented both at the surface and in the boundary layer, widespread cooling occurs at the surface and at the lowest one or two atmospheric levels. The increased negative buoyancy thus generated causes significant increases in the speed of katabatic winds blowing down the coastal slopes of Antarctica. Colder and stronger offshore winds lead to increased cooling of the Antarctic coastal waters. In a coupled model, this could impact on the production of sea ice and ocean‐bottom water. The modelled temperature changes appear to show both a direct response to changed boundary‐layer heat‐flux divergence and an indirect response as a result of the consequent changes to the low‐level

ISSN:0035-9009    

DOI:10.1002/qj.49712757304

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractFor a 17‐day period in late summer 1995 ammonia (NH3) fluxes over an extensive area of semi‐natural ungrazed short grassland were measured and modelled. The measurements were carried out at the Melpitz field research station near Torgau in the state of Saxony in eastern Germany (86 m above sea level, 51°31′N, 12°56′E). For the calculation of the fluxes the micrometeorological flux‐gradient technique was applied using a sensitive continuous‐flow wet annular denuder system for NH3at three heights. Both downward fluxes to the canopy and upward fluxes from the grassland to the atmosphere were observed. On the basis of the direction of the flux in relation to air concentrations, the ammonia canopy compensation point was estimated to vary within the range 0.2 to 3.0 μg m−3. Because the simple ‘canopy/surface resistance’ (Rc) model presupposes a surface concentration of zero for the observed trace gas, which cannot explain bidirectional fluxes, the results of the measurements were compared with a static ‘canopy compensation point‐cuticular resistance’ (Xc–Rw) model, which is able to quantify the stomatal compensation point (Xs) and allows bidirectional fluxes. The agreement between the model and the measured fluxes for the whole period was relatively good. Given realistic ratios of ammonium to proton concentrations (T = [NH4+]/[H+]) for the intercellular fluid of between 150 and 1000 the minimum in the difference was found for Γ ≈︁ 1000 (correlation coefficient r2= 0.23) and even bidirectional fluxes were obtained. Possible reasons for the differences between the measurements

ISSN:0035-9009    

DOI:10.1002/qj.49712757305

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA 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 NH3emission 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 NH3gas 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 NH3air concentration above the vegetation. The two‐layer canopy compensation point is applied to: (i) an oilseed rape canopy, in which NH3emission 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 NH3exchange in atmospheric transport models over a very wide range of veg

ISSN:0035-9009    

DOI:10.1002/qj.49712757306

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIn a previous study of two cyclones within the Fronts and Atlantic Storm Track EXperiment (IOP11 and IOP17), it was shown that upper‐level barotropic growth dominated the initial phase of the development. The cyclones later developed due to baroclinic energy conversion. These cyclones resemble Petterssen's Type B paradigm. This contribution presents the dynamics of archetypical Type B cyclogenesis by addressing the consequences of upper‐level barotropic growth before baroclinic cyclone development within the framework of idealized primitive‐equation simulations. Localized and finite‐amplitude upper‐level pressure perturbations have been designed to represent a variety of amplitudes and horizontal tilts of a localized barotropic trough structure. These are added to a zonal jet basic state, and it is shown that favourable initial situations, similar to the observed cyclones, lead to much stronger and faster cyclonic development compared with unfavourable ones. The most effective barotropic initial situation (without a vertical tilt and considering growth over a time‐scale of 48 hours) is one with a horizontal ‘tilt against the shear’ of an upper‐level trough on the cyclonic side of a jet associated with

ISSN:0035-9009    

DOI:10.1002/qj.49712757307

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractFluxes from the land surface into the atmosphere exhibit strong variability on diurnal and daily time‐scales during the Sahelian wet season. The influence of this variability on the atmosphere is examined here using a general circulation model. A land‐surface scheme with an improved description of the sparse vegetation which characterizes the Sahel is introduced, and compared with simulations using existing parameters. With smaller evaporation rates, the sparse vegetation results in a warmer and deeper planetary boundary layer (PBL) in better agreement with observations. The diurnal cycle in simulated rainfall is much weaker, due to less frequent triggering of daytime convection. The damping of diurnal variability is accompanied by enhanced daily variability, with a deep moist PBL developing over several days. Above the sparsely vegetated surface, there is also greater African easterly wave (AEW) activity, and more long‐lived rain events associated with it.A mechanism is proposed to explain the increased frequency of AEWs above sparser vegetation, based on wave composites in the model. Surface heating of the lower atmosphere is modulated strongly by AEWs. This occurs via a surface‐convection‐radiation feedback. Estimates of the boundary‐layer heat budget indicate that variations in surface heating can, to a large extent, account for the low‐level temperature signal characteristic of AEWs. By reinforcing the low‐level temperature anomalies, the surface may influence the meridional wind signature of the wave above the PBL. Further evidence of the close coupling between the land surface and atmosphere is found by examining the modulation of precipitation by AEWs. Surface heat and moisture fluxes influence the build‐up of convective available potential energy ahead of the wave trough. In this way, the properties of the surface can affect the timing and intensity of rainfall associ

ISSN:0035-9009    

DOI:10.1002/qj.49712757308

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA bulk mass‐flux convection parametrization for deep and shallow convection is presented that includes an efficient and straightforward treatment of numerics, moist thermodynamics and convective downdraughts. The scheme is evaluated in a single‐column model context for a tropical deep‐convective period and a trade‐wind cumulus case. Preliminary applications in a global numerical weather‐prediction model and a mesoscale model are also discussed.The results suggest that the present scheme provides reasonable solutions in terms of predicted rainfall, and tropical temperature and moisture structures. The application of the scheme to various scales is supported by the use of a convective available potential energy convective closure that assures a smooth interaction with the large‐scale environment and efficiently suppresses conditional instability of the second kind‐like spin‐up processes on the grid‐scale.Finally, the theoretical and practical limits of the present approach are discussed together with possible fu

ISSN:0035-9009    

DOI:10.1002/qj.49712757309

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe organization of convection by intraseasonal sea surface temperature (SST) anomalies is investigated to examine the role that atmosphere‐ocean coupling may play in the maintenance of the Madden‐Julian Oscillation. An atmospheric general‐circulation model with an ocean‐covered surface is used to investigate the response of tropical convection to idealized imposed intraseasonal SST anomalies, and the sensitivity of this response to their propagation speed. The convection is found to be organized on the spatial and temporal scales of the imposed SST anomalies and the location of the maximum in precipitation relative to the SST anomaly is in good agreement with observations, and different from that for a stationary SST anomaly. The magnitude of the precipitation anomalies increases with decreasing propagation speed of the SST anomalies. The role of the free‐tropospheric humidity is crucial for determining the location and magnitude of the precipitation

ISSN:0035-9009    

DOI:10.1002/qj.49712757310

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractBased 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

ISSN:0035-9009    

DOI:10.1002/qj.49712757311

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY