摘要:

AbstractThe paper describes the turbulence scheme implemented in the Meso‐NH community research model, and reports on some validation studies. Since the model is intended to perform both large‐eddy and mesoscale simulations, we have developed a full three‐dimensional scheme, based on the original method of Redelsperger and Sommeria. A prognostic equation for the turbulent kinetic energy is used, together with conservative variables for moist non‐precipitating processes. A particularity of the scheme is the use of variable turbulent Prandtl and Schmidt numbers, consistently derived from the complete set of second‐order turbulent‐moment equations. The results of three idealized boundary‐layer simulations allowing detailed comparisons with other large‐eddy simulation (LES) models are discussed, and lead to the conclusion that the model is performing satisfactorily.The vertical flux and gradient computation can be run in isolation from the rest of the scheme, providing an efficient single‐column parametrization for the mesoscale configuration of the model, if an appropriate parametrization of the eddy length‐scale is used. The mixing‐length specification is then the only aspect of the scheme which differs from the LES to the mesoscale configuration, and the numerical constants used for the closure terms are the same in both configurations. The scheme is run in single‐column mode for the same three cases as above, and a comparison of single‐column and LES results again leads to satisfactory results. It is believed that this result is original, and is due to the proper formulation of the parametrized mixing length and of the turbulent Prandtl and Schmidt numbers. In fact, a comparison of the parametrized mixing length with the length‐scale of the energy‐containing eddies deduced by spectral analysis of the LES

ISSN:0035-9009    

DOI:10.1002/qj.49712656202

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe intensity of acoustic backscatter from a monostatic acoustic sounder is used to compute the profile of the temperature structure parameterC T2. The average value ofC T2in low‐height turbulent stable layers (temperature inversions) is used to infer the local potential‐temperature gradient. The estimates of temperature jump in the first height inversion at the top of the unstable atmospheric boundary layer (ABL) are compared with the direct measurements of the temperature profile by a tethered balloon, and the indirect estimation from the wave period of the waves present on the inversion. The agreement of the estimations with the direct measurements is satisfactory (at least a 0.4 degC accuracy). Also, the‐4/3 similarity law of theC T2profile in the unstable ABL is used to obtain estimates of the surface heat flux,Qo. The comparison of these estimates of Qowith eddy correlation measurements shows the high accuracy of this method. This direct connection of theC T2profile withQocan also be used to calibrate the backscat

ISSN:0035-9009    

DOI:10.1002/qj.49712656203

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractDew variability during the autumn dewy season within a small arid drainage basin in the Negev Highlands, Israel, is studied. Dew measurements were carried out at 18 stations on four exposures, using the Cloth‐Plate Method (CPM) and Duvdevani dew gauges. The study also included periodical wind and substrate temperature measurements.Dew variability within the drainage basin was high. Average daily dew values obtained by the CPM were between 0.07 and 0.31 mm, whereas dew duration ranged between 1.6 and 4.1 hours per dewy morning. Dew amounts monitored by the Duvdevani gauges were lower, between 0.09 and 0.20 mm. Both methods show, however, consistent variability and correspond to a similar pattern. Whereas near‐ground dew measurements were the highest at the hilltops and at the bottom of the sun‐shaded northern and western exposures, wadi bed stations and, especially, the south facing midslope station obtained the lowest dew quantities.The near‐surface dew patterns are not in agreement with the classical model of both Geiger and Oke, which predicts high dew quantities at the wadi beds (due to nocturnal down‐slope wind) and at the lee side of the prevailing wind, i.e. the south‐facing midslope station. The low quantities at the south‐facing midslope station is explained by the paramount role of surface temperatures, whereas variability in radiational cooling is seen as responsible for the high near‐ground dew quantities at the hilltops and the low quantities at the wadis.This conclusion is supported by dew measurements al 40 cm above ground. Dew measurements at 40 cm above ground at the south‐facing midslope station and at both wadi beds were significantly higher (p<0.05) than at 0.7 cm above ground. Facilitating an efficient radiational cooling, and beyond the impact of the surface temperatures, dew measurements at this height correspond to the classical model, highlighting the important impact of surface temperatures and ventilation upon near‐ground dew condensation in an

ISSN:0035-9009    

DOI:10.1002/qj.49712656204

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractChanges in the characteristics of atmospheric aerosol over a marine environment are investigated by making regular spectral extinction measurements in the visible and near‐infrared region from a tiny island location. Minicoy (8.3°N, 73.04°E), situated in the Arabian Sea about 400 km due west of the southern tip of the Indian peninsula. The role of seasonally changing air‐mass type in causing a regular annual variation in the spectral optical depths is delineated. The association between aerosol optical depths, surface wind speed and rainfall is examined. An increase in wind speed causes an increase in optical depths, the effect is predominant when a marine air mass prevails. The impact of changes in wind speed on optical depths (due to sea‐spray production over the sea) is parametrized in the case that the island is influenced by a marine air mass. Columnar size distributions, retrieved from the spectral optical depths, in general, show a bimodal log‐normal distribution in the optically active size range. The accumulation mode is more sensitive to continental air‐mass types, while the coarse mode is influenced by the marine conditions. The coarse mode is sharper but its position is variable. Increase in wind speed leads to a remarkable enhancement in the concentration and relative abundance of coarse particles, particularly during the monsoon season. The mass loading and effective radius are well associated and depend on wind speed histories. The findings ar

ISSN:0035-9009    

DOI:10.1002/qj.49712656205

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe approach of an upper‐level trough is accompanied by a local ascent of the isentropic surfaces in the troposphere. The associated cooling and moistening destabilizes the atmosphere to convection. In this paper we attempt to quantify the maximum amount of destabilization as a function of the amplitude and horizontal scale of the trough. The trough is represented by an elongated potential‐temperature perturbation on the tropopause. Its structure, including the isentrope displacement it produces, is calculated from the balanced shear‐line solutions of Juckes. These solutions are used to estimate the amount by which mid‐ and lower‐tropospheric air masses are lifted when an upper‐tropospheric trough passes overhead on the assumption that isentropic potential‐vorticity anomalies are localized in the upper troposphere. The amount of destabilization is characterized by the maximum change in convective available potential energy (CAPE) and convective inhibition (CIN) brought about by the passage of the trough. The calculations seek to isolate one aspect of the interaction between an upper‐level trough and a tropical cyclone. We show that the changes in CAPE are significant in the Tropics (up to 83% for the soundings examined) and even more so in the middle latitudes (up to 130%) and that the CIN may be com

ISSN:0035-9009    

DOI:10.1002/qj.49712656206

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThere is observational evidence to suggest that at the time tropopause folds in extra‐tropical cyclones are descending into the mid troposphere there is frequently a destablization of the lower troposphere to moist convection. Here we consider the dynamical reasons for such a linkage using so‐called potential vorticity (PV) attribution concepts. In particular PV inversion is used to find the wind shear attributable to the tropopause fold itself. Then this shear is used to quantify the tendency to generate regions of potential instability. For an observed case of a modest tropopause fold it is found that the PV anomaly contained in the tropopause fold contributed substantially to the instantaneous convective destabilization in the sense of increasing the potential instability. Indeed, at some locations, it overcame a tendency to stabilize by the atmosphere without the fold. On the other hand, only a fraction of the region with increased potential instability experienced sufficient large‐scale ascent to lead to convection and it remains to be seen whether folds in general make a significant contribution to the generation of potential instability that can actually be realized. The importance of tropopause folds, or indeed of tropopause depressions in general in the context of convection, is that their small horizontal scale has the potential to focus and amplify any convective destabilization locally. This may account for the localization of outbreaks of severe convective weather which it is important to forecast accurately. Currently numerical weather‐prediction models often do not handle these small scales very acc

ISSN:0035-9009    

DOI:10.1002/qj.49712656207

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThis work is a theoretical and numerical study of the stability properties and scaling laws associated with an idealized radiative‐convective model. We find that the linear‐stability threshold in the model can be described by a radiative Rayleigh number, a parameter that incorporates radiative effects but otherwise resembles the classical Rayleigh number. The energy method is used to find a nonlinear‐stability threshold below which all perturbations, whether infinitesimal or finite‐amplitude, decay. The model behaviour when weakly nonlinear convection occurs is studied via the mean‐field equations. We find that changing the values of viscosity, thermal diffusivity, and radiative damping has only weak effects on the vertical convective heat flux, in contrast to the case for weakly nonlinear Rayleigh‐Bénard convection. Finally, we propose scaling laws for the vertical convective heat flux, vertical velocity, and temperature p

ISSN:0035-9009    

DOI:10.1002/qj.49712656208

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractWe revisit the parametrized moisture model of Garner and Thorpe and show it does indeed support realistic multicellular behaviour when an adequate (and commonly available) amount of instability is present in the initial environment. We also consider the dramatic impact that the simplified model's storms have on their upstream environments, and suggest that this is exaggerated by a fundamental shortcoming in the parametrized moisture framework.

ISSN:0035-9009    

DOI:10.1002/qj.49712656209

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThis study documents the precipitation and kinematic structure of a mature/mature‐to‐decaying, westward propagating, near‐equatorial oceanic mesoscale convective system (MCS) observed by airborne Doppler radars during TOGA‐COARE, the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. This system occurred on 12 December 1992 during the convectively active phase of an intraseasonal oscillation, and was followed, at its dissipating stage, by a redevelopment of convection that led to the MCS observed on 13 December 1992. These two successive MCSs were associated with a two‐day atmospheric disturbance. Radar‐deduced airflows at two time periods reveal many similarities with other tropical oceanic cloud clusters. In particular, a marked rear inflow jet associated with a midlevel cyclonic mesovortex was observed within the rear of the stratiform region.The mesovortex was better defined both in size and depth during the mature‐to‐decaying stage and was located more inside the system within a region of convergence, due to the westward propagation of the rear inflow. Countergradient transports of momentum normal to the mean orientation of the system at midlevels, and downgradient transports at lower and higher levels helped to intensify this rear inflow. These transports were mostly accomplished by eddy structures. Cyclonic vorticity was concentrated at the rear of the MCS and peaked at the centre of the observed closed wind circulation. As previously observed, stretching of the pre‐existing vertical vorticity was the dominant dynamical mechanism which helped to amplify the mesovortex at mid‐to‐upper levels, while tilting of the horizontal vorticity into the vertical was a lesser mechanism and had an opposite effect. In most respects, advective processes were negatively correlated with the stretching and tilting mechanisms, but not sufficiently to be balanced. The resulting tendency was an increase of cyclonic vorticity at low‐to‐mid leve

ISSN:0035-9009    

DOI:10.1002/qj.49712656210

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractHistorical records (approximately 100 years) of Indian summer monsoon rainfall and El Niño/Southern Oscillation (ENSO) indices show a strong negative correlation. This negative correlation is strongest for east Pacific sea surface temperature anomalies (SSTA) that occur during the months of December through to March, which is about three to six months immediately following the monsoon season (June to September). Based on this correlation, one is tempted to speculate that monsoon variability affects ENSO variability. However, it is well known that ENSO is phase locked to the annual cycle in that the largest SSTA occur at the end of the calendar year. In other words, an ENSO which originated well before the summer monsoon season will have its peak amplitude at the end of the calendar year. The purpose of this study is to explain the impact of the monsoon which has a strong seasonal preference on ENSO which has a life cycle of about 4 years.First, a 50‐year atmospheric general‐circulation model simulation with climatological SST is examined to determine the tropical Pacific wind‐stress anomalies that are associated with a variable monsoon but that are also independent of SST variability in the tropical Pacific. Using simple statistical techniques, it is found that a weak (strong) monsoon results in a weakening (strengthening) of the trade winds over the tropical Pacific. To examine how these 'monsoon‐forced wind‐stress anomalies' in the tropical Pacific affect ENSO, simulations were made with a simple coupled model that does not include the effects of a variable monsoon. The effects of the monsoon are then added in the coupled model by either specifying the strength of the monsoon or by parametrizing the strength of the monsoon in terms of the coupled‐model simulated SSTA in the east Pacific. Based on these coupled simulations, a variable monsoon enhances the ENSO variability, particularly three to six months after the monsoon ends, and can also serve as a trigger mechanism for ENSO. It is found that an ongoing warm (cold) ENSO event is made even warmer (colder) by a weak (strong) monsoon. Similarly, warm (cold) events are weakened by a strong (weak) monsoon. These results also reproduce the observed lag/lead ENSO‐monsoon relation where the maximum negative correlation between the monsoon and the SSTA in the east Pacific occurs 3‐6 months after th

ISSN:0035-9009    

DOI:10.1002/qj.49712656211

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY