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1.
The local features of transient kinetic energy and available potential energy were investigated using ECMWF(European Centre for Medium-Range Weather Forecasts) Interim Reanalysis data for the stratospheric sudden warming(SSW) event of January 2009.The Western Europe high plays important roles in the propagation of the energy from North America to Eurasian.When the Western Europe high appeared and shifted eastward,energy conversions increased and energy propagated from North America to Eurasian as a form of interaction energy flow.The baroclinic conversion between transient-eddy kinetic energy(Ke) and transient-eddy available potential energy(Ae) and the horizontal advection of geopotential height were approximately one order of magnitude less than Ke and Ae generation terms.So,these terms were less important to this SSW event. 相似文献
2.
在第(一)部分的基础上,进一步讨论辐散风动能和旋转风动能的收支以及这两种动能之间的转换过程。结果表明,尽管辐散风动能在总动能中所占比重很小,但它的变化与强对流天气过程的发生发展有着更为密切的关系。计算结果表明,在辐散风动能与旋转风动能的转换函数{KD,KR)中,B项(代表垂直运动与旋转风动能的垂直变化的耦合)是最大的转换项;在强对流区,反映涡管伸缩机制的A项也是一个很重要的转换项。就区域时间平均而言,有旋转风动能向辐散风动能(KR→KD)转换。 相似文献
3.
The limited-length-scale k-e{k-\varepsilon} model proposed by Apsley and Castro for the atmospheric boundary layer (Boundary-Layer Meteorol 83(1):75–98, 1997) is revisited with special attention given to its predictions in the constant-stress surface layer. The original model proposes a modification to the length-scale-governing e{\varepsilon} equation that ensures consistency with surface-layer scaling in the limit of small ℓ m/ℓ max (where ℓ m is the mixing length and ℓ max its maximum) and yet imposes a limit on ℓ m as ℓ m/ℓ max approaches one. However, within the equilibrium surface layer and for moderate values of z/ℓ max, the predicted profiles of velocity, mixing length, and dissipation rate using the Apsley and Castro model do not coincide with analytical solutions. In view of this, a general e{\varepsilon} transport equation is derived herein in terms of an arbitrary desired mixing-length expression that ensures exact agreement with corresponding analytical solutions for both neutral and stable stability. From this result, a new expression for Ce3{C_{\varepsilon3}} can be inferred that shows this coefficient tends to a constant only for limiting values of z/L; and, furthermore, that the values of Ce3{C_{\varepsilon3}} for z/L → 0 and z/L →∞ differ by a factor of exactly two. 相似文献
4.
D. Narayana Rao H. R. Singh J. R. Kulkarni A. Y. Chandrika S. Vijaya Bhaskara Rao 《Meteorology and Atmospheric Physics》2000,73(1-2):55-59
Summary Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to
study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke) in the normal monsoon season.
The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT)
has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There
are three dominant modes viz., 50–70, 30–40 and 10–20 day periodicity, which contain considerable fraction of energy and show
high degree of vertical variability. The peak amplitude of 50–70 day mode in u, 30–40 mode in v and eke were observed at 16–17 km
just below the tropopause level. The peak amplitudes of 30–40 day mode in u and 50–70 day mode in v were found in the height
region of 13–16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is
higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere,
and a part of the energy leaks into the stratosphere.
Received September 17, 1998/Revised September 26, 1999 相似文献
5.
The global energy cycle of stationary and transient atmospheric waves: Results from ECMWF analyses 总被引:3,自引:0,他引:3
Summary The role of stationary (monthly mean) and transient (departure from monthly mean) waves within the atmospheric energy cycle is examined using global analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for the period 1980–1987. Only January and July averages are considered.It is confirmed that planetary stationary waves are basically baroclinic. Their contribution to the globally averaged energy cycle of the atmosphere is comparable to that of the transient waves. In January they contribute about 40% to the baroclinic conversion (CA) from zonal mean to eddy available potential energy. Local values for the northern hemisphere even show a predominant role of the stationary wave conversions over those originating from transient waves. Part of the available potential energy of stationary waves (A
SE) is converted to kinetic energy by warm air rising and cold air sinking. Nonlinear energy conversion, which can be interpreted as destruction of stationary temperature waves by transients, is the second sink forA
SE. The order of magnitude of these two processes is similar.Barotropic nonlinear conversions, though negligible in the global average, reveal large conversion rates between the mean positions of the polar and the subtropical jets. Their orientation is suggestive of a tendency to increase stationary wave kinetic energyK
SE at its local minimum between the jets at the expense of the synoptic scale transients.While all terms of the energy cycle related to stationary waves reveal a predominance of the planetary scale (zonal wave numbers 1–3) transient waves are governed by synoptic scale waves (zonal wave numbers 4–9) only with respect to the baroclinic and barotropic conversions: a significant amount of transient wave energy (50% for the global average ofA
TE) is due to planetary scale waves.With 15 Figures 相似文献
6.
The impact of the PBL scheme and the vertical distribution of model layers on simulations of Alpine foehn 总被引:1,自引:0,他引:1
Summary This paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution
of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn
1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer
settings, which mainly differ in the height above ground of the lowest model level (z
1). Specifically, z
1 takes values of about 7 m, 22 m and 36 m, and the experiments with z
1 = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results
are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the
dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z
1 is examined for several different regions.
Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z
1 tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with
observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the
dependence of lee-side flow separation on z
1. However, the simulated 2 m-temperatures are closest to observations for the intermediate z
1 of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing
z
1 for all PBL schemes, turning a positive bias at z
1 = 36 m into a negative bias at z
1 = 7 m. The systematic z
1-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency
of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed
in the paper. On the other hand, a systematic z
1-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to
an unrealistic profile of the vertical mixing coefficient.
Correspondence: Günther Z?ngl, Meteorologisches Institut der Universitat München, 80333 München, Germany 相似文献
7.
利用多年逐月海温距平和区风应力距平观测资料,运用线性回归和EOF分析方法,分析了ENSO相联系的热带太平洋典型风应力异常场结构。结果显示,与ENSO线性相关的风应力异常场在时间尺度上表现为低频变化,在水平结构上主要表现为四个典型分布。 相似文献
8.
Annual methane emission from Finnish mires estimated from eddy covariance campaign measurements 总被引:4,自引:0,他引:4
K. J. Hargreaves D. Fowler C. E. R. Pitcairn M. Aurela 《Theoretical and Applied Climatology》2001,70(1-4):203-213
Summary Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N,
27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position
on CH4 emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with
surface temperature from which an activation energy of −99 kJ mol−1 was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in
three separate years, was calculated to be 5.5 ± 0.4 g CH4 m−2 y−1 of which 0.6 ± 0.1 g CH4 m−2 y−1 (11%) was released during the spring thaw which lasted 20 to 30 days.
The effect of global warming on the CH4 budget of the site was estimated using the central scenario of the SILMU (Finnish Research Programme on Climate Change) model
which predicts annual mean temperature increases of 1.2, 2.4 and 4.4 °C in 2020, 2050 and 2100, respectively. Maximum enhancements
in CH4 emission due to warming were calculated to be 18, 40 and 84% for 2020, 2050 and 2100, respectively. Actual increases may
be smaller because prediction of changes in water table are highly uncertain.
Received September 17, 1999 Revised October 16, 2000 相似文献
9.
Summary Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory,
Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing
the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and
wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong
correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between
the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths
of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January
and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with
a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection
can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their
intermittency and locations relative to floating balloons. 相似文献
10.
Modelling the response of glaciers to climate change by applying volume-area scaling in combination with a high resolution GCM 总被引:4,自引:0,他引:4
A seasonally and regionally differentiated glacier model is used to estimate the contribution that glaciers are likely to
make to global sea level rise over a period of 70 years. A high resolution general circulation model (ECHAM4 T106) is used
to estimate temperature and precipitation changes for a doubled CO2 climate and serves as input for the glacier model. Volume-area relations are used to take into account the reduction of glacier
area resulting from greenhouse warming. Each glacieriated region has a specified glacier size distribution, defined by the
number of glaciers in a size class and a mean area. Changes in glacier volume are calculated by a precipitation dependent
mass balance sensitivity. The model predicts a global sea level rise of 57 mm over a period of 70 years. This corresponds
to a sensitivity of 0.86 mm yr−1K−1. Assuming a constant glacier area as done in earlier work leads to an overestimation of 19% for the contribution to sea level
rise.
Received: 16 August 2000 / Accepted: 21 May 2001 相似文献
11.
Maintenance and oscillation mechanisms of summer tropical upper-tropospheric easterlies 总被引:1,自引:0,他引:1
The mechanisms of the maintenance and oscillation of 1982 summer tropical 200-hPa mean easterly flow and extra-long waves
are investigated in terms of the energy equations in wavenumber-frequency space. Calculation results show that the difference
in heating between land and sea and the boundary effect serve as the main source of energy; frictional dissipation as the
sink; the conversion of available potential energy into kinetic takes place dominantly in the waves of number 1–2 such transformation
is accomplished in just a small amount in zonal mean flow and therefore can be ignored because of the value.
In the interaction between wave and zonal mean flow, the latter loses its available potential and gains kinetic energy. The
tropical easterly belt over 20°N-5°S is found barotropically stable and that over 10°-5°S, unstable. The waves of number 2
and 1 manifest themselves a primary source and sink of kinetic energy, respectively, in the interplay between waves and between
zonal mean flow and wave.
It is found that zonal mean flow and the waves of number 1-2 have a roughly 40-and 20-day oscillational period of kinetic
energy, respectively, whose primary mechanism is the transfer of barotropic energy, the conversion of baroclinic energy, and
the boundary effect. 相似文献
12.
Comprehensive global energetics analysis is carried out for the NCAR CCM2 with different horizontal resolutions of R15, T42,
T63, and T106 to assess the effect of various model truncations on the global energetics characteristics in climate models.
Both the energy levels and energy transformations are examined over the zonal wave number domain during a northern winter
and summer. In addition to the simulated atmosphere, the ECMWF global analysis during 1986 to 1990 is analyzed for comparison
using the same diagnostic scheme. Previous studies have revealed that zonal kinetic energy is supplied by synoptic disturbances
in terms of the zonal-wave interactions of kinetic energy. According to our result, however, such an energy flow from eddies
to zonal motions is valid only for zonal wave numbers up to about 30. We find that the zonal-wave interactions of kinetic
energy change sign beyond wave number 30 where the energy is transformed from zonal to eddies for both the ECMWF and CCM2-T106.
The large-scale zonal motions are diffusive against the short waves beyond wave number 30, which may well be parameterized
by various forms of the diffusion schemes. We suggest from this result that the atmospheric disturbances with wave numbers
lower than 30 are necessary to represent accurately the two-way interactions between zonal and eddy motions, because these
waves can actively influence the behavior of the zonal motions. Based on this finding, we suggest that the model resolution
of R15 is inadequate for climate studies from the energetics point of view, and that resolution of T42 is the minimum requirement
to represent the general circulation adequately. Some other discrepancies are discussed in detail for the coarse resolution
climate models.
Received: 15 July 1996/Accepted: 3 January 1997 相似文献
13.
We demonstrate that a hemispherically averaged upwelling-diffusion energy-balance climate model (UD/EBM) can emulate the
surface air temperature change and sea-level rise due to thermal expansion, predicted by the HadCM2 coupled atmosphere-ocean
general circulation model, for various scenarios of anthropogenic radiative forcing over 1860–2100. A climate sensitivity
of 2.6 °C is assumed, and a representation of the effect of sea-ice retreat on surface air temperature is required. In an
extended experiment, with CO2 concentration held constant at twice the control run value, the HadCM2 effective climate sensitivity is found to increase
from about 2.0 °C at the beginning of the integration to 3.85 °C after 900 years. The sea-level rise by this time is almost
1.0 m and the rate of rise fairly steady, implying that the final equilibrium value (the `commitment') is large. The base
UD/EBM can fit the 900-year simulation of surface temperature change and thermal expansion provided that the time-dependent
climate sensitivity is specified, but the vertical profile of warming in the ocean is not well reproduced. The main discrepancy
is the relatively large mid-depth warming in the HadCM2 ocean, that can be emulated by (1) diagnosing depth-dependent diffusivities
that increase through time; (2) diagnosing depth-dependent diffusivities for a pure-diffusion (zero upwelling) model; or (3)
diagnosing higher depth-dependent diffusivities that are applied to temperature perturbations only. The latter two models can be run to equilibrium, and with a climate sensitivity of 3.85 °C, they give sea-level rise
commitments of 1.7 m and 1.3 m, respectively.
Received: 27 April 1999 / Accepted: 13 September 2000 相似文献
14.
Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February
1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations
of the wind vector u′, v′, w′ and the sonic temperature θ
s
′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the
zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h
(the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ
w
/u
* is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller
compared to other urban turbulence studies. The σθ/θ* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant
deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The
profile of friction velocity u
* is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement
height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and
as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities
S
w
(f)/S
u
(f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit
best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2.
Received September 26, 1997 Revised February 15, 1998 相似文献
15.
16.
L. D. D. Harvey 《Climate Dynamics》2000,16(7):491-500
This work uses an energy balance climate model (EBCM) with explicit infrared radiative transfer, parametrized tropospheric
temperature and humidity profiles, and separate stratosphere, troposphere, and surface energy balances, to investigate claims
that a downward redistribution of tropospheric water vapor in response to surface warming could serve as a strong negative
feedback on climatic change. A series of sensitivity tests is carried out using: (1) a variety of relationships between total
precipitable water in the troposphere and temperature; (2) feedbacks between surface temperature and the vertical distribution
of tropospheric water vapor at low latitudes; and (3) feedback between surface temperature or meridional temperature gradient
and lapse rate. Fixed relative humidity (RH) enhances the global mean surface temperature response to a CO2 doubling by only 50% compared to fixed absolute humidity, giving a response of 1.8 K. When water vapor is assumed to be redistributed
downward between 30°S–30°N such that a 1 K surface warming reduces total precipitable water above 600 hPa by 10%, the global
mean surface air temperature response is reduced to 1.2 K. Assuming a stronger downward redistribution in relation to surface
temperature change has a rapidly diminishing marginal effect on global mean and tropical surface temperature response, while
slightly increasing the warming at high latitudes due to the parametrized dependence of middle-to-high latitude lapse rate
on the meridional temperature gradient. A modest downward water vapor redistribution, such that absolute humidity in the upper
troposphere at subtropical latitudes is constant as total precipitable water increases, can reduce the tropical temperature
sensitivity to less than 1 K, while increasing the equator-to-pole amplification of the surface air temperature response from
a factor of about three to a factor of four. However, it is concluded that whatever changes in future GCM response might occur
as a result of new parametrizations of subgrid-scale processes, they are exceedingly unlikely to produce a climate sensitivity
to a CO2 doubling of less than 1 K even if there is a strong downward shift in the water vapor distribution as climate warms.
Received: 23 February 1998 / Accepted: 1 November 1999 相似文献
17.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献
18.
Lisa K. Whalley Kate L. Furneaux Tom Gravestock Helen M. Atkinson Catherine S. E. Bale Trevor Ingham William J. Bloss Dwayne E. Heard 《Journal of Atmospheric Chemistry》2007,58(1):19-39
A Laser Induced Fluorescence (LIF) instrument has been developed to detect iodine monoxide (IO) radicals in the atmosphere.
An all solid-state Nd:YAG pumped Ti:Sapphire laser operating at approximately 445 nm was used to excite the (2,0) band of
the IO A2Π3/2 ← X2Π3/2 electronic transition, with off-resonance fluorescence in the (2,5) band detected at 521 nm. The sensitivity of the instrument
was determined by calibration. IO (between 10 and 150 pptV) was generated following the 184.9 nm photolysis of N2O/CF3I/N2 mixtures with O3 actinometry used to determine the photolysis flux. The detection limit was determined to be 0.3 pptV for a 300 s integration
period, with an uncertainty of 23% (1σ). The instrument was deployed in August/September 2006 during the RHaMBLe (Reactive Halogens in the Marine Boundary Layer)
campaign in Roscoff, France. Located on a small jetty, a few metres from the water’s edge at high tide, the instrument measured
significant levels of IO on 11 days, with a maximum of 27.6 ± 3.2 pptV observed on one day (averaged over 10 s) representing
the highest IO mixing ratio recorded in the marine boundary layer to date. IO displayed a clear diurnal profile with a maximum
at low tide during the daytime. These results represent the first point measurements of IO in the atmosphere by LIF. 相似文献
19.
R. García-Herrera D. Barriopedro E. Hernández D. Paredes J. F. Correoso L. Prieto 《Meteorology and Atmospheric Physics》2005,90(3-4):225-243
Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological
settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region
between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics.
Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period,
with September being the most active period. They tend to develop during the late afternoon or early night, with preferred
eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km2 is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity
was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average
speeds. The average MCS precipitation rate was 3.3 mm·h−1 but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h−1 (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation
in the Mediterranean region during the convective season, but its impact varied according to the location.
Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean
Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern
portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h−1 towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θe) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern
Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10−10 s−2 over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection
were the most relevant forcing mechanisms during 2001. 相似文献
20.
Summary Sublayer-Stanton numbers, Bi, of heat and matter for the interfacial sublayer over aerodynamically smooth surfaces determined for forced convective conditions
by elementary and numerical integration are reviewed and evaluated. The results are based on Roth’s modified Heisenberg model
for the spectral energy transfer in the equilibrium range under locally isotropic conditions and the approaches of Reichardt,
Elser, Deissler, van Driest, Rannie, Sheppard, and Spalding for the normalized eddy diffusivity Km/v. The results substantiate that with the exception of Sheppard’s Km/v-approach all formulations are appropriate to provide sublayer-Stanton numbers with a sufficient degree of accuracy. From
a theoretical point of view the Km/v-relationships of Roth, Reichardt, van Driest, and Spalding are to be preferred, when a turbulent Prandtl number Prt = 1 is presumed.
Since within the framework of mesoscale meteorological modelling numerical integration techniques would consume too much CPU-time
because of the large number of near-wall grid points, a parameterization formula for the sublayer-Stanton number is proposed
and evaluated. Compared to the Bi
−1-results obtained by numerical integration, this kind of parameterization leads to a relative error of less than 5 percent
for roughness Reynolds numbers, ηr, ranging from 30 to 600.
Received January 2001 Revised November 5, 2001 相似文献