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Frontal substructures within the planetary boundary layer 总被引：2，自引：0，他引：2
A two-dimensional mesoscale model, extended by a TKE closure for the subgrid-scale terms and coupled with a soil model, is used to investigate the role of the Planetary Boundary Layer (PBL) for the development and the substructures of two different types of cold fronts. The effects of turbulent friction, large-scale (geostrophic) forcing and the diurnal variation of the terms of the surface energy balance (SEB) equation on the frontal development are studied by 10 different model runs. The ageostrophic cross-frontal circulation in the lowest two kilometres of a cold front results from friction as well as from large-scale forcing. The first one dominates the PBL processes and causes a special boundary-layer structure, which becomes apparent through the existence of seven characteristic zones defined for the x-z cross sections of potential temperature. The arrangement of these characteristic zones depends on the sense of rotation of the frictionally induced part of the ageostrophic circulation and hence on the direction of the along-front jet within the boundary layer. The daytime increase of the terms of the SEB equation for a midlatitude midsummer case leads to a strong enhancement of the frictionally induced cross-frontal circulation. The arrangement of the seven characteristic zones, however, is approximately conserved. 相似文献
2.
South Australian rainfall variability and climate extremes 总被引：1，自引：0，他引：1
Rainfall extremes over South Australia are connected with broad-scale atmospheric rearrangements associated with strong meridional
sea surface temperature (SST) gradients in the eastern Indian Ocean. Thirty-seven years of winter radiosonde data is used
to calculate a time series of precipitable water (PW) and convective available potential energy (CAPE) in the atmosphere.
Principle component analysis on the parameters of CAPE and PW identify key modes of variability that are spatially and seasonally
consistent with tropospheric processes over Australia. The correlation of the leading principle component of winter PW to
winter rainfall anomalies reveal the spatial structure of the northwest cloudband and fronts that cross the southern half
of the continent during winter. Similarly the second and third principle components, respectively, reveal the structures of
the less frequent northern and continental cloudbands with remarkable consistency. 850 hPa-level wind analysis shows that
during dry seasons, anomalous offshore flow over the northwest of Australia inhibits advection of moisture into the northwest,
while enhanced subsidence from stronger anticyclonic circulation over the southern half of the continent reduces CAPE. This
coincides with a southward shift of the subtropical ridge resulting in frontal systems passing well to the south of the continent,
thus producing less frequent interaction with moist air advected from the tropics. Wet winters are the reverse, where a weaker
meridional pressure gradient to the south of the continent allows rain-bearing fronts to reach lower latitudes. The analysis
of SSTs in the Indian Ocean indicate that anomalous warm (cool) waters in the southeast Indian Ocean coincide with a southward
(northward) shift in the subtropical ridge during dry (wet) seasons. 相似文献
3.
H. Kraus C. M. Ewenz M. Kremer J. M. Hacker 《Meteorology and Atmospheric Physics》2000,73(3-4):157-175
Summary In Southern Australia summertime deep cold fronts are frequently preceded by a shallow cold frontal line connected to a prefrontal
lower tropospheric trough. The advance of this line defines a “cool change” which in many cases causes severe weather events.
The goal of this paper is to analyze the multi-scale structure of these cool changes using aircraft observations and synoptic-scale
analyses. The aircraft measurements on cross-frontal tracks of horizontal lengths of up to 300 km are performed with an average
resolution of 3 to 4 m along the track. Thus a multi-scale analysis from micro-scale events up to the synoptic-scale phenomena
can be presented. All flights and thus all meso- and micro-scale analyses are performed over water only. The obviously very
different characteristics of the cool change structure elements over land are not investigated.
The synoptic analyses for one very typical case show a prefrontal trough as characterized by its position in relation to the
main deep cold front, its source region in Western Australia and its extent to the southeast. Fields of strong wind shear,
temperature gradients, vertical wind and Q-vectors are displayed.
The meso-β-scale x, z-cross-sections derived from two aircraft missions (data of the second one in brackets) show: a shallow cold front with a
160 (60) km wide transition zone in which the near surface potential temperature drops rather steadily by 9 °C (20 °C); a
shallow feeder flow topped by a strong inversion with a vertical gradient of potential temperature up to 5 °C/100 m between
the top of the feeder flow at 400 (200) m and 1500 (700) m; a cross-frontal circulation expressed by the ageostrophic wind
components u
ϕ,subscale and w with a center at 1200 m over the frontal edge of the feeder flow (for one mission only); a strong shear of the along-frontal
wind component v
ϕ with a large increase of the negative v
ϕ-values with height, which very well fits to the synoptic-scale view of the wave structure of the geostrophic wind (well-known
from the upper level synoptic charts) at different heights; a jet core of this along-frontal wind in the center of the cross-frontal
circulation, again for one mission only.
A very striking example of a micro-scale event is an approximately 1 km wide head of a frontal squall line. It shows dramatic
changes of all meteorological parameters. The event is displayed in a horizontal domain of 4 km with full resolution (∼ 4 m).
Derivatives of the measured parameters in the cross-frontal direction add information to the space series of the parameters
themselves. Deformation frontogenesis of potential temperature and specific humidity show very large values on the scale resolved
here. Fortunately the squall line could be sampled again at the same height, but in a somewhat degenerated state 1? h later.
Received September 3, 1999 Revised December 14, 1999 相似文献
4.
A. Becker K. Keuler C. M. Ewenz H. Kraus E. Schaller 《Meteorology and Atmospheric Physics》1997,62(3-4):201-214
Summary The influence of turbulent friction on the propagation of cold fronts is investigated by numerical simulations using a two-dimensional mesoscale model. We compare the frictional effect with the effects of large-scale shear forcing and energy conversion at the earth's surface and discuss the synergic effect of all three mentioned processes. There is no pure superposition of these effects indicating that nonlinear interaction plays a role. In addition it is possible to show that—depending on the along-front jet—friction does not necessarily slow down the front but can also accelerate it. The direction of the along-front jet within the planetary boundary layer (PBL) is crucial for that question.With 12 Figures 相似文献
5.
K. Stephan H. Kraus C. M. Ewenz J. M. Hacker 《Meteorology and Atmospheric Physics》1999,70(1-2):81-95
Summary This paper is a contribution to experimental meteorology: A sea-breeze front was investigated by aircraft observations and
thorough numerical analysis using an unprecedented number of runs crossing the same front within a timespan of . The 33 runs were flown in a situation of offshore geostrophic wind of 5 m/s in 1000 hPa and with the strategy of obtaining
information on the four-dimensional field (t=time, x=cross-coastal coordinate, y=coast-parallel coordinate, z=height): 9 runs in x-direction (and reverse) at different heights to yield x,z-cross-sections of the observed meteorological quantities (specific humidity q, potential temperature Θ and the components u, v and w of the wind velocity), assuming a frozen structure in time; the next 7 runs again in x-direction but all at the same level and on the same track to yield x,t-diagrams of the same quantities in order to study the temporal changes compared to those with x and z; the next 10 runs as a zig-zagging flight track crossing the front but drifting in y-direction, all at the same height, in order to obtain the y-dependency; andfinally 7 runs for another x,z-cross-sectional analysis, which can be compared to that evaluated from the runs at the beginning of the mission.
The paper describes the 4-dimensional dependencies in detail. Pure x-variations at constant z are expressed by VCM low-pass filtered space series (VCM=variance conserving multiresolution, according to Howell and Mahrt,
1994). The x,z-analyses are similar to those in Kraus et al. (1990) and Finkele et al. (1995) verifying these results. The comparison of
the x,z-studies gained from the data at the beginning and at the end of the mission show how the sea-breeze frontal area changes
its structure. The fluctuations (in time) revealed by the low-pass filtered x,t-runs (same track and same height) are smaller than the contour intervals chosen in the x,z-cross-sections. This shows, that the single runs, from which the x,z-cross-sections are constructed, reliably and significantly contribute to the interpolated structure. The paper also demonstrates
the overall development of the front within the 31/2 h of continuous observation. The x,y-fields demonstrate that the y-dependency of the various quantities is generally one order of magnitude smaller than the x-dependency and that the assumption of negligible y-dependency holds in the first order of approximation for a fairly homogeneous coast. Convective disturbances of a horizontal
scale of 1 to 4 km at the landward side of the front, embedded in the offshore flow and bouncing against the landward propagating
sea-breeze front, considerably contribute to variations of the frontal propagation speed and of the frontal shape and also
to changes of the parameters with the along-frontal coordinate y.
Received April 24, 1998 Revised November 3, 1998 相似文献
6.
Summary Frontogenesis is frequently described by theQ-vector (Hoskins et al., 1978), a term being composed of several derivatives of basic meteorological parameters and their products. Its distribution and especially the
H
·Q-fields are highly important to estimate frontogenesis and cross frontal circulation. Although theQ-vector (Hoskins et al., 1978) allows an easier assessment of the vertical wind forcing than the original omega equation of the quasi-geostrophic theory, it is still difficul to imagine the three-dimensional (3-d) spatial distribution ofQ and
H
·Q even for standard atmospheric fields. Thus there is a need to shed more light in theQ and
H
·Q-fields for special synoptic situations.This is done here by constructing analytical 3-d geostrophically balanced wind-and temperature fields, for which theQ-forcing (Qformed with the geostrophic wind) can easily be computed and presented. Three examples (see Sections 3 to 5) are discussed yielding typical and realistic (compared to known pattern) 3-d forcing distributions ofQ and
H
·Q. Within the simple analytical scheme used here their origin can casily be understood. These fields of a 2000×2000 km2 horizontal domain ranging up to 250 hPa are: A modified Bergeron deformation field containing a cold front (case I a) and a warm front (case I b); an upper tropospheric jet including a jet-parallel transition zone between warm and cold air (case II); and a circular low pressure circulation pattern with two fronts (case III).The paper presents these 3-d fields with the advantage that the analytical method is not affected by any kind of limited numerical resolution. It also shows how these fields degenerate with decreasing resolution if the analytical data are used in descrete form. This simulates working with discrete numerical data and demonstrates how narrow frontal zones of structure elements ofQ and
H
·Q considerably smooth out with increasing grid distances.With 17 Figures 相似文献
7.
Examination and parameterization of the root water uptake model from stem water potential and sap flow measurements 总被引：2，自引：0，他引：2
Yuting Yang Huade Guan John L. Hutson Hailong Wang Caecilia Ewenz Songhao Shang Craig T. Simmons 《水文研究》2013,27(20):2857-2863
A simple field‐based method for directly parameterizing root water uptake models is proposed. Stem psychrometers and sap flow meters are used to measure stem water potential and plant transpiration rate continuously and simultaneously. Predawn stem water potential is selected as a surrogate for root zone soil water potential to examine and parameterize the root water uptake–water stress response functions. The method is applied to two drooping sheoak (Allocasuarina verticillata) trees for a period of 80 days, covering both a dry season and a wet season. The results indicate that the S‐shape function is more appropriate than the Feddes piecewise linear function for drooping sheoak to represent the effect of soil moisture stress on its root water uptake performance. Besides, the water stress function is found to be not only a function of soil moisture but also dependent of the atmospheric demand. As a result, the water stress function is corrected for the effect of atmospheric conditions. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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