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1.
H. Linné B. Hennemuth J. Bösenberg K. Ertel 《Theoretical and Applied Climatology》2007,87(1-4):201-211
Summary Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations
by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar.
The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The
eddy-correlation method was applied, and error estimates of ±50 W/m2 for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between
60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with
low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements
are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux
profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes
from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different
structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer
a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from
the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase
in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about
zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease. 相似文献
2.
Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange
processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric
model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using
the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities.
Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study
vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured
net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the
investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface
fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method,
which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL
is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed
net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible
heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation
of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes,
but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the
LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the
LM scale. 相似文献
3.
Summary ?Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were
performed on 18 June 1998 during the LITFASS-98 field experiment. The area-averaged turbulent vertical fluxes of momentum,
sensible, and latent heat were determined on a 15 km × 15 km and a 10 km × 10 km flight pattern, respectively. The flights
were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus
clouds.
Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively
small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was
identified as a significant peak in some co-spectra. The turbulent fluxes of momentum and sensible heat at 80 m above the
ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed
for the latent heat flux, probably due to the vertical distribution of humidity in the boundary layer.
Statistical error analysis of the fluxes F showed that the systematic statistical error ΔF was one order of magnitude smaller than the standard deviation σ
F
. The difference between area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements was much smaller than
σ
F
, indicating that the systematic statistical error was possibly over-estimated by the usual method.
In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data.
A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent
and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements
on various surface types. Systematic discrepancies between airborne and ground-based measurements were not found.
Received June 18, 2001; revised December 21, 2001; accepted June 3, 2002 相似文献
4.
S. Khodayar N. Kalthoff M. Fiebig-Wittmaack M. Kohler 《Meteorology and Atmospheric Physics》2008,99(3-4):181-198
Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends
from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical
anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between
the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale
wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near
the coast and in the valley.
Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change
considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases
slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze
winds of 3–4 m s−1 prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night.
The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day,
no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about
150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above
the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection
in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In
the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection
of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence
is of minor relevance to the moisture budget of the boundary layer.
Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe,
Postfach 3640, 76021 Karlsruhe, Germany 相似文献
5.
Study on Physical Mechanism of Influence on Atmospheric Boundary Layer Depth in the Arid Regions of Northwest China 总被引:1,自引:0,他引:1 下载免费PDF全文
Using the sounding data of wind, temperature, and humidity in the boundary layer and micrometeorological data on the earth's surface observed in the same period in Dunhuang arid region of Northwest China,this paper researches characteristics of potential temperature, wind, and humidity profiles, confirms the structure and depth of thermodynamic boundary layer in Dunhuang region, and analyzses the relationship of depth of thermodynamic boundary layer with surface radiation, buoyancy flux as well as wind speed and
wind direction shear in the boundary layer. The results show that the maximum depth of diurnal convective boundary layer is basically above 2000 m during the observational period, many times even in excess of 3000 m and sometimes up to 4000 m; the depth of nocturnal stable boundary layer basically maintains within a range of 1000-1500 m. As a whole, the depth of atmospheric boundary layer is obviously bigger than those results observed in other regions before. By analyzing, a preliminary judgement is that the depth
of atmospheric thermodynamic boundary layer in Dunhuang region may relate to local especial radiation characteristics, surface properties (soil moisture content and heat capacity) as well as wind velocity shear of boundary layer, and these properties have formed strong buoyancy flux and dynamic forcing in a local region which are fundamental causes for producing a super deep atmospheric boundary layer. 相似文献
6.
I. Foyo-Moreno I. Alados F. J. Olmo J. Vida L. Alados-Arboledas 《Theoretical and Applied Climatology》2001,68(1-2):41-50
Summary Knowledge of ultraviolet radiation is necessary in different applications, in the absence of measurements, this radiometric
flux must be estimated from available parameters. To compute this flux under all sky conditions one must consider the influence
of clouds. Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type
of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received
at the Earth’s surface. This cloud radiative effect is different for the different solar spectral bands. In this work, we
analyse the cloud radiative effect over ultraviolet radiation (290–385 nm). This could be done by defining a cloud modification
Factor. We have developed such cloud modification Factor considering two different types of clouds. The efficiency of the
cloud radiative effect scheme has been tested in combination with a cloudless sky empirical model using independent data sets.
The performance of the model has been tested in relation to its predictive capability of global ultraviolet radiation. For
this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almería, a
seashore location (36.83° N, 2.41° W, 20 m a.m.s.l.), while the second one is located at Granada (37.18° N, 3.58° W, 660 m
a.m.s.l.), an inland location. The database includes hourly values of the relevant variables that cover the years 1993–94
in Almería and 1994–95 in Granada. Cloud cover information provided by the Spanish Meteorological Service has been include
to compute the clouds radiative effect. After our study, it appears that the combination of an appropriate cloudless sky model
with the cloud modification Factor scheme provides estimates of ultraviolet radiation with mean bias deviation of about 5%
that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar
spectrum provides evidence for the spectral dependency of the cloud radiative effect.
Received November 15, 1999 Revised September 11, 2000 相似文献
7.
根据2017、2019年7月塔克拉玛干沙漠腹地GPS探空和地面观测数据,利用位温廓线法等方法,对比分析了沙漠腹地夏季晴天和沙尘暴天气大气边界层结构变化特征。结果表明:晴天和沙尘暴天气大气边界层结构特征显著不同。晴天大气边界层各气象要素垂直分布较为均一,白天对流边界层深厚,高度接近5 km,夜间稳定边界层一般在500 m左右。沙尘暴天气边界层内位温和比湿垂直变化较小,风速较大,可达24.0 m/s,其白天对流边界层在1.5 km左右,夜间稳定边界层在1 km左右。晴天辐射强烈,地表升温迅速,湍流旺盛,是形成晴天深厚对流边界层的主要因素。大尺度天气系统冷平流的动力条件,以及云和沙尘减弱了到达地表的辐射强度是形成沙尘暴天气独特的大气边界层结构的主要因素。 相似文献
8.
Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main
data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were
provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from
212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from
19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27%
and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and
radiation efficiency declined with elevation at an annual mean of 1.15 Wm−2/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline
of 1.54 Wm−2/100 m and 0.34 Wm−2/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm−2 for the lowland site and 73 Wm−2 for the highest elevated site.
Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been
validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott
relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition
was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen
temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith
and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation,
cloud amount and screen temperature.
Received March 5, 2001 Revised October 29, 2001 相似文献
9.
W. H. Moores 《Boundary-Layer Meteorology》1982,22(3):283-294
Results derived from simultaneous measurements of turbulent heat flux and radiation convergence in the daytime convective boundary layer are presented. It is found that the effects of long-wave radiation result in a warming near the surface and cooling at higher levels, in good agreement with infra-red radiative transfer models. Heating rates, roughly 30% of those produced by turbulence, are observed as a result of the absorption of short-wave radiation in the lowest 1000 m of the atmosphere. 相似文献
10.
Although the residual layer has already been noted in the classical diurnal cycle of the atmospheric boundary layer,its effect on the development of the convective boundary layer has not been well studied. In this study, based on 3-hourly20 th century reanalysis data, the residual layer is considered as a common layer capping the convective boundary layer. It is identified daily by investigating the development of the convective boundary layer. The region of interest is bounded by(30°–60° N, 80°–120° E), where a residual layer deeper than 2000 m has been reported using radiosondes. The lapse rate and wind shear within the residual layer are compared with the surface sensible heat flux by investigating their climatological means, interannual variations and daily variations. The lapse rate of the residual layer and the convective boundary layer depth correspond well in their seasonal variations and climatological mean patterns. On the interannual scale, the correlation coefficient between their regional averaged(40°–50°N, 90°–110° E) variations is higher than that between the surface sensible heat flux and convective boundary layer depth. On the daily scale, the correlation between the lapse rate and the convective boundary layer depth in most months is still statistically significant during 1970–2012. Therefore, we suggest that the existence of a deep neutral residual layer is crucial to the formation of a deep convective boundary layer near the Mongolian regions. 相似文献
11.
南海季风建立前后珠江三角洲的陆气热量交换与热力边界层结构特征 总被引:4,自引:0,他引:4
根据动力与热力指标,2004和2005年南海季风建立前后可分成明显的4个阶段——季风建立前的雨期、非雨期;季风建立后的活跃期与非活跃期。对2004和2005年南海季风建立前后的广州番禺综合外场观测资料进行分析,得到了这4个阶段陆气热量交换与热力边界层的主要特征:净辐射与净短波辐射的变化趋势基本一致,净短波辐射与净长波辐射之比为3.49—4.81,净短波辐射是净辐射的主要贡献项,云量与降水是控制净短波辐射与净辐射的直接因素;季风活跃期间午间对流云系对太阳辐射衰减显著,造成了辐射各分量以及热通量的峰值区变窄,量值急剧变小;季风建立前后感热与潜热均是净辐射的主要消耗项,占净辐射的90%以上,潜热明显大于感热,2005年较2004年潜热的分配额有明显的增加,其原因可能与近地层的风速较大,总是维持向上的湿度梯度有关;季风建立前后除季风活跃期外边界层位温结构均具有明显的日变化特征,午间混合层可发展至1070m,而季风活跃期间午间混合层发展受到对流云释放潜热的抑制,导致季风活跃期混合层消失的现象,分析还发现季风建立前后各阶段夜间残余混合层均不明显。分析表明引起陆气能量过程及边界层热力结构差异的关键因素之一是云系与降水,加强边界层过程与降水宏微观过程相互作用的研究是深入认识陆气过程与边界层结构特征的关键。 相似文献
12.
Jeffrey S. Grabon Kenneth J. Davis Christoph Kiemle Gerhard Ehret 《Boundary-Layer Meteorology》2010,134(1):61-83
Airborne, light detection and ranging (lidar) backscatter observations of the convective boundary layer from the International
H2O Project (IHOP) in 2002 are analysed to study the structure of the transition zone; the backscatter gradient between the
convective boundary layer and free atmosphere. A new mathematical algorithm is developed and used to extract high-resolution
(15 m) transition-zone boundaries from 6,500 km (flight legs) of airborne observations. The cospectra of transition-zone boundaries
and its thickness indicate that thickness changes occur from boundaries moving in opposite directions (vertically) at small
wavelengths (<1 km), while at longer wavelengths (>1 km) both boundaries move coherently, with the lower boundary changing
altitude more rapidly. Daily probability distributions of the transition-zone thickness are positively skewed with a mode
of 60 m. The structure of the transition zone shows no dependence on the “overall” Richardson number, unlike the entrainment
zone. This study provides the first quantitative characterization of the structure of the instantaneous transition zone, a
contribution towards an improved understanding of convective boundary-layer entrainment. 相似文献
13.
A Study of the Internal Boundary Layer due to a Roughness Change in Neutral Conditions Observed During the LINEX Field Campaigns 总被引:1,自引:0,他引:1
Summary As an aspect of the LINEX field studies (1996–1997; Lindenberg near Beeskow, Germany), the characteristics of the internal
boundary layer (IBL) that is associated with a step change of the surface roughnesses in neutral constant stress layers was
investigated and is reported in this paper. Both smooth to rough (in 1996) and rough to smooth (in 1997) types of flow, have
been studied based upon the profiles of mean wind and temperature realised from a 10-m mast and eddy correlation measurements
taken at two levels (2 m and 5 m). Depending upon wind direction, the fetch at the site varied between 140 m and 315 m within
the wind sector (200° to 340°) used for the field investigations. The height of the IBL, δ, had been determined from the intersect
of the logarithmic wind-profiles below (< 2 m) and above (> 6 ) the interface. Values of δ obtained at the experimental site
compared fairly well to the existing theoretical/empirical fetch-height relationships of the form: δ=aċx
b
, where a, b, are empirical constants. The ratio for the friction velocities below and above the IBL as measured directly by the eddy
correlation techniques showed that for fetches less than 250 m there was an increase (decrease) of about 20% of the momentum
flux arising from the smooth to rough (rough to smooth) transitions. Influences of distant obstructions (e.g., bushes, pockets
of trees) on the surface flow were markedly important on the examined wind profiles and such can be indicative as multiple
IBLs.
Received September 1, 1997 Revised August 5, 1998 相似文献
14.
L. Cvitan 《Meteorology and Atmospheric Physics》2006,93(3-4):235-246
Summary The stability parameter μ is suggested as the one which is determinable with satisfying accuracy for routine application by
means of commonly accessible meteorological data at the Molve location (Croatia). The similarity functions applied for vertical
wind speed simulation in the planetary boundary layer (PBL) at Molve were useful for the determination of local stability
classes. Universal similarity functions were applied for unstable and neutral stability, whereas local similarity functions
were established for stable stratification. Wind speed simulations were performed using two types of wind models. The Monin-Obukhov
similarity theory was included in both types. However, it turned out that for the operative determination of the stability
of the 35 m deep lowest layer, the stability parameter μ was locally a better stability parameter than the Monin-Obukhov parameter
z/L. That was possibly because 35 m deep lowest layer sometimes (depending upon stability) includes a large proportion of
the Ekman layer and parameter μ is originally designed for the deeper part of PBL than z/L that is originally designed for
the surface layer. At Molve, the input data for local wind models as well as for the stability parameter μ were wind speed
at 35 m and temperature at 2 and 35 m above the ground. 相似文献
15.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(3):365-383
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under
consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe
the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z
m
(defined by the maximum of the potential temperature gradient) and its standard deviation σ
m
, the mixed-layer height z
i
(defined by the minimum of the potential temperature flux), entrainment velocity W
e, and the entrainment flux H
i
. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature
flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime,
and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured
values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer. 相似文献
16.
Dissimilarity of Scalar Transport in the Convective Boundary Layer in Inhomogeneous Landscapes 总被引:1,自引:1,他引:0
A land-surface model (LSM) is coupled with a large-eddy simulation (LES) model to investigate the vegetation-atmosphere exchange
of heat, water vapour, and carbon dioxide (CO2) in heterogeneous landscapes. The dissimilarity of scalar transport in the lower convective boundary layer is quantified
in several ways: eddy diffusivity, spatial structure of the scalar fields, and spatial and temporal variations in the surface
fluxes of these scalars. The results show that eddy diffusivities differ among the three scalars, by up to 10–12%, in the
surface layer; the difference is partly attributed to the influence of top-down diffusion. The turbulence-organized structures
of CO2 bear more resemblance to those of water vapour than those of the potential temperature. The surface fluxes when coupled with
the flow aloft show large spatial variations even with perfectly homogeneous surface conditions and constant solar radiation
forcing across the horizontal simulation domain. In general, the surface sensible heat flux shows the greatest spatial and
temporal variations, and the CO2 flux the least. Furthermore, our results show that the one-dimensional land-surface model scheme underestimates the surface
heat flux by 3–8% and overestimates the water vapour and CO2 fluxes by 2–8% and 1–9%, respectively, as compared to the flux simulated with the coupled LES-LSM. 相似文献
17.
Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.ST Systems Corporation, Lanham, MD, 20706, U.S.A. 相似文献
18.
Jens Bange Peter Zittel Thomas Spieß Jörg Uhlenbrock Frank Beyrich 《Boundary-Layer Meteorology》2006,119(3):527-561
The low-level flight method (LLF) has been combined with linear inverse models (IM) resulting in an LLF+IM method for the determination of area-averaged turbulent surface fluxes. With this combination, the vertical divergences of the turbulent latent and sensible heat fluxes were calculated from horizontal flights. The statistical errors of the derived turbulent surface fluxes were significantly reduced. The LLF+IM method was tested both in numerical and field experiments. Large-eddy simulations (LES) were performed to compare ‘true’ flux profiles with ‘measurements’ of simulated flights in an idealised convective boundary layer. Small differences between the ‘true’ and the ‘measured’ fluxes were found, but the vertical flux divergences were correctly calculated by the LLF+IM method. The LLF+IM method was then applied to data collected during two flights with the Helipod, a turbulence probe carried by a helicopter, and with the research aircraft Do 128 in the LITFASS-98 field campaign. The derived surface fluxes were compared with results from eddy-covariance surface stations and with large-aperture scintillometer data. The comparison showed that the LLF+IM method worked well for the sensible heat flux at 77 and 200 m flight levels, and also for the latent heat flux at the lowest level. The model quality control indicated failures for the latent heat flux at the 200 m level (and higher), which were probably due to large moisture fluctuations that could not be modelled using linear assumptions. Finally the LLF+IM method was applied to more than twenty low-level flights from the LITFASS-2003 experiment. Comparison with aggregated surface flux data revealed good agreement for the sensible heat flux but larger discrepancies and a higher statistical uncertainty for the latent heat flux 相似文献
19.
A Study of the Atmospheric Boundary Layer Structure During a Clear Day in the Arid Region of Northwest China 下载免费PDF全文
The local climate and atmospheric circulation pattern exert a clear influence on the atmospheric boundary layer (ABL) formation and development in Northwest China. In this paper, we use field observational data to analyze the distribution and characteristics of the ABL in the extremely arid desert in Dunhuang, Northwest China. These data show that the daytime convective boundary layer and night time stable boundary layer in this area extend to higher altitudes than in other areas. In the night time, the stable boundary layer exceeds 900 m in altitude and can sometimes peak at 1750 m, above which the residual layer may reach up to about 4000 m. The daytime convective boundary layer develops rapidly after entering the residual layer, and exceeds 4000 m in thickness. The results show that the deep convective boundary layer in the daytime is a pre-requisite for maintaining the deep residual mixed layer in the night time. Meanwhile, the deep residual mixed layer in the night time provides favorable thermal conditions for the development of the convective boundary layer in the daytime. The prolonged periods of clear weather that often occurs in this area allow the cumulative effect of the atmospheric residual layer to develop fully, which creates thermal conditions beneficial for the growth of the daytime convective boundary layer. At the same time, the land surface process and atmospheric motion within the surface layer in this area also provide helpful support for forming the particular structure of the thermal ABL. High surface temperature is clearly the powerful external thermal forcing for the deep convective boundary layer. Strong sensible heat flux in the surface layer provides the required energy. Highly convective atmosphere and strong turbulence provide the necessary dynamic conditions, and the accumulative effect of the residual layer provides a favorable thermal environment. 相似文献
20.
A numerical study of the influence of urban expansion on monthly climate in dry autumn over the Pearl River Delta, China 总被引:6,自引:0,他引:6
W. S. Lin C.-H. Sui L. M. Yang X. M. Wang R. R. Deng S. J. Fan C. S. Wu A. Y. Wang S. K. Fong H. Lin 《Theoretical and Applied Climatology》2007,89(1-2):63-72
Summary In this study, we employed a regional model to simulate the impact of urban expansion on monthly climate in Pearl River Delta
(PRD) region. Two experiments were performed by prescribing two different land covers in the PRD region. One land cover represents
vegetation in the 1970s which is derived from the United States Geological Survey (USGS) data with 24-category (hereafter
referred to as NU). The other land cover represents the current urban condition which is derived from remote sensing data
acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) in 2004 (hereafter referred to as HU). Using the two
land cover datasets, monthly climate of October 2004 was simulated, which was a very dry season in the PRD region. The results
obtained from the numerical simulation show a distinct difference in simulated shelter-level temperature, humidity, surface
fluxes and the height of planetary boundary layer (PBL) with two different land cover data sets being specified. The maximum
difference in simulated monthly mean temperature over urban areas was 0.9 °C. A large temperature difference was found in
urbanized area in Guangzhou, Dongguan, Zhongshan and Shenzhen. The monthly mean relative humidity in urban areas decreased
by 1.4% as a result of urban expansion (from 59.2% in NU to 57.8% in HU). The maximum decrease in mixing ratio was 0.4 g/kg
in Guangzhou and Dongguan, whereas the maximum decrease in relative humidity was 2.4%. There was an increase of sensible heat
flux in developed lands and the maximum increase was 90 W m−2. In contrast, latent hear flux in urban area decreased and the maximum decrease was 300 W m−2. In addition, the increase in mean height of PBL ranged from 20 to 80 m (HU compared with NU), and the maximum change of
the height was 180 m over urban area in city of Guangzhou. 相似文献