Evaluation of turbulent fluxes over Maitri,Antarctica

Turbulent fluxes have been evaluated for clear sunny days over the Indian Antarctic station, Maitri, using the basic meteorological data recorded at four levels of a 28 m tower. The data are supplemented with radiation data. The surface layer over Maitri remains thermally stratified during the hours of minimum solar insolation, the so-called nighttime period. The surface winds during this period are generally very strong resulting in high momentum fluxes. In particular, for high winds (>12 m s^–1), the temperature gradient is found to be less positive than for moderate winds (4 to 7 m s^–1). Solar insolation provided the daytime heating necessary for the diurnal variation of atmospheric stability, and hence, for the turbulent fluxes. Thus, on clear days daytime conditions are marked by upward transport of heat with reduced momentum flux, while stable nighttime conditions are marked by a downward heat flux with increased momentum fluxes.     

Tilt errors and precipitation effects in trivane measurements of turbulent fluxes over open water

For 390 ten-minute samples of turbulent flux, made with a trivane above a lake, the vertical alignment is determined within 0.1 ° through azimuth-dependent averaging. One degree of instrumental misalignment is found to produce an average tilt error of 9 ± 4% for momentum flux, and 4 ± 2% for heat flux. The tilt error in the vertical momentum flux depends mainly ons _u/u_*, and cannot be much diminished with impunity by high-pass pre-filtering of the turbulence signals. The effects of rain on trivane measurements of vertical velocity are shown to be negligible at high wind speeds, and adaptable to correction in any case.The normalized vertical velocity variance,s _w/u_*, appears to be proportional to the square root ofz/L for unstable stratification. For a wind speed range of 2 to 15 m s^–1, the eddy correlation stresses measured at 4- and 8-m heights can be reasonably well estimated by using a constant drag coefficientC _d=1.3 X 10^-3, while cup anemometer profile measurements give an overestimate of eddy stress at high wind speeds. A good stress estimate is also obtained from the elevation variance; it is suggested that trivane measurement of this variance might be made from a mobile platform, e.g., a moderately stabilized spar buoy.     

LLM – a nonhydrostatic model applied to high-resolving simulations of turbulent fluxes over heterogeneous terrain

Summary ?At the Deutscher Wetterdienst (DWD) an internal project named LITFASS was running to determine the representative turbulent fluxes of heat and momentum over heterogeneous land surfaces by observation and simulation. The project took advantage of the infrastructure of the Research Division at the DWD, where model research capacity is combined with the measurements made at and around the Meteorological Observatory Lindenberg. The paper describes the simulation component of the LITFASS-project. It consists of a high-resolving model, derived from the new operational non-hydrostatic, compressible Lokal-Modell (LM), which is denoted LLM (LITFASS-Lokal-Modell). The integration area covers the lower atmosphere in the vertical up to 3000 m with 39 model layers. The horizontal size of the integration area with 145 × 145 grid points (horizontal mesh width Δs = 96.5 m) corresponds to a typical grid box of a meso-scale model. The LLM has to operate under real meteorological conditions. Therefore, the LLM is driven by time-dependent measured vertical profiles of wind, temperature and humidity and surface-based measurements (of radiation, precipitation, soil properties) supported by satellite information. The profiles are available for a great variety of weather situations occurring during the simulation period (1–20 June 1998). First model results from extended 24 hour-integrations against different kinds of measurements are discussed. They reveal the LLM to become a promising validation instrument, from which a systematic, sustainable validation system can be established beyond LITFASS for improving parameterization schemes in the NWP models of the DWD. Received July 18, 2001; revised March 15, 2002; accepted May 30, 2002     

Comparison of areally averaged turbulent fluxes over non-homogeneous terrain: Results from the EFEDA-field experiment

In June 1991 the EFEDA-field experiment (ECHIVAL Field Experiment in a Desertification-Threatened Area) was carried out in the Spanish province Castilla-La Mancha, to improve the understanding of the interactions between the soil, the vegetation and the atmosphere.Here results of energy balance studies at the Barrax site are given, one of the three intensively studied experimental sites within Castilla-La Mancha. This area is characterized by a large fraction of irrigated fields (40%) while the remaining 60% was fallow land at the end of June 1991. The energy balances over these two characteristic land-use classes totally differ. While for the irrigated fields the evapotranspiration is dominant, for the non-irrigated fields the sensible and the soil heat fluxes dominate and the latent heat flux is nearly negligible.In order to achieve areally averaged turbulent fluxes, surface, SODAR and aircraft data have been used. Comparing the surface fluxes from all three facilities, it can be found that:The sensible heat flux estimation from SODAR (_w-method) gives reliable results when a calibration of _w is done with another independent system (e.g. aircraft).Aircraft measurements in conjunction with energy budget methods yield surface fluxes of sensible heat, which are about 20% lower than the areally averaged values calculated by the surface measurements. The areally averaged latent heat fluxes from aircraft and surface measurements agree better than the sensible heat fluxes.     

Comparison of NCEP turbulent heat fluxes with in situ observations over the south-eastern Arabian Sea

Turbulent surface heat fluxes (latent and sensible heat) are the two most important parameters through which air–sea interaction takes place at the ocean–atmosphere interface. These fluxes over the global ocean are required to drive ocean models and to validate coupled ocean–atmosphere global models. But because of inadequate in situ observations these are the least understood parameters over the tropical Indian Ocean. Surface heat fluxes also contribute to the oceanic heat budget and control the sea surface temperature in conjunction with upper ocean stratification and ocean currents. The most widely used flux products in diagnostic studies and forcing of ocean general circulation models are the ones provided by the National Centres for Environment Prediction (NCEP) reanalysis. In this study we have compared NCEP reanalysed marine meteorological parameters, which are used for turbulent heat fluxes, with the moored buoy observation in the south-eastern Arabian Sea. The NCEP latent heat flux (LHF) and sensible heat flux (SHF) derived from bulk aerodynamic formula are also compared with that of ship and buoy derived LHF and SHF. The analysis is being carried out during the pre-monsoon and monsoon season of 2005. The analysis shows that NCEP latent as well as sensible heat fluxes are largely underestimated during the monsoon season, however, it is reasonably comparable during the pre-monsoon period. This is largely due to the underestimation of NCEP reanalysis air temperature (AT), wind speed (WS) and relative humidity (RH) compared to buoy observations. The mean differences between buoy and NCEP parameters during the monsoon (pre-monsoon) period are ~21% (~14%) for WS, ~6% (~3%) for RH, and ~0.75% (0.9%) for AT, respectively. The sudden drop in AT during rain events could not be captured by the NCEP data and, hence, large underestimations in SHF. During the pre-monsoon period, major contribution to LHF variations comes from WS, however, both surface winds and relative humidity controls the LHF variations during the monsoon. LHF is mainly determined by WS and RH during the monsoon and, WS is the main contributor during the pre-monsoon.     

西太平洋热带海域湍流通量以及海面粗糙度和曳力系数的观测研究

本文利用1987年西太平洋热带海域考察期间系留气艇所取得的廓线资料,用相似理论半经验的通量-廓线关系,给出这一海域的湍流热通量和动量通量,并且对海面粗糙度长度Z_0,曳力系数C_D及其与风速的关系等作了初步的研究,给出了一些经验公式,并与其他研究者的研究结果作了比较。     

近地面层湍流通量观测误差的比较

本文根据误差理论分析并比较了在近地面层内各种间接确定湍流通量方法的观测误差。空气动力学法由于采用不同的通用函数将造成很大的误差。H扣除法确定潜热通量,当Bowen比较小时误差较小,Bowen比B>1之后误差随B的增大迅速增长。在相同的观测条件下间接法确定湍流通量以Bowen比法和组合法误差最小,实例计算表明组合法的精度最高。     

Estimation of turbulent sensible heat and momentum fluxes over a heterogeneous urban area using a large aperture scintillometer

The accurate determination of surface-layer turbulent fluxes over urban areas is critical to understanding urban boundary layer (UBL) evolution. In this study, a remote-sensing technique using a large aperture scintillometer (LAS) was investigated to estimate surface-layer turbulent fluxes over a highly heterogeneous urban area. The LAS system, with an optical path length of 2.1 km, was deployed in an urban area characterized by a complicated land-use mix (residential houses, water body, bare ground, etc.). The turbulent sensible heat (Q H) and momentum fluxes (τ) were estimated from the scintillation measurements obtained from the LAS system during the cold season. Three-dimensional LAS footprint modeling was introduced to identify the source areas ("footprint") of the estimated turbulent fluxes. The analysis results showed that the LAS-derived turbulent fluxes for the highly heterogeneous urban area revealed reasonable temporal variation during daytime on clear days, in comparison to the land-surface process-resolving numerical modeling. A series of sensitivity tests indicated that the overall uncertainty in the LAS-derived daytime Q H was within 20%-30% in terms of the influence of input parameters and the non-dimensional similarity function for the temperature structure function parameter, while the estimation errors in τ were less sensitive to the factors of influence, except aerodynamic roughness length. The 3D LAS footprint modeling characterized the source areas of the LAS-derived turbulent fluxes in the heterogeneous urban area, revealing that the representative spatial scales of the LAS system deployed with the 2.1 km optical path distance ranged from 0.2 to 2 km2 (a "micro-α scale"), depending on local meteorological conditions.     

Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment

Summary ?During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, 1998. Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7 km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99 m-tower significantly deviate from “idealised” profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels. Received June 6, 2001; revised January 15, 2002; accepted April 4, 2002     

Turbulent fluxes over inhomogeneous landscape

Mesoscale surface turbulent fluxes over a complex terrain surrounded by oceans have been investigated using a 3-D numerical mesoscale model, under conditions with and without synoptic flows. The study indicated that under synoptically calm condition, the allocation and intensity of mesoscale surface turbulent fluxes (MSTFs) were greatly impacted by the thermally forced mesoscale circulation (TFMC) over mesoscale heterogeneous landscape. The max-imum values of sensible (Hs) and latent (LE) heat fluxes were located over the convergent zones and considerably im-pacted by the soil wetness (M), but did not depend strongly on the atmospheric background thermal stability (β0). The simulated results suggested that the sensible heat flux was closely proportional to the square of wind speed in the surface layer. By the action of synoptic flow, the allocation of LE was shifted to downwind, its intensity increased.     

Nonstationarity of turbulent heat fluxes at Summit,Greenland

Turbulence data collected over a total of 25 days during two summers are used to describe processes responsible for the nonstationarity of turbulent sensible heat fluxes at Summit, Greenland. A stationarity test shows that about 40% of the data are classified as nonstationary. Three main factors are explored to account for the large fraction of nonstationary runs: (1) intermittency of turbulence in stable conditions, (2) changes in net all-wave radiation in response to cloud forcing, and (3) diurnal trends in stability. A classification procedure that accounts for the intermittent nature of turbulence shows that during stable, nonstationary conditions 50% of the total sensible heat flux is realized in 22% of the sampling time. Intermittency often occurs at Summit during periods characterized by weak and irregular horizontal winds in combination with strong stability. Rapid changes in net all-wave radiation in response to cloud forcing results in nonstationarity during unstable conditions. Between 0930–1130 and 1900–1930 UTC turbulent heat fluxes are not only small in magnitude but also typically change sign, with nonstationarity during these periods often as high as 65%. These results should help resolve some of the present uncertainties in obtaining reliable fluxes at this site, in particular under stable atmospheric conditions.     

The influence of water vapor fluctuations on turbulent fluxes

The influence of water vapor fluctuations on vertical turbulent fluxes is examined. It is shown that effects on density and, consequently, buoyancy are insignificant. However, because specific heat of air is a function of specific humidity, these fluctuations are found to influence sensible heat flux significantly. The critical parameter is the Bowen ratio, and a formulation relating the heat flux assuming dry air to the true value is given. The implications of this analysis to flux-gradient relationships in the surface layer are commented on.     

Inertial-dissipation methods and turbulent fluxes at the air-ocean interface

The use of high frequency atmospheric turbulence properties (inertial subrange spectra, structure function parameters or dissipation rates) to infer surface fluxes of momentum, sensible heat and latent heat is more practical for most ocean going platforms than direct covariance measurement. The relationships required to deduce the fluxes from such data are examined in detail in this paper and several ambiguities and uncertainties are identified. It is noted that, over water, data on water vapor properties (the dimensionless functions for the mean profile, the structure function parameter and the variance transport term) are extremely sparse and the influence of sea spray is largely unknown. Special attention is given to flux estimation on the basis of the structure function formalism. Existing knowledge about the relevant similarity functions is summarized and discussed in light of the ambiguities identified above.     

Direct measurement of turbulent fluxes on a cruising ship

The result of an attempt at the direct measurement of turbulent fluxes on the top of the mast of a cruising ship is presented. The three-dimensional components of wind relative to the ship measured by a sonic anemometer are corrected for ship motion; from these the fluxes of momentum, sensible heat and water vapor are computed using the outputs of a fine-wire thermocouple psychrometer. The observations were made by this method on the Northwestern Pacific. The results indicate that this technique is usable for determining the distribution of fluxes over the ocean.     

几种计算湍流能量和通量方法的比较

本文利用超声脉动风速仪和白金丝脉动温度仪,分别测量了三维风速u、v、w和温度T的瞬时脉动值.根据这些风、温高频测量值,用几种不同的计算方法计算了湍流能量和动量、热量通量.计算结果表明,由于大气中湍涡尺度很宽,对于不同尺度的湍流量,选用合理的计算方法,能够得到正确的计算结果,并节省计算时间.     

Observations of fluxes over heterogeneous surfaces

This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.A variance analysis is carried out to quantify the relative importance of surface heterogeneity and transient mesoscale motions on the variability of the turbulence fluxes. The momentum and ozone fluxes are more influenced by transient mesoscale motions while fluxes of heat, moisture and carbon dioxide are more influenced by surface heterogeneity. The momentum field is also influenced by a quasi-stationary mesoscale front and larger scale velocity gradients.For the present case, the mesoscale modulation of the turbulent flux is numerically more important than the direct mesoscale flux. This spatial modulation of the turbulent fluxes leads to extra Reynolds terms which act to reduce the area-averaged turbulent momentum flux and enhance the area-averaged turbulent heat flux.     

Spatial variability of turbulent fluxes and roughness lengths in HAPEX-MOBILHY

Surface-based and aircraft measured fluxes over the heterogeneous surface in HAPEX-MOBILHY are analyzed for the ten flight days when cloud cover above the boundary layer was minimal. The fair-weather climatology of the spatial variation of surface fluxes is estimated to provide an assessment of the generality of previous case studies appearing in the literature. For the 10-day averages, greater heating over the forest generates a forest breeze which leads to rising motion and a modest increase of boundary-layer cloud cover at the forest edge. The exchange coefficients and effective roughness lengths are computed for local averages (15 km scale) and for regional averages (100 km scale) intended to represent a range of grid sizes in numerical models of the atmosphere. The effective roughness length for momentum over the mixed agricultural region for both scales is on the order of 1 m, apparently due to bluff roughness effects associated with scattered trees, edges of small woods and other obstacles. This roughness length value is an order of magnitude larger than values used in numerical models for the same region, which are based on the dominant vegetation type. The spatially varying effective roughness length for heat is computed for use in those models which use surface radiation temperature to estimate surface heat flux. The effective roughness lengths for heat are found to be smaller than those typically used in numerical models of the atmosphere.     

Atmospheric response to soil-frost and snow in Alaska in March

Summary A hydro-thermodynamic soil-vegetation model including soil freezing/thawing (soil-frost) and snow-metamorphism has been integrated into the PennState/NCAR Mesoscale Meteorological Model MM5 in a two-way coupled mode. A hierarchy of simulations with and without the soil-frost module, each combined with and without the snow module, shows the influence of snow-cover and soil-frost on weather in Alaska. Herein the landscape is featured as it is typically by mesoscale models.Theoretical considerations suggest that organic soil types should be considered in mesoscale modeling because of their different thermal and hydrological behavior as compared to mineral soils. The Ludwig-Soret and Dufour effects are small, but increase appreciably during freezing/thawing and snow-melt.The snow and soil-frost processes have a demonstrable impact on the surface thermal and hydrological regimes and on the near-surface atmospheric conditions even on the short (synoptic) timescales. The presence of snow-cover results in a highly stable stratification. In cloud-free areas, the enhanced loss of radiant energy and cooling of the air over snow-cover lead to a positive feedback to relatively colder, drier conditions. In cloudy areas, a positive feedback to warmer, moister conditions develops over snow-cover. As the changes in atmospheric humidity and temperature caused by snow-cover propagate into the pressure field, sea level pressure is lower by more than 1hPa in the simulations with snow-cover. Although the effect of soil-frost alone is an order of magnitude smaller, the soil-frost snow system leads to an increase of the pressure difference to 1.2hPa. The changes in the pressure field alter wind speed and direction slightly.Soil-frost results in soil temperature differences of 2–5K in the upper soil layers, while snow results in differences of 3–10K. Soil-frost has a notably greater impact in cloud-free than cloudy areas. When a snow-cover is present, frozen soil enhances the insulating effect of a snow-cover in cloudy areas, but reduces it in cloud-free areas. In cloudy areas, soil-frost without snow-cover leads to cooler, drier atmospheric conditions relative to no frost. In cloudy areas, soil-frost under a snow-cover reduces the water supply to the atmosphere as compared to snow-covered conditions without soil-frost. The combined effects of soil-frost and snow increase precipitation locally by as much as 12.2mm/ 48h. If mesoscale modeling does not consider the soil-frost snow system, predicted water vapor fluxes will be too high in cloud-free areas, and too low in cloudy areas.