Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO₂ fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, andLE. This finding is consistent with results from several recent studies.     

On momentum flux and velocity spectra over waves

Data from a research tower in Lake Ontario are used to study the validity of Monin--Obukhov scaling in the marine atmospheric boundary layer under various wave conditions. It is found that over pure wind seas, the velocity spectra and cospectra follow established universal scaling laws. However, in the presence of swells outrunning weak winds, velocity spectra and cospectra no longer satisfy universal spectral shapes. Here, Monin–Obukhov similarity theory, and the classical logarithmic boundary layers, are no longer valid. It is further shown that, in the presence of such swells, the momentum flux can be significantly modified in comparison to pure wind sea values. The implications of these findings for bulk flux estimations and on the inertial dissipation method for calculating fluxes are discussed.     

Analysis of the teleconnectional structures of the 500-hPa height field over the NH in january

Based on the oblique mode scheme provided in the Appendix, and analysis is made of the teleconnectional structures (TNS) of the NH 500-hPa height field in January, resulting in a series of teleconnectional patterns re-presentative of the main features of TNS, of which the teleconnectional pattern WPAA (Western Pacific/Asian/ American) closely related to the East-Asian circulation is presented for the first time in this field of research. Contrast analysis indicates that TNS are so sensitive to the transition of seasons that a slight transition would bring about a considerable response. Hence the examination of the month-to-month TNS of the NH atmospher-ic circulations is indispensable.     

Eddy flux measurements over the ocean and related transfer coefficients

Eddy correlation measurements of vertical turbulent fluxes made during AMTEX 1975 are used to assess the reliability of flux prediction from established bulk transfer relations, using both surface-layer and planetary boundary-layer formulations. The surface-layer formulae predict momentum and latent heat fluxes to an accuracy comparable to the direct eddy correlation method, using transfer coefficients of C _DN (at 10m and in neutral conditions) increasing with wind speed, and a constant C _EN - 1.5 × 10 ^–3 . The data suggest C _CHN , for sensible heat, increases significantly with wind speed and is on average 30% lower than C _CEN The boundary-layer drag coefficient, C _GD , agrees within about 40% of recently published values using a vertically averaged geostrophic wind to the height of the lowest temperature inversion, corrected for trajectory curvature. Values of _* / from which C _CGH is derived, are in excellent agreement if the published values are modified to account for inappropriate surface temperatures used in their derivation. Preliminary values of C _GE are also presented.     

Comparison of flux measurements with open- and closed-path gas analyzers above an agricultural field and a forest floor

Comparison was made of the flux measurements of a closed-path CO₂/H₂O analyzer and an open-path H₂O analyzer above a clover field and the forest floor of a Douglas-fir stand. The attenuation of the gas concentration fluctuations caused by the sampling tube of the closed-path analyzer resulted in underestimation of the H20 flux above both surfaces. The degree of underestimation above the clover field depended on wind speed, but was smaller than that calculated from the transfer function for laminar flow in a circular tube and the scalar cospectrum in the neutral and unstable surface layer. Above the forest floor CO₂ fluctuations led those of H₂O by 0.7s. The implications of this are discussed regarding the determination of the time delay caused by the sampling tube of the closedpath analyzer. The day-time CO₂ efflux from the forest floor, averaged over three days, was 0.043 mg/(m²s).     

Radiative surface temperature and convective flux calculation over crop canopies

The analysis presented in this paper aims at a better understanding of the potential role of radiative temperature, as measured by a radiometer over crops, in sensible heat flux calculation. Defining radiative temperature as the mean temperature of the surfaces viewed by the radiometer (leaves and soil surface) and assuming that an Ohm's law type formula can be used to express sensible heat flux as a function of the difference between air temperature and radiative temperature, the aerodynamic resistance which divides this temperature difference has been analytically defined. The parameters which appear in the resistance expression depend essentially on wind velocity and canopy structure but also on the inclination angle of the radiometer. Finally an experimental verification is presented with data obtained over a potato crop.     

Estimation of the average surface heat flux over an inhomogeneous terrain from the vertical velocity variance

An indirect method of estimating the surface heat flux from observations of vertical velocity variance at the lower mid-levels of the convective atmospheric boundary layer is described. Comparison of surface heat flux estimates with those from boundary-layer heating rates is good, and this method seems to be especially suitable for inhomogeneous terrain for which the surface-layer profile method cannot be used.     

Large-scale variability modes of freshwater flux and precipitation over the Atlantic

 Precipitation (P) and freshwater (E-P) fluxes at the air-sea interface are investigated in the Atlantic Ocean sector using the reanalyses of the European Centre for Medium Range Weather Forecasts (ERA) and of the National Centers for Environmental Prediction (NCEP). A canonical correlation analysis method between these fields and sea level pressure (SLP) is used to identify patterns. We also test whether precipitation and freshwater fluxes can be reconstructed from SLP data. In the winter months, patterns associated with both the North Atlantic Oscillation (NAO) and the East Atlantic (EA) mode are identified. The signals are strong enough to be reconstructed from the reanalysis fields, and they correspond to a significant part of the variability. The NAO signal is more robust than the EA one. The NAO-related variability mode is also present when the monthly precipitation rate is averaged for the winter season and even for annual averages. However, in the later case, other variability of natural origin (for instance, ENSO variability) or noise from the model and assimilation system prevents the reconstruction of E-P associated with NAO from SLP variability. Difficulties are identified in the tropical Atlantic with a different behaviour of NCEP and ERA precipitation variability, especially near the Inter Tropical Convergence Zone (ITCZ). The ERA patterns suggest a NAO signature in the tropical Atlantic which has clear monthly patterns and indicates a link between the phase of NAO and changes in the position and intensity of ITCZ. However, the analysis of winter rainfall based on satellite and in situ data does not support the monthly tropical pattern of ERA precipitation although it suggests a relation between convection near 15°S and NAO during northern winter. Received: 10 February 2000 / Accepted: 7 May 2001     

Using MODIS and AVHRR data to determine regional surface heating field and heat flux distributions over the heterogeneous landscape of the Tibetan Plateau

In this study, a parameterization methodology based on Advanced Very High-Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), and in situ data is proposed and tested for deriving the regional surface heating field, sensible heat flux, and latent heat flux over a heterogeneous landscape. In this case study, this method is applied to the whole Tibetan Plateau (TP) area. Four sets of AVHRR data and four sets of MODIS data (collected on 17 January 2003, 14 April 2003, 23 July 2003, and 16 October 2003) were used in this study to make comparisons between winter, spring, summer, and autumn values. The satellite-derived results were also validated using the “ground truth” as measured in the stations of CAMP/Tibet (Coordinated Enhanced Observing Period (CEOP) and Asia–Australia Monsoon Project on the Tibetan Plateau). The results show that the surface heating field, sensible heat flux, and latent heat flux in the four seasons across the TP are in close accordance with its land surface status. These parameters range widely due to the strongly contrasting surface features found within the TP region. Also, the estimated surface heating field, sensible heat flux, and latent heat flux all agree with the ground truth data, and usually, the absolute percentage difference between the two sets of data is less than 10 % at the validation stations. The AVHRR results were also in agreement with the MODIS data, with the latter usually displaying a higher level of accuracy. We have thus concluded that the proposed method was successful in retrieving surface heating field, sensible heat flux, and latent heat flux values using AVHRR, MODIS, and in situ data over the heterogeneous land surface of the TP. Shortcomings and possible further improvements in the method are also discussed.     

Study of crop coefficient and the ratio of soil evaporation to evapotranspiration in an irrigated maize field in an arid area of Yellow River Basin in China

A field experiment was conducted in a maize field in 2006 in an arid area of the Yellow River Basin in China. The daytime evapotranspiration (ET_c) and soil evaporation beneath the maize canopy (E _g) were measured by Bowen ratio energy balance method and micro-lysimeters, respectively. The results showed that the total ET_c during maize growth season was 696 mm, and the maximum values occurred at about 90–140 days after sowing. The crop coefficient (K _c), which was calculated from the ratio of ET_c to reference evapotranspiration (ET₀), was quite different from the values reported by other researchers in similar climate areas, with average values of 0.34, 0.47, 1.0 and 0.9 for initial, development, mid-season and late-season stages, respectively. High correlations between leaf area index (LAI) and average K _c for every 4 days were obtained. The total E _g was 201.4 mm with average values ranged from 0.92 to 2.05 for four growth stages of maize; and accounted for around 28.9 % of ET_c. The ratio E _g/ET_c showed high negative relationship with LAI. These results were very important in precise management of irrigation for maize in Yellow River Basin areas.     

The breakup of marine boundary-layer clouds over an inhomogeneous sea surface temperature field

This numerical study examines the breakup of marine atmospheric boundary-layer (MABL) clouds through various physical processes over an inhomogeneous sea surface temperature (SST) field. Three regimes are identified under which the cloud layer will break up. (A) advection of drier air into the MABL for the California case. (B) daytime absorption of solar radiation, occurring most easily over the cold water. (C) mesoscale fluctuations in the flow, producing holes in the cloud layer.The budget study of these three situations concludes that large-scale subsidence, solar radiation, local mesoscale advection, and inhomogeneous surface fluxes cannot be neglected in modeling cloud breakup. This study also confirms the belief that the mixing process alone induced by evaporative entrainment is generally insufficient to predict the breakup of the cloud layer.Sections of this paper are based on an extended abstract by the author and Dr. Steven Stage for the Ninth Symposium on Turbulence and Diffusion held at Riso, Denmark, 1990.     

Eddy correlation measurement of CO2 flux using a closed-path sensor: Theory and field tests against an open-path sensor

We have examined the potential of using a closed-path sensor to accurately measure eddy fluxes of CO₂. Five inlet tubeflow configurations were employed in the experimental setup. The fluxes of CO₂ were compared against those measured with an open-path sensor. Sampling air through an intake tube causes a loss of flux, due to the attenuation of CO₂ density fluctuations. Adjustments need to be made to correct for this loss and to account for density effects due to the simultaneous transfer of heat and water vapor. Theory quantifying these effects is discussed.The raw CO₂ flux measured with the closed-path sensor was smaller than that measured with the open-path sensor by about 15% (on average) for the turbulent tubeflow configurations with a short (3 m) intake tube, by 31% for turbulent tubeflow with a longer (6 m) intake tube and by 24% for laminar tubeflow. The difference was, in part, caused by tube attenuation of the CO₂ density fluctuations and inadequate sensor time response. The elimination of the flux adjustment for the simultaneous transfer of sensible heat (i.e., the attenuation of ambient temperature fluctuations in the intake tube) generally accounted for the rest of this difference.The raw flux measured with the closed-path sensor was corrected for frequency response and density effects. Except in the case of laminar tubeflow, the corrected closed-path flux agreed consistently with the corrected open-path flux within a few percent (<5%). These results suggest that closed-path sensors, with appropriate corrections, can be used to measure CO₂ flux accurately. Recommendations are included on selecting an optimum flow configuration to minimize the effect of sampling air through a tube.Published as Paper No. 9938, Journal Series, Nebraska Agricultural Research Division.     

Measurements of flow over an elongated ridge and its thermal stability dependence: The mean field

Measurements of mean wind flow and turbulence parameters have been made over Cooper's Ridge, a 115 m high elongated ridge with low surface roughness. This paper describes measurements of the streamwise and vertical variations in the mean field for a variety of atmospheric stability conditions. In near-neutral conditions, the normalised speedup over the ridge compares well with measurements from Askervein (Mickleet al., 1988). The near-neutral results are also compared to an analytical flow model based on that of Huntet al. (1988a). Measured streamwise variations show less deceleration at the foot of the hill and slightly more acceleration at the crest of the hill than does the model. In non-neutral conditions, the speedup over the ridge reduces slightly in unstable conditions and increases by up to a factor of two in stable conditions. The model is modified to allow boundary-layer stability to change the upwind wind profile and the depths of the inner and middle layers. Such a modification is shown to describe the observations of speedup well in unstable and weakly stable conditions but to overestimate the speedup in moderate to strongly stable conditions. This disagreement can be traced to the model's overestimation of the upstream scaling velocity at the height of the middle layer through its use of a stable wind profile form which has greater shear than that of the observed profiles, in possible combination with the three-dimensionality of the ridge which would allow enhanced flow around, rather than over, the feature in more stable conditions.     

Connections between surface sensible heat net flux and regional summer precipitation over China

Using the observed monthly precipitation and NCEP (National Centers for Environmental Prediction)reanalysis surface flux data from 1951-2000, the connections between the seasonal SSHNF (Surface Sensible Heat Net Flux) over the Asian continent and the regional summer precipitation of China were examined.The patterns of collective and individual correlations were identified. The results indicate that the responseof the regional summer precipitation of China to the seasonal SSHNF over the study area varies according to region and season. The interannual variability of summer precipitation anomalies over Xinjiang, the northernmost Northeast China, and the North China Plain are most sensitive to the anomaly of the seasonal SSHNF. There are significant collective correlations between the interannual anomalies of the seasonal SSHNF and summer precipitation over these regions. In contrast, the Southeast Tibetan Plateau,Huaihe River Valley, and surrounding areas exhibit the least significant correlation. Significant individual correlations exist between the summer precipitation over the southernmost Northeast China, East Inner Mongolia, South of the Yangtze River and South China and the seasonal SSHNF in certain seasons over the following areas: near Lake Baikal and Lake Balkhash, near Da Hinggan Mountains and Xiao Hinggan Mountains, as well as the Tibetan Plateau.     

Coherent eddies and temperature structure functions for three contrasting surfaces. Part II: Renewal model for sensible heat flux

Sensible heat, latent heat, and other scalar fluxes cannot be measuredwithin short dense canopies, e.g., straw mulches, with standard approachessuch as eddy correlation, Bowen ratio-energy balance, aerodynamic, andvariance methods. However, recently developed surface renewal models, thatare based on the fact that most of the turbulent transfer within and abovecanopies is associated with large-scale coherent eddies, which are evidentas ramp patterns in scalar time series, offer a feasible solution. Wepresent a new air renewal model that calculates sensible heat flux atdifferent heights within and above a canopy from the average cubictemperature structure function, sampled at a moderate rate, and measuredaverage friction velocity. The model is calibrated and tested with datameasured above and within a Douglas-fir forest and above a straw mulch andbare soil. We show that the model describes half-hour variations ofsensible heat flux very well, both within the canopy and roughnesssublayers and in the inertial sublayer, for stable and unstable atmosphericconditions. The combined empirical coefficient that appears in the modelhas an apparently universal value of about 0.4 for all surfaces andheights, which makes application of the model particularly simple. Themodel is used to predict daytime and nighttime sensible heat flux profileswithin the straw mulch and within a small bare opening in the mulch.     

西太平洋及南海地区一次热带辐合带环流和结构的初步分析

本文对1974年8月上半月西太平洋及南海地区一次强热带辐合带的环流特征作了分析,给出这次热带辐合带的平均结构和天气分布。发现在热带辐合带上空,200毫巴对应有一个弱的孤立反气旋;除了热带辐合带及它北部副高之间有一个垂直经圈环流外,和它南部赤道缓冲带之间还有一个经圈环流,其下沉运动位于赤道缓冲带北侧;在热带辐合带涡旋的南部存在低层强西南气流,主要云区和降水区发生在这支气流的西北侧。本文并对主要云区和降水区为什么集中于热带辐合带南侧和热带辐合带上热带扰动的发展问题作了一些物理解释。     

Similarity of atmospheric Reynolds shear stress and heat flux fluctuations over a rough surface

Horizontal u and vertical w velocity fluctuations have been measured together with temperature fluctuations in the atmospheric surface layer, at a small height above a wheat crop canopy. Marginal probability density functions are presented for both individual fluctuations u, w, and for the instantaneous Reynolds stress uw, and heat fluxes w and u. Probability density functions of the velocity fluctuations deviate less significantly from the Gaussian form than the probability density of temperature. There appears to be closer similarity between statistics of the instantaneous heat fluxes than between the momentum flux and either of the heat fluxes investigated. The mean momentum flux receives equal contributions from the events referred to as ejections and sweeps in laboratory boundary layers. Sweeps provide the largest contribution to the heat fluxes.     

Seasonal and diurnal variations of coherent structures over a deciduous forest

Coherent structures in turbulent flow above a midlatitude deciduous forest are identified using a wavelet analysis technique. Coupling between motions above the canopy (z/h=1.5, whereh is canopy height) and within the canopy (z/h=0.6) are studied using composite velocity and temperature fields constructed from 85 hours of data. Data are classified into winter and summer cases, for both convective and stable conditions. Vertical velocity fluctuations are in phase at both observation levels. Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates that the horizontal motions deep in the canopy should lead the vertical motions by 90°. On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported. However, our large data set reveals that this flux fraction comes from a wide distribution that includes much higher fractions in its upper extremes. The separation distanceL _s between adjacent coherent structures, 6–10h, is comparable to that obtained in previous observations over short canopies and in the laboratory. Changes in separation between the summer and winter (leafless) conditions are consistent withL _s being determined by a local horizontal wind shear scale.     

Synergistic contribution of precipitation anomalies over northwestern India and the South China Sea to high temperature over the Yangtze River Valley

This study explores the characteristics of high temperature anomalies over eastern China and associated influencing factors using observations and model outputs. Results show that more long-duration (over 8 days) high temperature events occur over the middle and lower reaches of the Yangtze River Valley (YRV) than over the surrounding regions, and control most of the interannual variation of summer mean temperature in situ. The synergistic effect of summer precipitation over the South China Sea (SCS) region (18°-27°N, 115°-124°E) and the northwestern India and Arabian Sea (IAS) region (18°-27°N, 60°-80°E) contributes more significantly to the variation of summer YRV temperature, relative to the respective SCS or IAS precipitation anomaly. More precipitation (enhanced condensational heating) over the SCS region strengthens the western Pacific subtropical high (WPSH) and simultaneously weakens the westerly trough over the east coast of Asia, and accordingly results in associated high temperature anomalies over the YRV region through stimulating an East Asia-Pacific (EAP) pattern. More precipitation over the IAS region further adjusts the variations of the WPSH and westerly trough, and eventually reinforces high temperature anomalies over the YRV region. Furthermore, the condensational heating related to more IAS precipitation can adjust upper-tropospheric easterly anomalies over the YRV region by exciting a circumglobal teleconnection, inducing cold horizontal temperature advection and related anomalous descent, which is also conducive to the YRV high temperature anomalies. The reproduction of the above association in the model results indicates that the above results can be explained both statistically and dynamically.     

Measurement of methane flux over an evergreen coniferous forest canopy using a relaxed eddy accumulation system with tuneable diode laser spectroscopy detection

Very few studies have conducted long-term observations of methane (CH₄) flux over forest canopies. In this study, we continuously measured CH₄ fluxes over an evergreen coniferous (Japanese cypress) forest canopy throughout 1?year, using a micrometeorological relaxed eddy accumulation (REA) system with tuneable diode laser spectroscopy (TDLS) detection. The Japanese cypress forest, which is a common forest type in warm-temperate Asian monsoon regions with a wet summer, switched seasonally between a sink and source of CH₄ probably because of competition by methanogens and methanotrophs, which are both influenced by soil conditions (e.g., soil temperature and soil moisture). At hourly to daily timescales, the CH₄ fluxes were sensitive to rainfall, probably because CH₄ emission increased and/or absorption decreased during and after rainfall. The observed canopy-scale fluxes showed complex behaviours beyond those expected from previous plot-scale measurements and the CH₄ fluxes changed from sink to source and vice versa.