Bulk transfer coefficient over a snow surface

The drag coefficient C _D and the bulk transfer coefficient for sensible heat C _H over a flat snow surface were determined experimentally. Theoretical considerations reveal that C _D depends on the friction velocity u _* as well as on the geometrical roughness h of the snow surface. It is found that C _D increases with increasing u _* and/or h. The dependency of C _H on u _* and h is so small that it is possible to consider C _H as a constant for practical purposes: C _{H, 1} = 2.0 × 10^–3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at C _{E, 1} = 2.1 × 10^–3 for a reference height of 1 m.     

The Effect of Stratification on the Aerodynamic Roughness Length and Displacement Height

The roughness length, z _0u , and displacement height, d _0u , characterise the resistance exerted by the roughness elements on turbulent flows and provide a conventional boundary condition for a wide range of turbulent-flow problems. Classical laboratory experiments and theories treat z _0u and d _0u as geometric parameters independent of the characteristics of the flow. In this paper, we demonstrate essential stability dependences—stronger for the roughness length (especially in stable stratification) and weaker but still pronounced for the displacement height. We develop a scaling-analysis model for these dependences and verify it against experimental data.     

Air-sea bulk transfer coefficients in diabatic conditions

On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u ₁₀>3 m s^–1, the coefficient for sensible heat C _H is larger by about 10% than that for momentum C _D . For u ₁₀<5 m s^–1, however, the value of C _D exceeds the value of C _H , and for u ₁₀=15 m s^–1 it is shown that C _H 0.8C _D . It may be also proposed that 10³ C _D =1.11 to 1.70, 10³ C _E =1.18 to 1.30, and 10³ C _H =1.15 to 1.26 for a range of u ₁₀=4 to 20 m s^–1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae.     

N–PLS regression as empirical downscaling tool in climate change studies

Summary An N–PLS regression technique was tested as an empirical downscaling method. Average monthly near-ground air temperature (t), specific humidity (q), and sea-level pressure (p) fields across Central and Western Europe were used as predictors for average monthly air temperature (T), dew temperature (D), and precipitation amount (P) at 4 locations in Slovenia. The empirical downscaling models (EM) were developed by means of available predictand data from the ARSO archive and predictor data from the NCEP/NCAR reanalysis project, for the period 1951–2002, separately for single months. Using the combination of t and p as predictors, the EM for T explained from 73% to 95% of predictand variability, for D from 74% to 97% of predictand variability, and for P from 31% to 76% of predictand variability. The use of q as an additional predictor did not improve the quality of the EM considerably. Developed EM using p and t as predictors were applied to the results of 5 general circulation models (GCM): CSIRO/Mk2, CCC/CGCM2, UKMO/HadCM3, DOE-NCAR/PCM, and MPI-DMI/ECHAM4-OPYC3. Only the simulations based on SRES A2 and B2 emission scenarios were considered in our calculations. Available mean monthly values of predictors for the period 1951–2100 were used. All the projections of GCM results indicate an increase in T and D and decrease in P in the 21^st century at all 4 locations. The expected range of changes in T, D, and P is wide due to the different response of GCM to identical changes in the atmospheric composition, and represents a source of uncertainty in empirical downscaling results. Another important source of uncertainty in empirical downscaling studies, especially when temperature dependent predictors are used, is the problem of extrapolation. By using the proper mathematical approach for EM development we only reduce a part of the uncertainty related to the quality of empirical models that also strongly depend on the quality of input data and predictor selection. The N–PLS regression seems to be a suitable choice of mathematical method, as the feature selection from a large number of predictor time series is not predictand independent. Finally, any climate change and impact studies for the future are affected by many other uncertainties that we have to be fully aware of, while interpreting their results.     

Calculation of basin-scale surface fluxes by combining remotely sensed data and atmospheric properties in a semiarid landscape

Optical remote sensing data collected during the Monsoon '90 experiment in the Walnut Gulch Experimental Watershed in southern Arizona were used to estimate basin-scale surface temperature, net radiation (R _n) and soil heat flux (G). These were combined with several atmospheric boundary-layer (ABL) models to allow computation of basin-scale surface fluxes of sensible (H) and latent heat (LE). The calculated fluxes were compared to averages from a network of surface flux stations. One ABL model calculatedH using a bulk similarity approach for wind and temperature with remotely sensed surface temperature as the lower boundary condition. With basin-scale estimates ofR _n andG, LE was solved as a residual. The other ABL model applied atmospheric profiles from a series of soundings in the conservation equations of temperature and humidity in the mixed layer to computeH andLE directly. By combining theseH values withR _n andG, calculation ofLE by residual also was performed. The ABL-derivedH values differed from the averages from the surface network by roughly 20 and 30% for the bulk similarity and conservation approaches, respectively. ForLE, these same differences were around 10 and 70%. The disparity was reduced to nearly 30% for the conservation approach whenLE was solved as a residual. Days with significant spatial variation in surface soil moisture and/or cloud cover were associated with most of the disagreement between the ABL-derived and surface-based values. This was particularly true for conservation estimates ofLE. The bulk similarity method appeared less sensitive non-ideal environmental conditions. This may in part be due to the use of remotely sensed information, which provided a lower boundary value of surface temperature and estimates ofR _n andG over the study area, thereby allowing for residual calculations ofLE. Such information clearly has utility for assessing the surface energy and water balance at basin scale.     

Estimating the convective velocity scale for diffusion applications

A simple mixed layer model is used to derive the following expressions for the maximum (daily) convective velocity scale. w _*m = AQ _m ^1/2; w _*m = Bz _im The variables A and B are shown to vary within narrow limits thus allowing them to be treated as constants. This is very useful for routine computation of w _*m , an important variable for dispersion under unstable conditions, from estimates of either the kinematic surface heat flux Q _m (m^-1) or the maximum mixed layer height z _im .Analysis of observations made during the Minnesota boundary layer experiment shows that there is ample justification for assigning typical values to A and B in estimating w _*m.     

Momentum and Heat Transfer over Urban-like Surfaces

Momentum and heat transfer was examined for the urban-like surfaces used within the Comprehensive Outdoor Scale MOdel (COSMO) experiments. Simultaneous and comparative meteorological measurements were made over a pair of scale models with different block geometries. These data were analyzed to investigate the influence of height variations, obstacle elongation, and packing density, λ _p , of blocks on the aerodynamic properties. In addition, the robustness of theoretical expressions of bulk transfer coefficients for momentum and heat with respect to geometric parameters was examined. Our analyses showed: (1) the theoretical framework for the bulk transfer coefficient for momentum, C _m , and that for heat, C _h , was applicable for homogeneous building arrays, (2) the sensitivity of C _h to the surface geometry was smaller than that of C _m , (3) the transfer coefficients were increased by variations of block heights, but not by elongation of blocks, (4) first-order approximations of C _m and C _h for an array of blocks with two different heights can be made by applying simple theoretical assumptions to include the effects of height variation, and (5) variations of block heights increased the momentum flux significantly, but caused little change in the sensible heat flux. This can be explained by the feedback mechanism of aerodynamic– thermal interaction; aerodynamic mixing decreased both the advective velocity and the vertical temperature gradient.     

Determination of selected atmospheric aromatic hydrocarbons at remote continental and oceanic locations using photoionization/flame-ionization detection

A new gas chromatographic technique with a modified photoionization detector connected in series with a conventional flame ionization detector was used to determine low concentrations of atmospheric hydrocarbons in remote atmospheres. Average mixing ratios of five aromatic hydrocarbons measured between 42°N and 30°S latitude in the Pacific Ocean in October/November 1983 were highest in the Northern Hemisphere. The average mixing ratios in the northern and southern marine atmospheres were 49±25 ppt (n=35) and 10±2 ppt (n=21) for benzene, 20±12 ppt (n=32) and 5.6±1.6 ppt (n=12) for toluene, 7.6±3.7 ppt (n=35) and 3.7±1.6 ppt (n=21) for ethylbenzene, 25±12 ppt (n=35) and 13±5 ppt (n=20) for the sum of m- and p-xylenes, and 14±6 ppt (n=35) and 6.6±3.0 ppt (n=21) for o-xylene, respectively. The first latitudinal gradients for these five aromatic compounds are reported. Benzene and toluene mixing ratios measured between July 1982 and October 1983 at a rural, mid-latitude continental site in eastern Washington state gave average values of 226±108 ppt and 133±84 ppt, respectively, with higher wintertime than summertime benzene levels. These continental samples gave calculated air mass ages averaging six days based on benzene-to-toluene ratios.     

Long term (100 yr) climatic trends for agriculture at selected locations in Canada

Long term (100 yr) trends of 17 selected annual climatic parameters were studied for 5 locations in Canada. Simple linear correlation coefficients (r) were computed for each variable with time. Temporal variations were not uniform over space, as might be expected over a large continent. Some variables differed significantly with time, while others did not. Forage aridity indices (FAI), or seasonal forage water deficits, had significant negativer values at Agassiz and Ottawa, indicating decreasing aridity over time, but no such trends were evident at the other 3 locations. Stations in western Canada were characterized byr values (mostly significant atP = 0.01) which indicated trends to earlier last spring frost (SF), later growing season end (GSE), later first fall frosts (FF) and increased frost-free period (FFP), growing degree-days (GDD) and Corn Heat Units (CHU). Stations in eastern Canada did not exhibit the same warming trends. No warming trends were observed in January mean temperature (JAN).Five-year moving averages and standard deviations were calculated and plotted forFAI, FF, GDD andGSL (growing season length). The climatic attributes were extremely variable, making detection of warming or cooling trends difficult. TheFAI for the last 30 yr or more at Agassiz and for the last 20 yr at Ottawa was well below the 100 yr normal. At Indian Head,FAI values were high during the 1930's and again around 1960, reflecting the drought conditions which occurred at those times. The other 3 variables showed a tendency to slightly increasing values over the past 50 yr at Agassiz, Indian Head, Brandon and Ottawa. Little, if any, systematic change in these elements was evident at Charlottetown. Standard deviations (S _d) fluctuated widely at all locations, with generally little evidence to suggest that the climate has become more or less variable. Cumulative frequency distribution for the total period and the first 70 yr were compared to the last 30 yr. There was an increased frequency of lowerFAI values at 2 locations for the 30-yr period.GDD increased significantly at lower probabilities at Indian Head. Differences in frequency distributions were generally slight for all other variables at the 5 locations.     

2009 年我国汛期降水形势总结与三个常用模式预报效果检验

利用实况24小时降水、形势场资料及T213、T639、Japan模式降水、形势场的预报资料,对2009年汛期(5-9月,下同)中国降水时空分布进行分析,并对T213、T639、Japan三个常用模式对2009年汛期的天气形势、降水及其影响系统的预报做主客观检验,以期得出2009年汛期降水分布特点及三个模式的降水预报效果对比.结果表明:(1) 2009年汛期华南地区降水量为全国之最,长江中下游和西南东部地区其次,东北和华北地区再次.(2)从TS评分看,Japan模式的小雨～大雨量级评分较高,T639模式暴雨～大暴雨量级评分较高；T213模式对华北地区暴雨、大暴雨量级降水预报评分高于Japan和T639模式.(3)从降水预报偏差看,T213模式对华北预报明显偏强,T639模式对华北预报强度较为适中,两模式对其他区域中等以下强度降水预报偏强,对强降水预报偏弱；T639对中等以下强度降水预报偏强程度明显小于T213,而对强降水除华南和东北区域外,预报偏弱程度明显大于T213；Japan模式预报偏差随降水量级增大而减小,对大雨以上各量级预报均明显偏弱,且偏弱程度明显大于T213、 T639.(4)由代表性形势场预报检验结果可知,除T213对500hPa高度场、850hPa温度场预报效果好于其他两模式外,各模式预报效果相差不大.(5)三个模式对500 hPa副高总体预报偏东、偏北、偏强,但Japan预报效果明显好于T213、T639.(6) T639模式对台风和低涡的预报相对较好,T213较差.     

Evapotranspiration from a boreal forest drainage basin using an energy balance/eddy correlation technique

Meteorological techniques were used to monitor evapotranspiration (ET) at two sites in a boreal forest drainage basin located in southeastern Manitoba, Canada. An energy balance method was used in which net radiation (R _N ) and ground heat flux (G) were measured directly. Sensible heat flux (H) was measured by the eddy correlation technique using a propeller anemometer and a fine-wire thermocouple. The energy components were calculated hourly on-line, and data were collected reliably over a five-month period.The R _N and H instruments were mounted above the forest canopy and simultaneous measurements of H at heights of 12 and 6 m were in good agreement. Measurements at an open bare rock site indicated that G could be a substantial fraction of the daily RN at some locations, but over longer time periods, it was a small fraction and, therefore, was ignored.The two measurement locations represented upland (open bedrock/jack pine forest) and lowland (aspen/willow forest) sites in the drainage basin. The mean daily value of R _N - H at the upland site was 0.57 times the value at the lowland location owing to differences in R _N , H, and G. The mean ratio of daily H/R _N was 0.6 for the upland site and 0.4 for the lowland site. A basin-wide ET was calculated by weighting the values for the two sites in proportion to their areas. The measured ET agreed well with precipitation minus runoff for the basin. Differences between these two quantities in summer and fall were attributed to water release and storage by the ground, respectively.Issued as AECL-9153.     

Estimating the Monin-Obukhov length in the stable boundary layer for dispersion calculations

Analysis of data collected during the Prairie Grass, Kansas and Minnesota experiments reveals the following empirical relationship between the Monin-Obukhov length L and the friction velocity u _*: L = Au _* ², A = 1.1 × 10³s²m^-1. This result combined with the formulation for the height of the stable boundary layer h suggested by Zilitinkevich (1972) leads to h u _* ^3/2 f–^1/2 where f is the Coriolis parameter. Data from the Minnesota study (Caughey et al., 1979) provide ample support for this expression.These empirical equations for L and h are useful for routine dispersion estimates during stable conditions.     

Uncoupled multi-layer model for the transfer of sensible and latent heat flux densities from vegetation

The sensible heat flux density C and the latent heat flux density E are coupled in the case of a multi-layer model of vegetation. Therefore two linearly independent combinations of C and E, the enthalpy flux density H and the saturation heat flux density J, are introduced. Two electrical analogues, for H and J, are designed. They are equivalent to the resistance scheme for C and E, but uncoupled. Penman's formulas for C and E, which are applicable only to single-layer models, can be expressed equivalently in terms of H and J. This version of Penman's formulas can be extended easily to multi-layer canopies.     

Measurement of Prandtl Number as a Function of Richardson Number Avoiding Self-Correlation

The empirical dependence of turbulence Prandtl number (Pr) on gradient Richardson number (Ri) is presented, derived so as to avoid the effects of self-correlation from common variables. Linear power relationships between the underlying variables that constitute both Pr and Ri are derived empirically from flux and profile observations. Pr and Ri are then reconstructed from these power laws, to indicate their interdependence whilst avoiding self-correlation. Data are selected according to the stability range prior to regression, and the process is iterated from neutral to higher stability until error analysis indicates the method is no longer valid. A Butterworth function is fitted to the resulting Pr ⁻¹(Ri) regression to give an empirical summary of the analysis. The form suggests that asymptotically Pr ⁻¹ decreases as Ri ^3/2. Scatter in the data increases above Ri ~ 1, however, indicating additional constraints to Pr are not captured by Ri alone in this high stability regime. The Butterworth function is analytic for all Ri > 0, and may be included in suitable boundary-layer parameterisation schemes where the turbulent diffusivity for heat is derived from the turbulent diffusivity for momentum.     

Notes from council

A wide set of size‐distribution models of haze and fog droplets expressed in terms of the modified gamma distribution function have been used for evaluating the proportionality coefficient b in the empirical relationship between visibility V and liquid water content LWC, that is V=b (LWC)‐ ^2/3, as proposed by Eldridge (1966). The evaluations of b turn out to be consistent with the values proposed in the literature and give evidence for its close dependence on the shape‐parameters of the droplet size distribution, especially as regards the mode radius and the width of the larger‐droplet wing. Three average relationship curves are proposed respectively for dense haze, “dry and cold” fog and “wet and warm” fog.

Corresponding to these cases, three correlation lines are presented between b and the ratio of the volume extinction coefficient at 3.70 μm wavelength to that at 0.55 μm, evaluated for the same uniform path in hazy and foggy atmospheres. Applied to measurements of infrared and visible transmission such correlation lines give the possibility of estimating the most proper value of b to be used in Eldridge's relationship for different atmospheric conditions.     

Flux-Variance Method for Latent Heat and Carbon Dioxide Fluxes in Unstable Conditions

Applied previously to momentum and heat fluxes, the present study extends the flux-variance method to latent heat and CO₂ fluxes in unstable conditions. Scalar similarity is also examined among temperature (θ), water vapour (q), and CO₂ (c). Temperature is adopted as the reference scalar, leading to two feasible strategies to estimate latent heat and CO₂ fluxes: the first one relies on flux-variance similarity relations for scalars, while the second is based on the parameterization of relative transport efficiency in terms of scalar correlation coefficient and a non-dimensional quantity. The relationship between the θ-to-q transport efficiency (λ _{θ q} ) and θ-q correlation coefficient (R _{θ q} ) is used to describe the intermediate hydrological conditions. We also parameterize the θ-to-c transport efficiency (λ _{θ c} ) as a function of the θ-c correlation coefficient (R _{θ c} ) by introducing a new non-dimensional ratio (α). The flux-variance method is a viable technique for flux gap-filling, when turbulence measurements of wind velocity are not available. It is worth noting that the extended method is not exempt from a correction for density effects when used for estimating water or carbon exchange.     

Radar Quantitative Precipitation Estimation Based on the Gated Recurrent Unit Neural Network and Echo-Top Data

The Gated Recurrent Unit(GRU) neural network has great potential in estimating and predicting a variable. In addition to radar reflectivity(Z), radar echo-top height(ET) is also a good indicator of rainfall rate(R). In this study, we propose a new method, GRU＿Z-ET, by introducing Z and ET as two independent variables into the GRU neural network to conduct the quantitative single-polarization radar precipitation estimation. The performance of GRU＿Z-ET is compared with that of the other three meth...     

The dependence of the bulk Richardson number on stability in the surface layer

Relationships between the bulk Richardson number, B, and other stability parameters were derived for the atmospheric surface layer. Nomograms were constructed, relating the Monin-Obukhov stability parameter, z/L, to B. The nomograms and the graph of Golder (1972) were used to establish various schemes for classifying z/L, B and Ri, in terms of the Pasquill stability classes.     

Surface Energy Balance Measurements Above an Exurban Residential Neighbourhood of Kansas City,Missouri

Previous measurements of urban energy balances generally have been limited to densely built, central city sites and older suburban locations with mature tree canopies that are higher than the height of the buildings. In contrast, few data are available for the extensive, open vegetated types typical of low-density residential areas that have been newly converted from rural land use. We made direct measurements of surface energy fluxes using the eddy-covariance technique at Greenwood, a recently developed exurban neighbourhood near Kansas City, Missouri, USA, during an intensive field campaign in August 2004. Energy partitioning was dominated by the latent heat flux under both cloudy and near clear-sky conditions. The mean daytime Bowen ratio (β) values were 0.46, 0.48, and 0.47 respectively for the cloudy, near clear-sky and all-sky conditions. Net radiation (R _n ) increased rapidly from dawn (−34 and −58W m⁻²) during the night to reach a maximum (423 and 630W m⁻²) after midday for cloudy and near clear-sky conditions respectively. Mean daytime values were 253 and 370W m⁻², respectively for the cloudy and near clear-sky conditions, while mean daily values were 114 for cloudy and 171W m⁻² for near clear-sky conditions, respectively. Midday surface albedo values were 0.25 and 0.24 for the cloudy and near clear-sky conditions, respectively. The site exhibited an angular dependence on the solar elevation angle, in contrast to previous observations over urban and suburban areas, but similar to vegetated surfaces. The latent heat flux (Q _E ), sensible heat flux (Q _H ), and the residual heat storage ΔQ _s terms accounted for between 46–58%, 21–23%, and 18–31% of R _n , respectively, for all-sky conditions and time averages. The observed albedo, R _n , and Q _E values are higher than the values that have been reported for suburban areas with high summer evapotranspiration rates in North America. These results suggest that the rapidly growing residential areas at the exurban fringe of large metropolitan areas have a surface energy balance that is more similar to the rural areas from which they were developed than it is to the older suburbs and city centres that make up the urban fabric to which they are being joined.     

Variation on PAR to global solar radiation ratio along altitude gradient in Naeba Mountain

Summary We investigated the ratio of photosynthetically active photon flux (Q _p ) to global solar radiation (R _s ) at three sites along different altitudes in Naeba Mountain, Japan at various temporal scales based on 3 years measurement data (1999–2001). The lowest values of the ratio ever reported were found for all sites on both an hourly and a daily scale. A similar slight diurnal pattern was found for all sites based on the monthly mean hourly values of the ratio. However, different sites exhibited different seasonal courses. Statistically significant altitude dependencies were found for the hourly Q _p /R _s under both clear and cloudy weather conditions (t-test, P < 0.001). For clear weather conditions, the hourly Q _p /R _s exhibited an increasing trend with altitude at an average rate of 3.6% per km. The increasing rate was more noticeable below 900 m (8.8% per km) than above (0.7% per km). The inverse trend was found for hourly Q _p /R _s with the altitude under cloudy weather conditions. The hourly Q _p /R _s decreased from 550 m to 1500 m at a rate of 1.8% per km. Again, a major decrease occurred below 900 m, which had the rate of 4.2% per km, compared with 0.2% per km over 900 m. Although the same tendencies were noted for daily Q _p /R _s , under clear sky conditions, they were not as statistically significant as the hourly counterpart (t-test, P < 0.021). The increasing rate of Q _p /R _s at this scale under clear weather conditions was near that of the hourly rate, but below the 900 m rate was reduced to near half of the hourly rate (4.7% per km). And the rate over 900 m increased to 2.7% per km. On the other hand, statistically significant altitude effect was noted for the daily under cloudy weather conditions (t-test, P < 0.002). A rapidly decreasing rate was found for it along the altitude gradient. The reason was due to the large decreasing rate below 900 m (12.4% per km). But again similar to the hourly Q _p /R _s , a very small decreasing rate of daily Q _p /R _s was found over 900 m under cloudy weather conditions. These results suggest the necessity of considering the altitude dependency of Q _p /R _s in future studies.