Field measurement of small ozone fluxes to snow,wet bare soil,and lake water

Eddy-correlation measurements over snow, wet bare soil, and lake water indicate very small vertical ozone fluxes. Adjustments to the small vertical fluxes are needed to take into account the effect of mean Stefan flow associated with evaporation at the surface and the effects of correlation between density variations and vertical wind fluctuations. For snow, the residual resistance calculated for the surface is about 34 s cm^-1, indicating that the maximum deposition velocity is abut 0.03 cm s^-1. For cold bare soil well saturated with water, the surface resistance is about 10 s cm^-1 (maximum deposition velocity of about 0.1 cm s^-1). The highest resistances obtained are for transfer to the surface of Lake Michigan, yielding values near 90 s cm^-1 for resistance (0.01 cm s^-1 for deposition velocity).Work supported by the U.S. Dept. of Energy and the U.S. Environmental Protection Agency.     

Interaction Between Wind and Snow Surface

The horizontal and vertical wind velocity fluctuations were measured using two sonic anemometers at a height of 135 cm above a snow surface under a transverse snow wave-forming condition. A snow-wave was formed when the wind at a height of 1 m blew at a speed of more than 7 m s⁻¹ after an approximate accumulation of from 10 to 20 cm of new snow on a snowfield. For example, when a snow-wave had a wavelength of 10 m and a wave height of 15 to 20 cm, the measured horizontal and vertical velocity components showed that they had a frequency peak of 0.7 Hz in coherence and co-spectrum corresponding to this wavelength. The results suggest that wind turbulence and snow-wave formation interact with each other.     

Interaction between wind and snow surface

The horizontal and vertical wind velocity fluctuations were measured using two sonic anemometers at a height of 135 cm above a snow surface under a transverse snow wave-forming condition. A snow-wave was formed when the wind at a height of 1 m blew at a speed of more than 7 m s^–1 after an approximate accumulation of from 10 to 20 cm of new snow on a snowfield. For example, when a snow-wave had a wavelength of 10 m and a wave height of 15 to 20 cm, the measured horizontal and vertical velocity components showed that they had a frequency peak of 0.7 Hz in coherence and co-spectrum corresponding to this wavelength. The results suggest that wind turbulence and snow-wave formation interact with each other.     

Ground observed climatology and trend in snow cover phenology across China with consideration of snow-free breaks

Accurate understanding of snow cover phenology and its changes is important to hydrological processes and climate system. Having recognized the potential uncertainties in remote sensing snow cover products, we used daily snow depth observations from 514 meteorological stations across China to investigate the spatiotemporal variations in snow cover phenology during 1970–2014. Climatologically, the snow cover onset date (D_o) and end date (D_e) as well as the number of snow cover days (D_s) depended on latitude at most stations outside of the Tibetan Plateau (TP). For the high-elevation stations, which were mainly in the TP, multiple snow-free breaks (SFBs) during the cold season made D_s insensitive to D_o and D_e. Furthermore, the number of SFBs (D_b) increased significantly with the rise in elevation, explaining why higher altitudes in TP did not necessarily have greater D_s values despite the earlier D_o and later D_e values. From 1970 to 2014, most stations in China exhibited delayed D_o and advanced D_e due mainly to the increased temperature, but such trends were significant at only 10.5% and 15.4% of the stations, respectively. During the same period, shortened D_s primarily occurred south of ~ 40° N, whereas the opposite ones dominated north of ~ 40° N. Most stations (except those in Hexi Corridor) with significant growth in D_s were characterized by delayed D_o and advanced D_e. Such a phenomenon of “increased snow cover days during shortened cold season” was due to the significant shrinkage in D_b values. The spatial pattern of the trends in annual total snow depth overall follows that of D_s, suggesting that the D_s, when takes SFBs into consideration, could be an indicator of variations of snow water resources in China. The trends in D_o, D_e and D_s were not elevation dependent in TP.

     

Transport of Snow by the Wind: A Comparison Between Observations in Ad��lie Land, Antarctica, and Simulations Made with the Regional Climate Model MAR

For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Adélie Land, Antarctica. A regional climate model featuring a coupled atmosphere/blowing snow/snowpack model is forced laterally by meteorological re-analyses. The vertical grid spacing was 2 m from 2 to 20 m above the surface and the horizontal grid spacing was 5?km. The simulation was validated by comparing the occurrence of blowing snow events and other meteorological parameters at two automatic weather stations. The Nash test allowed us to compute efficiencies of the simulation. The regional climate model simulated the observed wind speed with a positive efficiency (0.69). Wind speeds higher than 12 m s ^?1 were underestimated. Positive efficiency of the simulated wind speed was a prerequisite for validating the blowing snow model. Temperatures were simulated with a slightly negative efficiency (?0.16) due to overestimation of the amplitude of the diurnal cycle during one week, probably because the cloud cover was underestimated at that location during the period concerned. Snowfall events were correctly simulated by our model, as confirmed by field reports. Because observations suggested that our instrument (an acoustic sounder) tends to overestimate the blowing snow flux, data were not sufficiently accurate to allow the complete validation of snow drift values. However, the simulation of blowing snow occurrence was in good agreement with the observations made during the first 20 days of January 2010, despite the fact that the blowing snow flux may be underestimated by the regional climate model during pure blowing snow events. We found that blowing snow occurs in Adélie Land only when the 30-min wind speed value at 2 m a.g.l. is >10 m s ^?1. The validation for the last 10 days of January 2010 was less satisfactory because of complications introduced by surface melting and refreezing.     

Vertical ozone fluxes and related deposition parameters over agricultural and forested landscapes

The spatial variability and temporal behavior of the vertical flux of ozone have been investigated from turbulence measurements collected on aircraft flight legs in the daytime period during two consecutive summer experimental field programs. The data were obtained during horizontal flight legs conducted over agricultural crops and forested land in three different regions of the eastern United States.Results from individual experimental cases and statistics derived from all cases in each region are presented. Ozone flux generally exhibited a significant height dependency. The strongest negative (downward) fluxes in the lowest-level flight legs were primarily attributed to the uptake of ozone by the surface and vegetative cover. Fluxes were near-zero in the middle of the convective boundary layer (CBL) in the afternoon period. As ozone flux was proportional to concentration, slightly stronger fluxes were found in low-level urban plume segments where ozone concentrations were 10–20 ppb higher than in the surrounding area. The derived deposition velocity showed no such bias as a function of position across the urban plume. Ozone flux differences were not apparent between the more heavily forested sections and the primarily agricultural cropland areas in these regions. During the afternoon period when no clear temporal trend was evident, means from values obtained below 0.15Z _i (Z _i being the CBL height) were -0.421 and -0.431 ppb m^-2 s^-1 for ozone flux and 0.81 and 0.82 cm s^-1 for the derived mean deposition velocity in the southeastern Pennsylvania and central Ohio areas, respectively. These experimental results for ozone provide support to a dry deposition parameterization module which computes grid-area averaged deposition velocities for use in regional-scale models.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.     

Sensitivity of ADOM dry deposition velocities to input parameters: A comparison with measurements for SO2 and NO2 over three land use types

Abstract

Dry deposition velocity measurements of SO₂ and NO₂ over a deciduous forest, a carrot field and a snow surface are compared with estimates obtained from the dry deposition module in the regional Eulerian Acid Deposition and Oxidant Model (ADOM). The comparison with measurements taken in the fall and winter shows large model overestimates, sometimes as large as a factor of 5. The NO₂ estimates are particularly poor and support existing evidence that models that employ the constant flux assumption for NO₂ are inadequate. The canopy and the snow surface resistances are the largest contributors to the total resistances for SO₂ and NO₂, except for situations in which some of the snow turns into liquid water, when the aerodynamic resistance becomes important.

Increasing the magnitudes, taken from measurements, of the ADOM original values for the stomatal, cuticle, ground and snow resistances and decreasing the NO₂ mesophyll resistance and the Leaf Area Index (LAI) yield improved model results, particularly for SO₂, reducing the error by almost a factor of 5 at times. The new estimates compare favourably with those from a model that includes Wesely's canopy resistance parametrization. Over snow the NO₂ estimates are improved by as much as a factor of 6. Observed deposition velocities for SO₂ vary from 0 to 0.65 cm s^?2 over a deciduous forest, 0 to 0.60 cm s^?2 over a carrot field and are generally less than 0.05 cm s^?2 over snow.     

A theory for the scalar roughness and the scalar transfer coefficients over snow and sea ice

Although the bulk aerodynamic transfer coefficients for sensible (C _H ) and latent (C _E ) heat over snow and sea ice surfaces are necessary for accurately modeling the surface energy budget, they have been measured rarely. This paper, therefore, presents a theoretical model that predicts neutral-stability values of C _H and C _E as functions of the wind speed and a surface roughness parameter. The crux of the model is establishing the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces on the basis of a surface-renewal model in which turbulent eddies continually scour the surface, transferring scalar contaminants across the interface by molecular diffusion. Matching these interfacial sublayer profiles with the semi-logarithmic inertial sublayer profiles yields the roughness lengths for temperature and water vapor. When coupled with a model for the drag coefficient over snow and sea ice based on actual measurements, these roughness lengths lead to the transfer coefficients. C _E is always a few percent larger than CH. Both decrease monotonically with increasing wind speed for speeds above 1 m s^–1, and both increase at all wind speeds as the surface gets rougher. Both, nevertheless, are almost always between 1.0 × 10^–3 and 1.5 × 10^–3.     

Extraordinary blowing snow transport events in East Antarctica

In the convergence slope/coastal areas of Antarctica, a large fraction of snow is continuously eroded and exported by wind to the atmosphere and into the ocean. Snow transport observations from instruments and satellite images were acquired at the wind convergence zone of Terra Nova Bay (East Antarctica) throughout 2006 and 2007. Snow transport features are well-distinguished in satellite images and can extend vertically up to 200 m as first-order quantitatively estimated by driftometer sensor FlowCapt?. Maximum snow transportation occurs in the fall and winter seasons. Snow transportation (drift/blowing) was recorded for ~80% of the time, and 20% of time recorded, the flux is >10^?2 kg m^?2 s^?1 with particle density increasing with height. Cumulative snow transportation is ~4 orders of magnitude higher than snow precipitation at the site. An increase in wind speed and transportation (~30%) was observed in 2007, which is in agreement with a reduction in observed snow accumulation. Extensive presence of ablation surface (blue ice and wind crust) upwind and downwind of the measurement site suggest that the combine processes of blowing snow sublimation and snow transport remove up to 50% of the precipitation in the coastal and slope convergence area. These phenomena represent a major negative effect on the snow accumulation, and they are not sufficiently taken into account in studies of surface mass balance. The observed wind-driven ablation explains the inconsistency between atmospheric model precipitation and measured snow accumulation value.     

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.     

Fluxes of gases and particles above a deciduous forest in wintertime

Eddy-correlation measurements of the vertical fluxes of ozone, carbon dioxide, fine particles with diameter near 0.1 m, and particulate sulfur, as well as of momentum, heat and water vapor, have been taken above a tall leafless deciduous forest in wintertime. During the experimental period of one week, ozone deposition velocities varied from about 0.1 cm s^–1 at night to more than 0.4 cm s^-1 during the daytime, with the largest variations associated primarily with changes in solar irradiation. Most of the ozone removal took place in the upper canopy. Carbon dioxide fluxes were directed upward due to respiration and exhibited a strong dependence on air temperature and solar heating. The fluxes were approximately zero at air temperatures less than 5 °C and approached 0.8 mg m^–2 s^–1 when temperatures exceeded 15 °C during the daytime. Fine-particle deposition rates were large at times, with deposition velocities near 0.8 cm s^–1 when turbulence levels were high, but fluxes directed upward were found above the canopy when the surface beneath was covered with snow. Diffusional processes seemed to dominate fine-particle transfer across quasilaminar layers and subsequent deposition to the upper canopy. Deposition velocities for particulate sulfur were highly variable and averaged to a value small in magnitude as compared to similar measurements taken previously over a pine forest in summer.     

Precipitation drives interannual variation in summer soil respiration in a Mediterranean-climate, mixed-conifer forest

Predictions of future climate change rely on models of how both environmental conditions and disturbance impact carbon cycling at various temporal and spatial scales. Few multi-year studies, however, have examined how carbon efflux is affected by the interaction of disturbance and interannual climate variation. We measured daytime soil respiration (R _s) over five summers (June–September) in a Sierra Nevada mixed-conifer forest on undisturbed plots and plots manipulated with thinning, burning and their combination. We compared mean summer R _s by year with seasonal precipitation. On undisturbed plots we found that winter precipitation (PPT_w) explained between 77–96% of interannual variability in summer R _s. In contrast, spring and summer precipitation had no significant effect on summer R _s. PPT_w is an important influence on summer R _s in the Sierra Nevada because over 80% of annual precipitation falls as snow between October and April, thus greatly influencing the soil water conditions during the following growing season. Thinning and burning disrupted the relationship between PPT_w and R_s, possibly because of significant increases in soil moisture and temperature as tree density and canopy cover decreased. Our findings suggest that R _s in some moisture-limited ecosystems may be significantly influenced by annual snowpack and that management practices which reduce tree densities and soil moisture stress may offset, at least temporarily, the effect of predicted decreases in Sierran snowpack on R _s.     

Downdrafts from tropical oceanic cumuli

Simultaneous observations at up to six levels through a depth of 1100 m from a tethered balloon and instrumented ship's boom of 49 gust fronts associated with convective clouds are used to describe the mean and the detailed characteristics of tropical oceanic cumuli outflow at the surface and in the boundary layer.The changes in temperature and velocity of the maritime tropical gust front are about 50%; of the changes observed in mid-latitude convective outflows. Vertical velocities at the gust front l m s^-1 are continuous through the lower 500 m for time periods up to 8 min implying vertical displacements of at least 500 m entirely sufficient at the observed humidities (q >- 15 g kg^-1) to initiate new cloud growth. Horizontal and vertical vorticity in the order of 10^-3 s^-1 to 10^-2 s^-1 is observed at the gust front.     

北京延庆山区降雪云物理特征的垂直观测和数值模拟研究

基于风廓线雷达、云雷达、粒子谱仪、微波辐射计和自动站等垂直观测设备,结合中尺度数值模式WRF对2017年3月23~24日北京延庆海坨山地区的一次降雪过程进行了观测和数值模拟研究。研究结果表明:垂直探测仪器结合中尺度数值模式可以获得降雪的宏观结构和微物理信息,有助于对降雪的深入研究。此次降雪过程由中高层西南及偏南暖湿气流与低层东南偏冷空气交汇造成动力和水汽辐合抬升形成,4~5 km高度处的风切变有利于降雪的增强。上升气流有助于水汽的输送、冰雪转化以及雪晶凝华、聚合,冰晶数浓度中心对应着上升运动顶部。然而此次降雪云系低层过冷云水含量不足,降雪回波<20 dBZ,回波顶高<7 km,雪花垂直下落速度<2 m s-1,地面降水量大值与低层强回波区对应。降雪粒子谱分布范围较窄,以直径1 mm左右的小粒子为主,相态主要为干雪,基本不存在混合相态。     

Annual variation of surface roughness obtained from wind profile measurements

Summary Profile measurements of wind and temperature have been performed at the Agricultural University of Norway on a routine basis since 1997. 10-min. averages are stored in a database together with other relevant meteorological parameters. The database can be used to determine the seasonal variation of surface aerodynamic roughness, showing the growth of grass between cutting during the growing season, the effect of snowfall and the melting of snow etc. However, careful screening of the data must be conducted before reliable estimates can be made. The main objective of this study is to establish simple practical rules for filtering out unreliable datasets for the evaluation of the surface roughness parameter z₀, and to present its annual variation. The resulting values for the summer period agree well with values found in the literature for homogenous grass covered surfaces. In the transition periods during autumn and spring, and during wintertime in mild weather conditions, the surface is generally non-homogenous with a mixture of snow patches, ponds of melting water and shrubs of withered grass. The results show that the mechanical interaction between a non-homogeneous land surface and the boundary layer flow can be described by one roughness parameter, with a numeric value somewhere in between the ideal values for the different surface characteristics. Another use of the database is to investigate drainage flow and the relationship between drainage flow, prevailing wind direction and the mean vertical velocity of the air. Most micrometeorological studies of the fluxes of heat and water vapour in the surface layer, assume the mean vertical velocity to be zero, focusing on eddy fluxes and thereby excluding any transport in the mean flow. In certain situations, this may lead to serious errors. This work shows that convergence of horizontal flow leads to an upward movement of air, which is enhanced if the prevailing direction of the wind opposes the outflow of the cold drainage winds from the area.     

A study of a CASP storm: Analysis of radar data

Abstract

The morphology and time evolution of a winter storm is studied using radar data taken during the Canadian Atlantic Storms Program (CASP). The vertical motions that generate the snow are derived from reflectivity measurements. The study reveals a banded structure in the precipitation pattern with ascending and descending air associated with the bands. Vertical velocities averaged over the radar‐covered area reach values close to 1 m s^‐1 . The region of large‐scale uplifting moves horizontally at about half the horizontal air velocity. A persistent precipitation pattern results from the continuously renewed air within the uplifting region.     

Momentum and scalar transfer coefficients over aerodynamically smooth antarctic surfaces

Vertical profiles of wind speed, temperature and humidity were used to estimate the roughness lengths for momentum (z ₀), heat (z _H ) and moisture (z _Q) over smooth ice and snow surfaces. The profile-measurements were performed in the vicinity of a blue ice field in Queen Maud Land, East Antarctica. The values ofz ₀ over ice (3·10^–6 m) seem to be the smallest ever obtained over permanent, natural surfaces. The settling of snow on the ice and the loss of momentum at saltating snow particles serve as momentum dissipating processes during snow-drift events, expressed as a strong dependence ofz ₀ on u_#.The scalar roughness lengths and surface temperature can be evaluated from the temperature and humidity profile measurements if the ratioz _H /z _Q is specified. This new method circumvents the difficult measurement of surface temperature. The scalar roughness lengths seem to be approximately equal toz0 for a large range of low roughness Reynolds numbers, despite the frequent occurrence of drifting snow. Possible reasons for this agreement with theory of non-saltating flow are discussed.     

THE APPROACH TO REMOTE SENSING OF WATER VAPOR BASED ON GPS AND LINEAR REGRESSION Tm IN EASTERN REGION OF CHINA

The approach to remote sensing of water vapor by using global positioning systems(GPS) isdiscussed.In order to retrieve the vertical integrated water vapor(IWV) or the precipitable water(PW),the weighted "mean temperature" of the atmosphere,T_m would be estimated to the specificarea and season.T_m depends on surface temperature,tropospheric temperature profile,and thevertical distribution of water vapor.The surface temperature dependence is borne out by acomparison of T_m and the values of surface temperature T_s using radiosonde profiles of BeijingStation(No.54511) throughout 1992.The analysis of radiosonde profiles spanning a one-yearinterval(1992) from sites in eastern region of China with a latitude range of 20-50°N and alongitude range of 100-130°E yields the coefficients a and b of a linear regression equation T_m=a+bT_s.     

Experimental studies of the development of quasi-two-dimensional turbulence in stably stratified fluid

Quasi-two-dimensional turbulence was generated by towing an array of vertical cylinders through a tank which was filled with a two-layer stratified fluid. Sugar and Epsom salts were used, to give matching refractive indices for the two layers. The interface between the two layers was seeded with approximately 1000 neutrally buoyant particles. The evolution of this quasi-two-dimensional turbulence was visualized by photographing the fluorescent particles illuminated by a horizontal laser sheet traversing in the vertical direction. The three-dimensional particle velocity was obtained by digitizing the streaks. The evolution of the velocity correlations, length scales, one-dimensional and two-dimensional velocity and vorticity spectra were obtained for N = 5.72 s⁻¹, N = 4.43 s⁻¹, and N = 2.55 s⁻¹ (where N is the Brunt-Väisälä frequency). The results showed the physical process of inverse energy cascading and the formation of dominant vortical structures under the influence of density stratification. Compared with idealized two-dimensional turbulence, the flow is highly dissipative at high N, as a result of the frictional dissipation between the interface and the unstratified layers.     

The Surface Energy Budget and Its Impact on the Freeze-thaw Processes of Active Layer in Permafrost Regions of the Qinghai-Tibetan Plateau

The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes,the interaction between surface energy budget and freeze-thaw processes.The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations.Annual average net radiation(R_n)for 2010 was 86.5 W m^-2,with the largest being in July and smallest in November.Surface soil heat flux(G₀)was positive during warm seasons but negative in cold seasons with annual average value of 2.7 W m^-2.Variations in R_n and G₀ were closely related to freeze-thaw processes.Sensible heat flux(H)was the main energy budget component during cold seasons,whereas latent heat flux(LE)dominated surface energy distribution in warm seasons.Freeze-thaw processes,snow cover,precipitation,and surface conditions were important influence factors for surface energy flux.Albedo was strongly dependent on soil moisture content and ground surface state,increasing significantly when land surface was covered with deep snow,and exhibited negative correlation with surface soil moisture content.Energy variation was significantly related to active layer thaw depth.Soil heat balance coefficient K was>1 during the investigation time period,indicating the permafrost in the Tanggula area tended to degrade.