ModellingNO conversion by using probability density functions

An expression for concentration fluctuations in a smoke plume is derived from airborne measurements ofNO _X. A linear relation between the standard deviation of the fluctuations around a Gaussian concentration profile and the average gradient in the concentrations is assumed. With this relation the probability density function of expectedNO ₂ concentrations at 3 km from a source ofNO _X is modelled under the assumption of photostatic equilibrium, and is compared with measurements. A parametrisation for the concentration fluctuations of std(C)= 26(+/–7)*dc/dr is proposed (r in metres). CalculatedNO ₂ distributions are in reasonable agreement with the measurements and the averageNO ₂ concentration appeared not to be affected by the concentration fluctuations in theNO _X concentration. The spatial resolution of all measurements was 40 m.     

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.     

Analysis of the origins and implications of the18O content of stratospheric water vapor

Factors influencing the¹⁸O content of stratospheric H₂O are reviewed in order to provide a theoretical framework for the interpretation of measurements of this quantity, which are now becoming available. Depletions in¹⁸O of 5–10% in stratospheric H₂O are expected based on the known correlation between that of D and¹⁸O in tropospheric H₂O and observed measurements of large (typically 50%) depletions of D in stratospheric H₂O. H₂O formed in the stratosphere as a result of oxidation of CH₄ can be expected to reflect primarily the¹⁸O content of stratospheric O₂, which is the same as that of tropospheric O₂ (slightly enhanced with respect to standard mean ocean water). Thus, a reduction in the¹⁸O depletion is expected with increasing altitude, but not a large enhancement in¹⁸O in upper stratospheric H₂O as found in recent far infrared measurements. The observed large enhancement of¹⁸O in stratospheric O₃ is not expected to be reflected in stratospheric H₂O. Necessary laboratory data for the improved quantification of these effects are reviewed.     

极地与青藏高原地区NOX的冰雪来源

NO_x在雪-气之间的交换能够影响到极地大气边界层的大气成分和大气化学过程,增强极地大气边界层的氧化性,并可能影响对冰雪大气成分记录的解释。近年来,人们认识到冰雪在光照下释放NO_x是极地大气边界层NO_x的一个重要来源。从以下几方面对大气边界层NO_x的冰雪来源进行综述：NO_x冰雪来源的观测和实验证据、冰雪释放NO_x的机制和影响参数、极地NO_x浓度和通量以及极地冰雪NO_x化学过程对环境的影响。近年来,笔者已在处于北半球中纬度的青藏高原观测到冰雪表面光照下释放出比极地高1个量级浓度水平的NO_x,因此需更深入的科学研究揭示其对青藏高原大气氧化性的影响。     

Flux determination over a smooth surface under strongly unstable conditions

Careful micrometeorological measurements on an empty parking lot allowed determination of the surface fluxes of sensible heatH and of momentum by applying profile equations derived from Monin-Obukhov similarity theory with two sets of the stability correction function for momentum _m and sensible heat _h . These fluxes were compared with reference values ofH independently determined by means of an eddy correlation technique. In general, better agreement was found betweenH values derived from profiles with the stability functions of Brutsaert (1992) and referenceH values, than when the Businger-Dyer functions were used to deriveH. The disagreement in the latter comparison was especially serious under strongly unstable conditions, with the value ofy=–z/L (wherez is the height andL is the Obukhov length) larger than 10. A closer look at the procedure for calculatingH from the profiles revealed that the large differences between theH values derived with these two different versions of the stability correction functions were caused by the small differences of the _h values, and not by the larger differences of the _m values. This result stems from the strong sensitivity of the resultingH values on the choice of _h .     

The equilibrium constant of NO2 with N2O4 and the temperature dependence of the visible spectrum of NO2: A critical review and the implications for measurements of NO2 in the polar stratosphere

Measurements of stratospheric NO₂ by ground-based visible spectrometers rely on laboratory measurements of absorption cross-sections. We review low-temperature laboratory measurements, which disagree by amounts claimed to be significant. Our recalculation of their errors shows that in general disagreements are not significant and that errors in the ratios of cross-sections at low to room temperature are between ±3% and ±8.8%. Of these errors, up to ±3.5% was contributed by errors in the equilibrium constant,K _p, in those measurements where the pressure was above 0.1 mbar.We review measurements and calculations ofK _p, which were accurate to ±5% from 300 to 233 K. Each method was potentially flawed. For example, infrared measurements of the partial pressure of NO₂ ignored the dependence of absorption on total pressure. From thermodynamic theory, formulae forK _pcan be derived from expressions for the variation of heat capacity with temperature. Contrary to common belief, coefficients in the formulae used by spectroscopists were not derived from the thermodynamic quantities. Rather, they were fitted to measurements or to calculations. Hence, they are empirical and it is dangerous to extrapolate below 233 K, the lowest temperature of the measurements.There are no measurements of NO₂ cross-sections below 230 K. Extrapolation of these cross-sections to analysis of measurements of NO₂ at the low temperatures of the Arctic and Antarctic stratosphere is also dangerous. For satisfactory analysis of polar spectra, the NO₂ cross-sections should be measured at temperatures down to 190 K with a relative accuracy of ±1%. This difficult experiment would need a cell of minimum length 32 m whose length can be adjusted. Because their effects are circular, many errors cannot be removed simply. Although circular errors also arise in the measurements ofK _pand of the infrared spectrum, their weights differ from those in the visible spectrum. The optimum experiment might therefore simultaneously measure the visible and infrared spectra andK _p.     

An experimental study of aerosol scavenging by hexagonal plate ice crystals

The scavenging efficiency, E, of small hexagonal plate ice crystals for aerosol particles has been measured in a series of laboratory cloud experiments. The ice crystal diameters, D μm ranged from 35 to 150 μm with aerosol particles in the range 4 to 6 μm. An ice crystal replication technique made possible the individual examination of more than 43m500 individual crystals from which the relation: log₁₀ E = 1.554 − 1.047 log₁₀D was established with values of E in the range 0.2–0.9 corresponding to the range of crystal sizes investigated. For some crystal collectors, values of E extended above unity and this was attributed to wake capture, oscillations and the extended sweep out path of the ice crystals associated with their spiral fall pattern.     

Estimation of probable maximum precipitation for a 2-day duration: Part 2 — north Indian region

Summary In a previous study the authors have estimated the probable maximum precipitation (PMP) for a 2-day duration using Hershfield's formulaX _PMP =X _n +k _{m n} for stations in the southern Indian region. In this paper, the study is extended to estimate the PMP for stations in the north Indian region, north of 20°N. Maximum annual 2-day rainfall data for an 80-year period, from 1901, were obtained for 286 stations in the region. A mathematical relationship between the frequency factor (k _m ) and the mean annual extreme rainfall (X _n ) was developed to bek _m = 18.15 exp (–0.0448X _n ). This equation was used to obtaink _m for different values ofX _n and, subsequently, to estimate 2-day PMP values for the 286 stations. Using these PMP estimates, a generalised chart was prepared, showing the spatial distribution of 2-day PMP. It was found that 2-day PMP over the north Indian region varied from 60 cm to 130 cm, and the average ratio of the 2-day PMP to the highest observed 2-day rainfall was found to be 1.91. The results show that there have been instances when almost 2-day point PMP have occurred at some of the stations in the region. These results indicate that the statistically estimated PMP rainall are not therefore, mere theoretical estimates, but they can occur under optimum meteorological conditions.With 4 Figures     

Momentum and sensible heat fluxes calculated by the dissipation technique during the Ocean Storms Project

High frequency measurements of wind velocity and temperature were made during the Ocean Storms Project in November 1987. The dissipation method was applied to the resulting time series in order to determine friction velocities,u _*, and the characteristic temperature scale,t _*, at 1-min intervals. These values were then compared to the 1-min mean wind speed and air-sea temperature differences to determine relationships for the drag coefficient (C _d ) and Stanton number (C _h ). The drag coefficient was comparable to other values reported in the literature, although the variation with wind speed was greater than reported by other investigators. An examination of the residual time series indicated a systematic low frequency periodicity of about 2-hr duration which was attributed to a fluctuating wind interacting with the surface gravity wave field. The temperature fluctuations did not produce meaningful estimates ofC _h for stable conditions. For unstable conditions, a value of 1.09±0.02×10^–3 was found.     

欧亚北部冬季增雪“影响”我国夏季气候异常的机理研究:欧洲冬季增雪的重要作用

在作者前期研究(穆松宁和周广庆, 2012)的基础之上, 本文主要利用了EOF分析和相关分析, 对欧亚北部冬季新增雪盖面积(Total Fresh Snow Extent, 记为T_FSE)与我国夏季气候异常之间"隔季相关"的机理进行了更进一步的探讨, 主要目的在于寻找T_FSE气候效应的冬季增雪面积关键区。结果表明, 虽然欧洲中纬冬季增雪面积(文中均以T_FSE-1表示)与我国夏季气候异常的关系不很显著, 但其变化维系了上述"隔季相关"的物理途径, 其变化对T_FSE的气候效应而言起关键作用, 但是, 亚洲中纬冬季增雪面积(文中均以T_FSE-2表示)的贡献尚不清楚;另外, 对T_FSE的气候效应而言, 起作用的实际上是欧亚中纬冬季增雪面积(文中均以T_FSE-1-2表示), 其不但与我国夏季气候异常具有更显著的"隔季相关", 而且这种"隔季相关"还具有和T_FSE非常相似的、但更为清晰的物理途径, 因此, 在气候预测的意义上, T_FSE-1-2可替代T_FSE。     

The surface energy balance of a snow cover: comparing measurements to two differentsimulation models

Summary  We compared two one-dimensional simulation models for heat and water fluxes in the soil-snow-atmosphere system with respect to their mathematical formulations of the surface heat exchange and the snow pack evolution. They were chosen as examples of a simple one-layer snow model and a more detailed multiple-layer snow model (SNTHERM). The snow models were combined with the same one-dimensional model for the heat and water balance of the underlying soil (CoupModel). Data from an arable field in central Sweden (Marsta), covering two years (1997–1999) of soil temperature, snow depth and eddy-correlation measurements were successfully compared with the models. Conditions with a snow pack deeper or shallower than 10 cm and bare soil resulted in similar discrepancies. The simulated net radiation and sensible heat flux were in good agreement with that measured during snow-covered periods, except for situations with snowmelt when the downward sensible heat flux was overestimated by 10–20 Wm⁻². The results showed that the uncertainties in parameter values were more important than the model formulation and that both models were useful in evaluating the limitations and uncertainties of the measurements. Received November 1, 1999 Revised April 20, 2000     

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 numerical study of boundary-layer dynamics in a mountain valley

The characteristics of dynamics and thermodynamics of the atmospheric boundary layer in a part of the Colorado River Valley, centered around Lake Mohave, have been investigated by analysis of measurements conducted during a field program in late spring and early summer of 1986 and a series of numerical simulations by a three-dimensional second-moment turbulence-closure model. The model was validated against measurements described in a companion article (Engeret al., 1993). According to airsonde measurements performed on eight nights, the depth of the surface inversion was around 200 m with an average temperature gradient of about 30 K km^–1. Analysis of acoustic sounder data collected during one month revealed significant diurnal variations ofU andV wind-speed components related to slope and valley flows, respectively. Some of the dynamics properties have been explained by the simulation results. It has been shown that the appearance of supergeostrophic southerly valley flow is associated with the westerly component of the geostrophic flow. Since a westerly component of the geostrophic wind is quite common for this area in summer, this effect also explains the frequently observed southerly valley flow in summer. Elevated minima of the measured wind speed around valley ridges appear to be related to the interaction of conservation of momentum in theX andY directions. The critical direction of the geostrophic wind relevant for reversal of up-valley flow to down-valley flow has also been studied. The critical direction is about 300° for one of the measurement sites and, depending on the angle between valley axis and south-north direction, the critical direction is expected to vary by about 15–20°. The scale analysis of the simulated equations of motion and turbulence kinetic energy emphasizes the strong impact of meandering of the flow due to actual topographic complexity.     

A Parameterization of Bowen Ratio with Respect to Soil Moisture Availability

The Bowen ratio (B) is impacted by 5 environmental elements: soil moisture availability, m, the ratio of resist-ances between atmosphere and soil pores, ra/rd, atmospheric relative humidity, h, atmospheric stability, ΔT, and environment temperature. These impacts have been investigated over diverse surfaces, including bare soil, free water surface, and vegetation covered land, using an analytical approach. It was concluded that: (a) B is not a continuous function. The singularity exists at the condition αhcb=h, occurring preferably in the following conditions: weak turbulence, stable stratified stability, dry soil, and humid air, where hcb, defined by Eq.(11) is a critical variable. The existence of a singularity makes the dependence of B on the five variables very complicated. The value of B approaches being inversely proportional to m under the conditions m≥mfc (the soil capacity) and / or ra/rd→0. The proportional coefficient changes with season and latitude with relatively high values in winter and over the poles; (b) B is nearly independent of ra/rd during the day. The impact of m on B is much larger as compared to that of ra/rd on B, (c) when h increases, the absolute value of B also increases; (d) over bare soil, when the absolute surface net radiation increases, the absolute value of B will increase. The impact of RN on B is larger at night than during the day, and (e) over plant canopy, the singularity and the dependcies of B on m, ra , and h are modified as compared to that over bare soil. Also (i) during the daytime unstable condition, m exerts an even stronger impact on B, at night, however, B changes are weak in response to the change in m; (ii) the value of B is much more sensitive in response to the changes of turbulent intensity; (iii) the B response to the variation of h over a vegetation covered area is weaker; and (iv) the singularity exists at the condition hcp=h instead of αhcb=h as over bare soil, where hcp is defined by Eq.(49). The formulas derived over bare soil also hold the same when applied to free water bodies as long as they are visualized as a special soil in which the volumetric fraction of soil pore is equal to one and are fully filled with water. Finally, the above discussions, are used to briefly study the impact on the thermally induced mesoscale circulations.     

Spectral scaling in a high reynolds number laboratory boundary layer

Spectra and co-spectra of the streamwise (u) and normal or vertical (w) velocity fluctuations have been measured in the inner region of a large Reynolds number laboratory boundary layer over a rough wall. There is reasonable evidence of ak ₁ ^–1 range in theu spectrum (wherek ₁ is the streamwise wavenumber). Such a range results from an overlap between a spectral region dominated by largescale, inactive motion, which scales on the boundary-layer thickness, and a region dominated by smaller-scale, active motion which scales on the distance from the wall. Spectra ofw, anduw cospectra, scale in a manner consistent with the dominance by active motion. The present spectral data do not support local isotropy over the inertial subrange. A comparison between measuredw spectra and calculations based on isotropy indicates that the inertial subrange anisotropy is only slightly affected by the magnitude of the non-dimensional mean shear.     

Measurements of stratospheric HO2 and NO2 by matrix isolation and ESR spectroscopy

Vertical profiles of stratospheric HO₂ and NO₂ concentrations were determined using matrix isolation and ESR. Up to 10 different samples per flight were collected in situ by a balloon borne cryosampler. Free radicals and trace constituents which are condensable at 68 K are trapped in a polycristalline H₂O or D₂O matrix. After collection, the samples are stored at a temperature below 83 K until they are analysed in the laboratory by X-band ESR spectroscopy at 4 K. The HO₂ and NO₂ were identified and calibrated by comparison with standard samples collected in the laboratory under typical stratospheric sampling conditions. From several flights over Southern France (44°N) we obtained two profiles of the stratospheric NO₂ mixing ratio. One, from 21 October 1982, agrees well with previous measurements. The other, from 8 October 1981, is lower by one order of magnitude. The few HO₂ data obtained around 35 km altitude agree with previous measurements. An isolated measurement at 17 km altitude is one order of magnitude higher than the model predicted HO₂ concentration.     

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.     

An analytical exericise to elucidate quasi-geostrophic frontogenesis

Summary Frontogenesis is frequently described by theQ-vector (Hoskins et al., 1978), a term being composed of several derivatives of basic meteorological parameters and their products. Its distribution and especially the _H ·Q-fields are highly important to estimate frontogenesis and cross frontal circulation. Although theQ-vector (Hoskins et al., 1978) allows an easier assessment of the vertical wind forcing than the original omega equation of the quasi-geostrophic theory, it is still difficul to imagine the three-dimensional (3-d) spatial distribution ofQ and _H ·Q even for standard atmospheric fields. Thus there is a need to shed more light in theQ and _H ·Q-fields for special synoptic situations.This is done here by constructing analytical 3-d geostrophically balanced wind-and temperature fields, for which theQ-forcing (Qformed with the geostrophic wind) can easily be computed and presented. Three examples (see Sections 3 to 5) are discussed yielding typical and realistic (compared to known pattern) 3-d forcing distributions ofQ and _H ·Q. Within the simple analytical scheme used here their origin can casily be understood. These fields of a 2000×2000 km² horizontal domain ranging up to 250 hPa are: A modified Bergeron deformation field containing a cold front (case I a) and a warm front (case I b); an upper tropospheric jet including a jet-parallel transition zone between warm and cold air (case II); and a circular low pressure circulation pattern with two fronts (case III).The paper presents these 3-d fields with the advantage that the analytical method is not affected by any kind of limited numerical resolution. It also shows how these fields degenerate with decreasing resolution if the analytical data are used in descrete form. This simulates working with discrete numerical data and demonstrates how narrow frontal zones of structure elements ofQ and _H ·Q considerably smooth out with increasing grid distances.With 17 Figures     

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.