A numerical study of contrail development

In this study, the formation of a contrail from an aircraft flying near the tropopause is simulated using a three-dimensional mesoscale atmospheric model including a very complex scheme of parameterized cloud microphysical processes. Two different primary ice nucleation parameterizations for deposition nucleation, condensation freezing, and contact freezing are applied. The model-predicted ice concentrations are compared to data measured during the International Cirrus Experiment (ICE), 1989.     

The retrieval of cloud microphysical properties using satellite measurements and an in situ database

By combining AVHRR data from the NOAA satellites with information from a database of in situ measurements, large-scale maps can be generated of the microphysical parameters most immediately significant for the modelling of global circulation and climate. From the satellite data, the clouds can be classified into cumuliform, stratiform and cirrus classes and then into further sub-classes by cloud top temperature. At the same time a database of in situ measurements made by research aircraft is classified into the same sub-classes and a statistical analysis is used to derive relationships between the sub-classes and the cloud microphysical properties. These two analyses are then linked to give estimates of the microphysical properties of the satellite observed clouds. Examples are given of the application of this technique to derive maps of the probability of occurrence of precipitating clouds and of precipitating water content derived from a case study within the International Cirrus Experiment (ICE) held in 1989 over the North Sea.     

Physical,radiative, and dynamical processes within a nighttime marine stratus cloud

Observations taken by aircraft and conventional platforms are used to investigate dynamical, physical, and radiative processes within a marine stratus cloud during the Canadian Atlantic Storms Program (CASP) II field project which took place over the east coast of Canada. Stratus which formed over the ocean on February 6, 1992 during the nighttime, is studied to analyze cloud top and base processes. The cloud was supercooled during the study period. Fluctuations and fluxes are calculated along constant flight altitude legs approximately 100 km long in space. The scales of structures larger than 5 km are removed from the analysis using a running average technique. Droplet spectra obtained by a forward scattering spectrometer probe (FSSP) were used in a 1-D radiative transfer model to calculate infrared (IR) fluxes and radiative heating rates. A heat conservation equation was used to estimate vertical air velocity (w _a ) within the cloud. The results showed that, because of a warmer ocean surface, significant moisture and heat were transferred from the ocean surface to the boundary layer. The cloud base was at about 400 m height and the top was at about 1.4 km.w _a at the cloud base was estimated about 5 cm s^–1. Strong IR cooling rate at the cloud top was calculated to be 75°C day^–1 for a 100 m thick layer. Negative skewness inw _a , suggesting narrow downdrafts, was likely due to radiative cooling at the cloud top. The entrainment velocity was found to be about 1.5 cm s^–1 at cloud top. Mean moisture and heat fluxes within the cloud were estimated to be comparable to those from the ocean surface. Vertical air velocity at the cloud top due to radiative cooling was found to be about –40 cm s^–1.     

The Baltic Sea Experiment BALTEX: A brief overview and some selected results of the authors

The mechanisms responsible for the transfer of energy and water within the climate system are under worldwide investigation within the framework of the Global Energy and Water Cycle Experiment (GEWEX) to improve the predictability of natural and man-made climate changes at short and long ranges and their impact on water resources. Five continental-scale experiments have been established within GEWEX to enable a more complete coupling between atmospheric and hydrological models. One of them is the Baltic Sea Experiment (BALTEX).In this paper, the goals and structure of BALTEX are outlined. A short overview of measuring and modelling strategies is given. Atmospheric and hydrological model results of the authors only are presented. These include also the validation of precipitation using station measurements as well as validation of modelled cloud cover with cloud estimates from satellite data. Furthermore, results of a large-scale grid based hydrological model to be coupled to atmospheric models are presented.This research has never been possible without the contribution of research groups and operational institutions from all 10 member countries. We concentrate here on results obtained at the GKSS research center.     

The Global Atmospheric Electrical Circuit and Climate

Evidence is emerging for physical links among clouds, global temperatures, the global atmospheric electrical circuit and cosmic ray ionisation. The global circuit extends throughout the atmosphere from the planetary surface to the lower layers of the ionosphere. Cosmic rays are the principal source of atmospheric ions away from the continental boundary layer: the ions formed permit a vertical conduction current to flow in the fair weather part of the global circuit. Through the (inverse) solar modulation of cosmic rays, the resulting columnar ionisation changes may allow the global circuit to convey a solar influence to meteorological phenomena of the lower atmosphere. Electrical effects on non-thunderstorm clouds have been proposed to occur via the ion-assisted formation of ultra-fine aerosol, which can grow to sizes able to act as cloud condensation nuclei, or through the increased ice nucleation capability of charged aerosols. Even small atmospheric electrical modulations on the aerosol size distribution can affect cloud properties and modify the radiative balance of the atmosphere, through changes communicated globally by the atmospheric electrical circuit. Despite a long history of work in related areas of geophysics, the direct and inverse relationships between the global circuit and global climate remain largely quantitatively unexplored. From reviewing atmospheric electrical measurements made over two centuries and possible paleoclimate proxies, global atmospheric electrical circuit variability should be expected on many timescales.     

Effect of atmospheric gases,surface albedo and cloud overlap on the absorbed solar radiation

Recent studies have provided new evidence that models may systematically underestimate cloud solar absorption compared to observations. This study extends previous work on this “absorption anomaly” by using observational data together with solar radiative transfer parameterisations to calculate f_s (the ratio of surface and top of the atmosphere net cloud forcings) and its latitudinal variation for a range of cloud types. Principally, it is found that (a) the zonal mean behaviour of f_s varies substantially with cloud type, with the highest values obtained for low clouds; (b) gaseous absorption and scattering can radically alter the pattern of the variation of f_s with latitude, but gaseous effects cannot in general raise f_s to the level of around 1.5 as recently determined; (c) the importance of the gaseous contribution to the atmospheric ASR is such that whilst f_s rises with surface albedo, the net cloud contribution to the atmospheric ASR falls; (d) the assumed form of the degree of cloud overlap in the model can substantially affect the cloud contribution to the atmospheric ASR whilst leaving the parameter f_s largely unaffected; (e) even large uncertainties in the observed optical depths alone cannot account for discrepancies apparent between modelled and newly observed cloud solar absorption. It is concluded that the main source of the anomaly may derive from the considerable uncertainties regarding impure droplet microphysics rather than, or together with, uncertainties in macroscopic quantities. Further, variable surface albedos and gaseous effects may limit the use of contemporaneous satellite and ground-based measurements to infer the cloud solar absorption from the parameter f_s.     

Convective heat transfer in magmas near the liquidus

Natural convection in magmas at subliquidus temperatures is analyzed using Bingham plastic and power-law rheology models. Heat flux measurements were obtained at liquidus and subliquidus temperatures for degassed basaltic lava at atmospheric pressure. These measurements of heat flux ranged from 2 to 40 kW/m² and were obtained using two different types of convective heat flux probes. The agreement between the two different instruments and the theoretical calculations is excellent. A noticeable change in the trend of the convective heat flux data is observed in the vicinity of the liquidus temperature. Subliquidus convective heat flux rates (6–15 kW/m²) are attractive for energy extraction applications.     

Radiative properties of cirrus clouds in NOAA 4 VTPR channels: Some explorations of cloud scenes from satellites

A one-dimensional spectral infrared radiative transfer model has been developed for atmospheres containing cirrus clouds and absorbing gases above, below and within the cloud. The transfer model takes into consideration the inhomogeneity of the cloudy atmosphere, the gaseous absorption in scattering cloud layers and the wavenumber dependence of radiative transfer. In addition, the cirrus cloud is further divided into a number of sub-layers to account for the non-isothermal and inhomogeneous cloud characteristics. Single-scattering properties for ice crystals are calculated assuming ice cylinders 200 and 60 m in lenght and width, respectively, randomly oriented in a horizontal plane. The spectral infrared transfer program is applied to VTPR channels of the NOAA 4 satellite to simulate upward radiances in cirrus cloud conditions.Comparisons between satellite observed and theoretically simulated upward radiances are carried out for selected cirrus cloud cases. Incorporating atmospheric profiles obtained from radiosonde and the observed cloud information into the spectral transfer program, we show a systematic agreement between observed and computed upward radiances. Systematic reduction patterns of the upward radiance caused by the increase of the cloud ice content are clearly demonstrated for VTPR channels employing tropical and midlatitude atmospheric profiles. Having the quantitative relationships between upward radiances and ice contents, procedures are described for the inference of the cloud ice content and cloud amount. The proposed method has been successfully applied to the three cirrus cloud cases.     

Stochastic analysis of neutrally stratified cirrus layer

Two cirrus cloud systems observed during the winter of 2001 at the Southern Great Plains site of the Atmospheric Radiation Measurements program in Oklahoma, USA are studied because of the distinct neutrally stratified layers formed within the clouds. Observations are obtained with 35 GHz millimeter-wave radar and backscattering cross-section η(t) signals within radar-reflectivity restricted sublayers of the clouds are analyzed. The neutrally stratified layers of cirrus clouds are known to be associated with the existence of generating cells. The statistics of radiative properties within the neutrally stratified layers is obtained to be non-Gaussian and time-dependent. The purpose of this research is to derive a model of the cloud-generating cells layer in cirrus based on the statistics of observations. The Fokker–Planck equation approach provides suitable framework to treat non-Gaussian, time-dependent probability density functions (pdfs) such as those found for the η(t) signals. It is shown that the deviations from Gaussianity of radiative properties of the neutrally stratified generating cells layer in cirrus can be modeled by linear stochastically perturbed dynamics with multiplicative noise statistics. Because the multiplicative noise is often identified with state-dependent variations of stochastic feedbacks from unresolved system components it is expected that derived stochastic model will be useful for parameterization of cirrus in global circulation models (GCMs).     

Chemical ozone loss in the Arctic vortex in the winter 1995–96: HALOE measurements in conjunction with other observations

Severe chemical ozone loss has been detected in the Arctic in the winter and spring of 1995–96 by a variety of methods. Extreme reductions in column ozone due to halogen catalysed chemistry were derived from measurements of the Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite in the Arctic vortex. Here, we discuss further aspects of the HALOE observations in the Arctic over this period. Potential problems, both in the data themselves and in the methodology of the data analysis are considered and the reason for the differences between the Arctic ozone losses deduced from HALOE data version 17 and 18 is analysed. Moreover, it is shown that HALOE measurements in the Arctic in winter and spring 1995–96 compare well with observations by other ground-based and satellite instruments.     

Spectral Aerosol Transmittance in the Ultraviolet and Visible Spectra in Athens,Greece

This work investigates the spectral atmospheric transmittance due to aerosols in the urban environment of Athens during a period of one year. The spectral transmittance due to aerosols is derived using measurements of spectral direct-beam solar irradiance in the 310–575 nm spectral band. This derivation is accomplished by using a radiative transfer model for estimating the partial spectral atmospheric transmittance functions due to Rayleigh scattering, and absorption by ozone, nitrogen dioxide and water vapor. The seasonal and diurnal variation of the aerosol transmittance is investigated and the results are discussed with a view to air pollution sources, meteorological factors and topographic characteristics of the Athens basin.     

On the regional climatic impact of contrails: microphysical and radiative properties of contrails and natural cirrus clouds

The impact of contrail-induced cirrus clouds on regional climate is estimated for mean atmospheric conditions of southern Germany in the months of July and October. This is done by use of a regionalized one-dimensional radiative convective model (RCM). The influence of an increased ice cloud cover is studied by comparing RCM results representing climatological values with a modified case. In order to study the sensitivity of this effect on the radiative characteristics of the ice cloud, two types of additional ice clouds were modelled: cirrus and contrails, the latter cloud type containing a higher number of smaller and less of the larger cloud particles. Ice cloud parameters are calculated on the basis of a particle size distribution which covers the range from 2 to 2000 m, taking into consideration recent measurements which show a remarkable amount of particles smaller than 20 m. It turns out that a 10% increase in ice cloud cover leads to a surface temperature increase in the order of 1K, ranging from 1.1 to 1.2K in July and from 0.8 to 0.9K in October depending on the radiative characteristics of the air-traffic-induced ice clouds. Modelling the current contrail cloud cover which is near 0.5% over Europe yields a surface temperature increase in the order of 0.05 K.     

短波红外三维亚像元低层水云散射效应对二氧化碳反演的影响分析

由于大气短波红外辐射传输散射的复杂性,目前二氧化碳物理反演算法通常假定为晴空平面平行大气.然而通常的云检测算法难以识别视场内亚像元低层水云存在的场景,直接影响了二氧化碳遥感产品的精度.本文基于三维矢量辐射传输计算方法模拟了三维亚像元水云在短波红外1.6 μm波段的散射效应,并估算了忽略此散射效应可能引入的二氧化碳反演偏差.结果表明：1.6 μm波段云区表现为强反射特征,阴影区表现为减弱特性.忽略三维云散射后,云区视场辐射模拟结果偏低,且相对偏差量随地表反射率的减小、太阳天顶角的升高和云层光学厚度的增大而增大;而阴影区辐射模拟结果将偏高,相对偏差量随太阳天顶角升高、云层光学厚度的增大而增大;在云区和阴影区吸收通道都表现了相对更大的偏差.估算结果显示在低反射率、高太阳天顶角和高云层光学厚度场景中, 晴空平面平行假设可能引入高达10~15 ppmv反演误差,远高于观测需求.如何剔除三维云散射影响的观测,或者在反演算法中修正三维云散射效应对于提高二氧化碳反演精度将非常重要.

     

Reducing the biases in shortwave cloud radiative forcing in tropical and subtropical regions from the perspective of boundary layer processes

Biases in shortwave cloud radiative forcing (SWCF), which cause overestimates in tropical regions and underestimates in subtropical marine stratocumulus regions, are common in many climate models. Here, two boundary layer processes are investigated in the atmospheric model GAMIL2, entrainment at the top of the boundary layer and longwave radiative cooling at the top of stratocumulus clouds, in order to reduce biases and reveal the mechanisms underlying these processes. Our results show that including the entrainment process in the model can reduce negative SWCF biases in most tropical regions but increases positive SWCF biases in subtropical marine stratocumulus regions. This occurs because entrainment reduces the low-level cloud fraction and its cloud liquid water content by suppressing the vertical turbulent diffusion in the boundary layer and decreasing the relative humidity when warm and dry free atmosphere is entrained in the boundary layer. Longwave radiative cooling at the top of stratocumulus clouds can enhance turbulent diffusion within the stratocumulus-topped boundary layer. When combined with the entrainment process, longwave radiative cooling reduces the positive SWCF biases in subtropical marine stratocumulus regions that are observed using the entrainment process alone. The incorporation of these two boundary layer processes improves the simulated SWCF in tropical and subtropical regions in GAMIL2.     

The establishment of the national gravity standard base net of Egypt (N.G.S.B.N.-77)

The earliest gravity measurements in Egypt were carried out by the Anglo-Egyptian Oil Company using Holweck-Lejay pendulum in the period 1937–1940.The measurement comprised few stations (about 41) in restricted areas along both the Western and Eastern coasts of the Gulf of Suez.During the period 1950–1951 Prof. G. Woolard made 21 gravimetric measurements in Egypt as a part of the world-wide gravity base net. Thi was refeered to Potsdam absolute value which was considered as the World Gravity Datum.In 1974, a new world-wide gravity net was published (I.G.S.N.71) which introduced a new concept for the world gravity reference datum, and consequently has replaced the Potsdam System. Since then, it was recommended that a national gravimetric network of base stations in Egypt on the basis of the I.G.S.N.-71 be established within the frame of establishing a gravity map of Egypt.In the period from January 1975, The General Petroleum Company carried out 624 gravity observations throughout the country. These measurements have been combined with the I.G.S.N.-71 values to form 65 gravity base points which in turn from the N.G.S.B.N. (Fig. 1).     

Contributions of H2S to the atmospheric sulfur cycle

H₂S is a most important biogenic sulfur compound with regard to the atmospheric sulfur cycle. Our present knowledge of the spatial and temporal distribution of this trace gas is rather incomplete owing to unreliable analytical methods. Therefore, a new method for the analysis of H₂S in the g-range was applied. This paper deals with the results of ground- and aircraft measurements of H₂S in unpolluted air over swamps and tidal flats. Based on the measured vertical distributions a removal coefficient of 2.3×10^–5 sec^–1 and an average lifetime of 12 hours were calculated. Some conclusions of the contribution of H₂S to the atmospheric sulfur budget are added.     

云顶以下臭氧对TOMS反演臭氧总量的影响及反演方法的理论研究

目前紫外后向散射反演臭氧总量的所有算法,都将云考虑成不透明的Lambertian反射体,并假定云顶有效反照率不随波长的变化而变化.然而本文通过模拟计算发现,由于云散射、瑞利散射、臭氧吸收三种作用的综合结果,云顶的有效反照率是与波长相关的,即使光学厚度比较大的云,辐射也可由云顶继续向下传输,因而会受到云顶以下臭氧吸收的影响.用V7方法进行反演,模拟计算结果表明：云的出现使得云顶以下,特别是云内光程增强,导致紫外波段的臭氧吸收衰减增大,所以反演出的臭氧总量值比真实值偏大,本文称这种现象为“云吸收效应”,并讨论了该效应的影响因子.最后,在辐射传输模拟的基础上建立一套反演算法,大大减弱了“云吸收效应”的影响.     

Anatomy of 1986 Augustine volcano eruptions as recorded by multispectral image processing of digital AVHRR weather satellite data

Eighteen digital AVHRR (advanced very high resolution radiometer) data sets from NOAA-6 and NOAA-9 polar-orbiting satellites recorded between 27 March and 7 April 1986 depict the eruptive activity of Augustine volcano, located 280 km SW of Anchorage, Alaska. The synoptic view (resolution of either 1.1 or 4.4 km), frequent coverage (often twice a day), and multispectral coverage (five bands: 0.58–0.68; 0.72–1.1; 3.55–3.93; 10.5–11.3; and 11.5–12.5 m) makes the AVHRR broadly applicable to analyzing explosive eruption clouds. The small scale of the Augustine activity (column heights of 2–13 km and eruption rates of 2x10⁶–8x10⁷ metric tonnes/day) facilitated intensive multispectral study because the plumes generally covered areas within the 550x550 km area of one easily manipulated image field. Hourly ground weather data and twice-daily radiosonde measurements from stations surrounding the volcano plus numerous volcanological observations were made throughout the eruption, providing important ground truth with which to calibrate the satellite data. The total erupted volume is estimated to be at least 0.102 km³. The pattern of changing eruption rates determined by satellite observations generally correlate with more detailed estimates of explosion magnitudes. Multispectral processing techniques were used to distinguish eruption clouds from meteorological clouds. Variable weather during the Augustine eruption offered an opportunity to test various trial algorithms. A ratio between thermal IR channels four and five, served to delineate the ashbearing eruption plumes from ordinary clouds. Future work is needed to determine whether the successful multispectral discrimination is caused by wavelength-dependent variable emission of silicate ash or reflects a spectral role of sulfuric acid aerosol in the plume.