Modelling radiation quantities and photolysis frequencies in the troposphere

STAR (System for Transfer of Atmospheric Radiation) was developed to calculate accurately and efficiently the irradiance, the actinic flux, and the radiance in the troposphere. Additionally a very efficient calculation scheme to computer photolysis frequencies for 21 different gases was evolved. STAR includes representative data bases for atmospheric constituents, especially aerosol particles. With this model package a sensitivity study of the influence of different parameter on photolysis frequencies in particular of O₃ to Singlet D oxygen atoms, of NO₂, and of HCHO was performed. The results show the quantitative effects of the influence of the solar zenith angle, the ozone concentration and vertical profile, the aerosol particles, the surface albedo, the temperature, the pressure, the concentration of NO₂, and different types of clouds on the photolysis frequencies.Notation I _A(, ) actinic flux - I _H(, ) irradiance - L(, , , ) radiance - wavelength - azimuth angle - cosine of zenith angle - _s cosine of solar zenith angle - optical depth - _s scattering coefficient - _c extinction coefficient - _o single scattering albedo - p _mix mixed phase function - g _mix mixed asymmetry factor - J _gas photolysis frequency     

Scavenging Efficiency of ‘Aerosol Carbon’ and Sulfate in Supercooled Clouds at Mt. Sonnblick (3106 m a.s.l., Austria)

Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and aerosol carbon to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (_SO) ranged from 0.52 to 0.99 with an average of 0.80. Aerosol carbon was scavenged less efficiently with an average value (_AC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both _SO and _AC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, _SO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of 0.3 g/m³. In the case of aerosol carbon, we obtained a more gradual increase of _AC up to an LWC of 0.5 g/m³. At higher LWCs, _{_} remained relatively constant at 0.60. As the differences between _SO and _A varied across the LWC range observed at SBO, we assume that part of the aerosol carbon was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.     

Measurements of the spectral aerosol optical depth using a sun photometer

Summary A sun photometer fitted with 9 narrowband interference filters from 368 nm to 1024 nm was used to determine the aerosol optical depth at 8 measuring stations in Europe. Of the 9 filters, 4 were not included in the standard equipment of the sun photometer. Given the temporal instabilities of the calibration factors of these 4 filters, meaningful results for the aerosol optical depth can be achieved only if calibration and measurements follow each other closely. Calibration was performed using the Langley plot method. For wavelengths < 500 nm, the measured aerosol optical depths were compared with the results of a high resolution spectrometer. Broadband aerosol optical depths, centred at 427 nm, were determined with an actinometer and were also used for comparison. Generally, there was good agreement of the results. To characterize the aerosol optical depths, the Ångström parameters and were used. The individual measuring stations showed a clustered distribution of and values.With 6 Figures     

Measurements of solar radiation and estimation of optical depth in the high Andes of Peru

Summary During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths () and irradiances at the top of the atmosphere. In addition, water vapor optical depths (_wv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (_ray) and ozone (_oz) optical depths were obtained from published tabulations. Subtracting _ray, _oz, and _wv from yielded estimates of aerosol optical depth _aer. The components _ray and _oz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; _aer also decreases towards the higher elevations. Particularly pronounced is the decrease of _aer and from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.With 4 Figures     

Environmental aerosols and their effect on the Earth’s local fair-weather electric field

Summary The Earths local fair-weather electric field is significantly affected by small ions present in the atmosphere. These ions are typically smaller than 0.001µm and occur in concentrations from 500 to 600cm^–3 in air. Attachment to larger aerosol particles may severely decrease the mobility of these atmospheric ions resulting in an increased local electric field. The number concentration of environmental aerosol particles in the size range 0.1 to 5.0µm was measured with two automatic laser scattering particle counters. The Earths electric field was monitored with an electric fieldmeter. Measurements were made in clean air and in an environment highly polluted by wood smoke. The electric field was found to be positively correlated to the aerosol number concentration. During one 24-hour period of measurement, the electric field increased from 180 to about 280Vm^–1 as the number concentration of aerosols larger than 0.1µm increased from about 2000 to 9000cm^–3. The number concentrations of aerosols larger than 0.1 and 0.3µm were both found to be positively correlated with the Earths electric field with correlation coefficients of 70% and 61%, respectively.Present address: School of Physical Sciences, Queensland University of Technology, Brisbane 4001, Australia.     

Effects of tropospheric aerosols on the solar radiative heating in a clear atmosphere

Summary The broadband solar absorptivity concept is employed to parameterize the aerosol absorption effect. The solar radiation model developed by Liou and his associates was modified to incorporate the parameterization of solar radiative transfer in an aerosol layer. Comparison of the results from this method with other schemes exhibits close agreement. A Sahara dust storm case was also chosen to test the performance of the present model, and the computed heating rate profiles agree well with calculations based on optical properties derived from observations for both clear and dust cases. In general, enhanced heating due to aerosol absorption of solar flux occurs particularly in the lower troposphere (below 5 km). The heating rate is independent of the scattering partition factor (), but the planetary albedo increases with . Further study shows that the aerosol heating is sensitive to the surface albedo (r _s ) and to the cosine of the solar zenith angle (µ ₀). The decrease inr _s and/or increase inµ ₀ lower the solar heating rate, the planetary albedo and the atmospheric absorptivity, but raise the surface absorptivity due to reduced multiple reflection between the atmosphere and surface.With 9 Figures     

Elemental Composition of Mineral Aerosol Generated from Sudan Sahara Sand

Eighteen soil samples from central Sudan were fractionated by dry sieving ina size fraction from <45 m to >300 m while aerosols generatedfrom these soils were fractionated in the particle size range from 0.25 mto >16 m. The elemental concentrations of soil samples were determinedby energy-dispersive X-ray fluorescence, while the elemental concentrationsof generated aerosols were analysed by particle-induced X-ray emission. Theelements Al, K and Rb show a slight positive fractionation with decreasingparticle size throughout the particle size range studied. The concentrationsof Ca, Mn, Fe, Sr and Y are maximum in the small soil size fraction (<45m) and decrease for the coarse soil size fractions, while in the mineralaerosol particle sizes (0.25– > 16 m) the concentrations remainmore or less constant. The size distributions for Cr, Ti and Zr show a maximumin the particle size range 45–100 m and the concentrations of theseelements decrease sharply in the aerosol fraction down to 16 m to remainconstant in the smaller aerosol fractions.Enrichment factors for the elements were calculated relative to five referencematerials: average crustal rock, average soil, the investigated Sahara bulksoil, the finest fraction of this soil and the aerosol generated from thissoil, and using four reference elements: Al, Si, Ti and Fe. The enrichmentfactors were found to vary significantly depending on the choice of thereference material or the reference element. The enrichment factors for theSudan mineral aerosol were almost identical to those for Khartoum atmosphericaerosol but different from those for Namib mineral aerosol and Israelatmospheric aerosol following dust storms. Multivariate display methods(cluster analysis, principal component analysis and linear discriminantanalysis) were applied to the element ratios in the mineral aerosol from theSahara and Namib and this showed that these mineral aerosol can bedifferentiated into different groups. An attempt was also made to relate themineral aerosol to its parent soil through the use of these multivariatetechniques and the elemental ratios in both the mineral aerosols and the bulksoils (Namib and Sahara). It was also possible using the elemental ratios andthe multivariate display methods to associate the crustal component to themineral aerosol generated from the Sahara.     

AN E – ε – ℓ TURBULENCE CLOSURE SCHEME FOR PLANETARY BOUNDARY-LAYER MODELS: THE NEUTRALLY STRATIFIED CASE

The standard E – model generates aplanetary boundary layerthat appears to be much too deep. The cause of theproblem is traced to the equation for the dissipationrate () of turbulent kinetic energy (E), specifically theparameterization of dissipation production anddestruction. In the context of atmosphericboundary-layer modelling, we argue that a part of thedissipation production should be modelled as the inputto the spectral cascade from the energy-containingpart of the spectrum, with a characteristic length , while the equilibrium imbalancebetween local production and destruction ofdissipation is modelled as proportional toE2/E, as in the standard model. Wepropose an E – – turbulence closurescheme, in which both the mixing length, m, and are prescribed. The importance ofthe equation is diminished, though itstill determines the dissipation rate in the Eequation.     

Stable Isotope Ratios: Hurricane Olivia

The oxygen and hydrogen isotopic compositions of rains from HurricaneOlivia (1994) in the eastern Pacific were measured. The rains werecollected on 24 and 25 September during airplane flights conducted at anelevation of 3 km. Hurricane Olivia peaked in intensity to a category-4storm between the two dates. Isotope ratios of rains from HurricaneOlivia were markedly lower ( ¹⁸O = –13.9to –28.8) than that of rain collected from a thunderstormat an elevation of 2.3 km outside the influence of Olivia (¹⁸O = –3.8). A distinct decrease in isotoperatios from the first day to the next ( ¹⁸O =–18.4 to –21.9) in Hurricane Olivia wasattributed to decreased updraft velocities and outflow aloft. Thisshifted the isotopic water mass balance so that fewer hydrometeors werelifted and more ice descended to flight level. A decrease in the averagedeuterium excess from the first day to the next (d = 15.5 to 7.1)was attributed to an increase in the relative humidity of the watervapor `source' area. We hypothesize that the `source' region for therain was in the boundary layer near the storm center and that becausethe hurricane was at peak intensity prior to the second day the relative humidity was higher.     

Red dust rain within the Spanish Mediterranean area

Dust rain belongs to the climatological conditions of the Iberian Peninsula's Mediterranean seaboard. Traditionally known as muddy' or bloody rains, red dust rain has become a topical issue as a result of more frequent rainfall of this kind in recent years. In spite of the difficulties that studying this phenomenon involves, owing to the lack of systematically kept records at meteorological observatories, details are provided of the high frequency of such phenomena within the Spanish Mediterranean area, using records from other phenological sources. The study analyzes the chemical composition of red dust rain and the atmospheric factors giving rise to such phenomena. Whether there is a relationship betwen the higher frequency of dust rain and possible changes in global atmospheric circulation is as yet unclear.     

A Green Planet Versus a Desert World: Estimating the Maximum Effect of Vegetation on the Land Surface Climate

We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (desert world), values representative of a desert are used for the land surface parameters for all non glaciated land regions. At the other extreme, a second simulation is performed (green planet) in which values are used which are most beneficial for the biosphere's productivity. Land surface evapotranspiration more than triples in the presence of the green planet, land precipitation doubles (as a second order effect) and near surface temperatures are lower by as much as 8 K in the seasonal mean resulting from the increase in latent heat flux. The differences can be understood in terms of more absorbed radiation at the surface and increased recycling of water. Most of the increase in net surface radiation originates from less thermal radiative loss and not from increases in solar radiation which would be expected from the albedo change. To illustrate the differences in climatic character and what it would imply for the vegetation type, we use the Köppen climate classification. Both cases lead to similar classifications in the extra tropics and South America indicating that the character of the climate is not substantially altered in these regions. Fundamental changes occur over Africa, South Asia and Australia, where large regions are classified as arid (grassland/desert) climate in the desert world simulation while classified as a forest climate in the green planet simulation as a result of the strong influence of maximum vegetation on the climate. This implies that these regions are especially sensitive to biosphere-atmosphere interaction.     

The recent characteristics of Asian dust and haze events in Seoul, Korea

Summary The study on the characteristics of aerosol in Seoul during springtime from 1998 to 2003 is performed by the size-resolved number concentrations of aerosol. Asian dust events occur in spring most frequently, but it has been often observed in wintertime since 1999. Since 2000, the number of Asian dust days has been increasing, and the intensity has been more severe until 2002. However, there were only 3 dust days in Seoul during the spring of 2003, since the synoptic cyclone was relatively not intense enough to rise and transport dust to Korean peninsula, and the air stream was usually tiled to north of Korean peninsula. In addition, the precipitation was relatively plentiful and the air temperature was cold enough not to keep dry soil condition.Haze is the suspended particles in the air, reducing visibility by scattering light, and it is often a mixture of aerosols and photochemical smog. Dry particles with diameters of the order of 0.1µm, are small enough to scatter short wavelengths of light. Haze occurs well in winter and spring, and severe haze is observed in the afternoon. The occurrence frequency of haze has been decreasing since 2000 except in May of 2003.During Asian dust events from 1998 to 2003, the number concentration of aerosol with diameters from 0.3µm to 0.5µm decreases notably, but that larger than 1µm increases rapidly. On the other hand, for the haze events the number concentration from 0.3µm to 0.5µm increases notably, but that larger than 1µm decreases.     

Climate modelling for the nesting scientist

This paper was originally submitted as a background essay to an early meeting concerned with the development of the GARP Climate Dynamics Sub-Programme. It was an attempt to put the scientific problems into a context which, for the benefit of administrators, leads to a fairly natural structuring of relevant research activity. In its specific references to GARP, the essay has been somewhat overtaken by events - but the concepts are still relevant at this time when it remains unclear as to how the climate problem is to be solved.

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Observed variations of σθ and σΦ in the nocturnal drainage flow in a deep valley

The variations of and in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average and in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.In the drainage flow, the hourly average varies inversely with wind speed according to the relation u 0.7ms^-1. The vertical standard deviation is much less enhanced by complex terrain than the horizontal standard deviation. The observed values are predicted fairly well by the local similarity theory.Oak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 andOak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 and     

Interpretation of Mauna Loa atmospheric transmission relative to aerosols,using photometric precipitable water amouts

Precipitable water measurements made coincident in time and space with direct broadband solar irradiance measurements are used in conjunction with an atmospheric transmission model to derive a parameter whose major dependence is on total aerosol extinction. Irradiance measurements are used to calculate an atmospheric transmission factor (ATF) that is independent of the instrument calibration and the extraterrestrial solar constant. The dependency of the ATF on precipitable water is determined using LOWTRAN5, an atmospheric transmission model with high spectral resolution. Precipitable water measurements are then used to adjust the measured ATF to correspond to an ATF value obtained for a constant precipitable water amount. The remaining variability in the adjusted ATF is due mostly to aerosol extinction. The technique is applied to a 6-year period (1978–1983) for clear-sky mornings at Mauna Loa, Hawaii (MLO). MLO ATF aerosol residuals are compared with independently measured monochromatic aerosol optical depth. Results show that the ATF aerosol residual is nearly equal to the 500 nm aerosol optical depth prior to the eruption of E1 Chichon, at which time a nonlinear time-dependent relationship between the two quantities is evident. ATF aerosol residuals reflect the spectrally integrated aerosol influence on transmission and, therefore, could indicate better than monochromatic optical depth the radiation balance perturbations due to aerosols. The 6-year precipitable water record for MLO, determined from a dual-channel sunphotometer, has a mean value of 0.3 cm. An annual cycle in precipitable water is evident, as is a 4-month 5-standard-deviation drought from December 1982 through March 1983.     

Northern hemisphere atmospheric response to changes of atlantic ocean SST on decadal time scales: a GCM experiment

Analyses indicate that the Atlantic Ocean seasurface temperature (SST) was considerably colder at the beginning than in the middle of the century. In parallel, a systematic change in the North Atlantic sea-level pressure (SLP) pattern was observed. To find out whether the SST and SLP changes analyzed are consistent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low-resolution (T21) atmospheric GCM was performed. Two perpetual January simulations were conducted, which differ solely in the Atlantic Ocean (40° S-60° N) SST: the cold simulation utilizes the SSTs for the period 1904–1913; the warm simulation uses the SSTs for the period 1951–1960. Also, a control run with the model's standard SST somewhat between the cold and warm SST was made. For the response analysis, a rigorous statistical approach was taken. First, the null hypothesis of identical horizontal distributions was subjected to a multivariate significance test. Second, the level of recurrence was estimated. The multivariate statistical approaches are based on hierarchies of test models. We examined three different hierarchies: a scale-dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the cold and warm experiments indicates a signal in the geostrophic stream function that in the S-hierarchy is significantly nonzero and highly recurrent. In the A-hierarchy, the low level temperature field is identified as being significantly and recurrently affected by the altered SST distribution. The SLP signal is reasonably similar to the SLP change observed. Unexpectedly, the upper level stream-function signal does not appear to be significantly nonzero in the B-hierarchy. If, however, the pairs of experiments warm versus control and cold versus control are examined in the B-hierarchy, a highly significant and recurrent signal emerges. We conclude that the cold versus warm response is not a small disturbance that would allow the signal to be described by eigenmodes of the linear system. An analysis of the three-dimensional structure of the signal leads to the hypothesis that two different mechanisms are acting to modify the model's mean state. At low levels, local heating and advection are dominant, but at upper levels the extratropical signal is a remote responce to modifications of the tropical convection.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil.AWI Publication no. 254     

Relationships of photosynthetically active radiation and shortwave irradiance

Summary Hourly measurements of solar irradiance in the wave band excluding photosynthetically active radiation (PAR) and solar irradiance (SI) were made over a 12-month period at the National Observatory of Athens, for obtaining the ratios of PAR to SI. These irradiance ratios exhibit dependence on sky conditions, with slightly larger values being observed under cloudy skies and seasonal variations, attributable to changes in local air mass climatology. The highest values have been obtained during the growing season (April–September). The mean annual value of 0.473 observed for the irradiance ratio in the PAR band compares favorably with values reported in the literature for different locations over a wide geographical area.With 2 Figures     

Gas-Phase Ozone Oxidation of Monoterpenes: Gaseous and Particulate Products

Atmospheric oxidation of monoterpenes contributes to formation of tropospheric ozone and secondary organic aerosol, but their products are poorly characterized. In this work, we report a series of outdoor smog chamber experiments to investigate both gaseous and particulate products in the ozone oxidation of four monoterpenes: -pinene, -pinene, ³-carene, and sabinene. More than ten oxygenated products are detected and identified in each monoterpene/O₃ reaction by coupling derivatization techniques and GC/MS detection. A denuder/filter pack sampling system is used to separate and simultaneously collect gas and aerosol samples. The identified products, consisting of compounds containing carbonyl, hydroxyl, and carboxyl functional groups, are estimated to account for about 34–50%, 57%, 29–67%, and 24% of the reacted carbon mass for -pinene, sabinene, -pinene, and ³-carene, respectively. The identified individual products account for >83%, 100%, >90%, and 61% of the aerosol mass produced in the ozone reaction of -pinene, sabinene, -pinene, and ³-carene. The uncertainty in the yield data is estimated to be ±50%. Many of the products partition between gas and aerosol phases, and their gas-aerosol partitioning coefficients are determined and reported here. Reaction schemes are suggested to account for the products observed.     

Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons

Measurements of aerosol formation during thephotooxidation of -pinene, -pinene,d-3-carene, d-limonene, ocimene, linalool, terpinene-4-ol, andtrans-caryophyllene were conducted in anoutdoor smog chamber. Daylight experiments in thepresence of and dark experiments withelevated ozone concentrations were performed. Theevolution of the aerosol was simulated by theapplication of a gas/particle absorption model inconnection with a chemical reaction mechanism. Thefractional aerosol yield is shown to be a function ofthe organic aerosol mass concentration andtemperature. Ozone and, for selected hydrocarbons, theNO₃ reaction of the compounds were found torepresent efficient routes to the formation ofcondensable products. For initial hydrocarbon mixingratios of about 100 ppb, the fractional aerosol yieldsfrom daylight runs have been estimated to be 5%for open-chain hydrocarbons, such as ocimene andlinalool, 5–25% for monounsaturated cyclicmonoterpenes, such as -pinene, d-3-carene, orterpinene-4-ol, and 40% for a cyclic monoterpenewith two double bonds like d-limonene. For the onlysesquiterpene investigated, trans-caryophyllene, afractional aerosol yield of close to 100% wasobserved. The majority of the compounds studied showedan even higher aerosol yield during dark experimentsin the presence of ozone.     

A Comparative Analysis of Transpiration and Bare Soil Evaporation

Transpiration E^v and bare soil evaporation E^b processes are comparatively analysed assuming homogeneous and inhomogeneous areal distributions of volumetric soil moisture content . For a homogeneous areal distribution of we use a deterministic model, while for inhomogeneous distributions a statistical-deterministic diagnostic surface energy balance model is applied. The areal variations of are simulated by Monte-Carlo runs assuming normal distributions of .The numerical experiments are performed for loam. In the experiments we used different parameterizations for vegetation and bare soil surface resistances and strong atmospheric forcing. According to the results theE^v()-^Eb() differences are great, especially in dry conditions. In spite of this, the available energy flux curves of vegetation A^v() and bare soil A^b() surfaces differ much less than the E^v() and ^Eb() curves. The results suggest that E^v is much more non-linearly related to environmental conditions than ^Eb. Both E^v and ^Eb depend on the distribution of , the wetness regime and the parameterization used. With the parameterizations, ^Eb showed greater variations than E^v. These results are valid when there are no advective effects or mesoscale circulation patterns and the stratification is unstable.