摘要:

Convective clouds and thunderstorms inject planetary boundary layer air with high concentrations of ozone (O3) and O3precursors into the free troposphere. By altering the chemistry and radiative balance of the upper troposphere, these local actions can have global consequences. In order to extrapolate from individual observations to general trends we have devised a method to identify weather patterns conducive to convection in the southern or lower Great Plains in early summer and applied this method to meteorological and chemical data from a series of research flights carried out in June 1985. Previous studies have noted that weather patterns in the lower Great Plains in this season are characterized by alternating pulses of polar and maritime air masses and very frequent episodes of violent convection, usually in maritime conditions in advance of a surface cold front or along the frontal zone itself. In this study, a set of selection criteria is applied to surface and upper air meteorological data from central Oklahoma to distinguish the two meridional phases characteristic of this region: maritime and polar. A deep layer of moist, southerly flow and convective instability is encountered in the maritime regime, while the polar phase is convectively stable throughout the midtroposphere and much less conducive to convection. For the period 1980–1985 both maritime and polar regimes occur with a frequency of about 35% in May, while the maritime phase becomes dominant in June (53% maritime versus 20% polar). Within the maritime regime, conditions conducive to the development of severe storms are characterized by the presence of a dry, low‐level inversion that tends to inhibit scattered midday convection over a wide region while simultaneously enhancing the probability of larger thunderstorms and mesoscale convective systems. This “modified maritime” category accounts for approximately 25% of the total cases in May and in June. The instances of frontal passage account for approximately 15% of the total cases. Profiles, from the surface to 200 mbar, made on 18 flights of the NCAR Sabreliner in the 1985 PRESTORM project, are categorized according to the selection criteria. Part 2 of this study presents representative concentrations of ozone, carbon monoxide, and reactive nitrogen compounds as a function of altitude (0–12 km) for each category and discusses the implications of these

ISSN:0148-0227    

DOI:10.1029/92JD01375

年代:1992

数据来源: WILEY

摘要:

It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a pristine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3precursor concentrations; and (3) initial amount of background free tropospheric NOx. Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30% more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.

ISSN:0148-0227    

DOI:10.1029/92JD01716

年代:1992

数据来源: WILEY

摘要:

Hydrogen peroxide, formaldehyde, and a measure of organic hydroperoxides are presented from the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). MLOPEX was conducted from May 1 to June 4, 1988, at the Mauna Loa Observatory on the island of Hawaii. A modified dual‐enzyme serial‐coil H2O2/ROOH method was used to quantify these species. A second enzyme method was used to measure CH2O. The location and meteorology at the Mauna Loa Observatory site permitted 35 days of measurements to be made in free tropospheric air and in modified marine boundary layer air. Average concentrations of H2O2, ROOH, and CH2O were 1050, 140, and 100 pptv in free tropospheric, or down‐slope air. In upslope air, or modified marine boundary layer air, average concentrations were 900, 150, and 190 pptv. Maximum concentrations for all three species were experienced during a two day “photochemical haze” episode and were 3230, 440, and 450 pptv for H2O2, ROOH, and CH2O. H2O2was depleted in air which had recently been processed by cloud or precipitation. The measured concentrations of these three species were comparable to prior measurements in “well‐aged” air, but were lower than previous models have predicted. Part of this discrepancy may be due to the treatment of heterogeneous removal processes, dry and wet deposition, in these models. The measured ratio of ROOH to H2O2is significantly different than present theory predicts, with ROOH as measured being approximately a fact

ISSN:0148-0227    

DOI:10.1029/92JD00268

年代:1992

数据来源: WILEY

摘要:

A two‐dimensional chemical/transport model of the middle atmosphere is used to assess the importance of chemical heterogeneous processes both in the polar regions (on polar stratospheric clouds (PSCs)) and at other latitudes (on sulfate aerosols). When conversion on type I and type II PSCs of N2O5into HNO3and of ClONO2into reactive forms of chlorine is taken into account, enhanced ClO concentrations lead to the formation of a springtime “ozone hole” over the Antarctic continent. No such major reduction in the ozone column is found in the Arctic region. When conversion of nitrogen and chlorine compounds is assumed to occur on sulfate particles present in the lower stratosphere at all latitudes, significant perturbations in the chemistry are also found. For background aerosol conditions, the concentration of nitric acid is enhanced and agrees with observed values, while that of nitrogen oxides is reduced and agrees less than if heterogeneous processes are ignored in the model calculations. The concentration of the OH radical is significantly increased. Ozone number density appears to become larger between 16 and 30 km but smaller below 16 km, especially at high latitudes. The ozone column is only slightly modified, except at high latitudes where it is substantially reduced if the ClONO2conversion into reactive chlorine is taken into account. After a large volcanic eruption such as that of Mount Pinatubo in June 1991, these changes are further exacerbated. The ozone budget in the lower stratosphere becomes less affected by nitrogen oxides but is largely controlled by the ClOxand HOxchemistries. A substantial decrease in the ozone column is predicted as a result of the Pinatubo eruption, mostly in winter at midlatitudes and high latitudes. The predicted values depend on the assumption made for the evolution of the aerosol surface area density but is expected to be of the order of 10% at mid‐latitudes in February and March 1992. An enhanced mixing ratio of NO2of the order of 100 parts per trillion by volume and a reduced mixing ratio of NO2below 25 km should be detected outside the polar vortex, especially in air masses with high levels of volcanic a

ISSN:0148-0227    

DOI:10.1029/92JD02021

年代:1992

数据来源: WILEY

摘要:

We retrieve mixing ratio profiles of O3, H216O, H217O, H218O, HF, and HCl from far‐infrared thermal emission observations of the limb in the 80–220 cm−1spectral region. The observations were made with a balloon‐borne Fourier transform spectrometer as a part of the 1983 Balloon Intercomparison Campaign (BIC‐2). A subset of the data was analyzed previously using the method in the work ofTraub et al. [1982, 1991]; in the present paper we use an alternative method of calibration and analysis, given byAbbas et al. [1985]. The retrieved constituent profiles are compared with the measurements made with other instruments on the BIC‐2 flights. The results for the concentrations of H217O and H218O obtained in this study indicate no isotopic enhancement or depletion with a standard deviation o

ISSN:0148-0227    

DOI:10.1029/92JD02083

年代:1992

数据来源: WILEY

摘要:

A zonally averaged transport and chemistry model for atmospheric trace species is developed, based on an existing statistical‐dynamical climate model. The modeled region is a single hemisphere from 1000 mbar to 240 mbar. The model is applied to a study of the budget of tropospheric ozone, in particular to an estimation of the relative importance of the stratospheric and in situ photochemical sources. A parameterization of cross‐tropopause exchange is developed based on the concept of tropopause folding. It is used to provide transfers of air and ozone across the upper boundary. A basic tropospheric chemistry scheme is also incorporated, featuring methane oxidation and subsequent reactions, carbon monoxide oxidation and oxygen‐hydrogen reactions. Nitrogen oxide reactions are reduced to a minimum by specification of NOxdistribution in space and time. Nonmethane hydrocarbon reactions are omitted. Transport and surface deposition of relevant species are performed.The model produces ozone distributions which are in general agreement with observations and other modeling studies. These are analyzed by considering, χχ, the mixing ratio of ozone of stratospheric origin with respect to the “complete” ozone mixing ratio. The sources and sinks of χχare parameterized in terms of the relevant processes, and it can be treated as a quasi‐conservative variable with respect to transports. Zonally averaged fields of this mixing ratio are produced and show the influence of ozone of stratospheric origin to be very small (5 % at most) close to the surface, in this zonally averaged analysis. At 300 mbar, in middle to high latitudes, the influence is more significant (up to about 50 % in high‐latitude winter). These results are in accord with the very limited obser

ISSN:0148-0227    

DOI:10.1029/92JD01834

年代:1992

数据来源: WILEY

摘要:

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear‐sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid‐latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid‐latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than

ISSN:0148-0227    

DOI:10.1029/92JD01631

年代:1992

数据来源: WILEY

摘要:

We describe a new line‐by‐line (LBL) algorithm for radiative excitation in infrared bands in a non local thermodynamic equilibrium (non‐LTE) planetary atmosphere. As a specific application, we present a predictive model for the terrestrial CO215‐μm emission that incorporates this generic algorithm, and validate the model by comparing its results with emission spectra obtained in a limb‐scanning rocket experiment. The radiative‐excitation algorithm has certain unique features. Being a completely monochromatic calculation, it includes the detailed layer‐by‐layer variation of the shapes of the individual lines in its evaluation of atmospheric transmittivity; and, being an iterative algorithm, it avoids the need to construct and invert large matrices, so that a fine layering scheme can be implemented. It also includes a simple correction procedure to minimize the most serious error due to having discrete layers. These features contribute to accurate radiative transfer results and reliable atmospheric cooling rates. For altitudes above 40 km, we present results of model calculations of CO2(v2) vibrational temperatures, 15‐μm limb spectral radiances, and cooling rates, for the main band as well as for weaker hot and isotopic bands. We calculate the excitation and deexcitation rates due to different processes, including the radiative pumping due to individual atmospheric layers. We compare the predicted limb radiance with earthlimb spectral scans obtained in the SPIRE rocket experiment over Poker Flat, Alaska, and get excellent agreement as a function of both wavelength and tangent height. This constitutes the first validation of a long‐wavelength CO2non‐LTE emission model using an actual atmospheric data set, and it verifies the existence of certain aeronomic features that have only been predicted by models. It also constrains the previously unknown value of the very important rate constant for deactivation of the CO2bending mode by atomic oxygen to the range of 5–6 × 10−12cm3/(mol s) at mesospheric and lower thermospheric temperatures. We discuss the significance of this large value for the terrestrial and Venusian thermospheres. We also discuss the convergence rate of the iterative scheme, the model's sensitivity to the background atmosphere, the importance of the lower boundary surface contribution, and the effects of the choice of the layer thickness and

ISSN:0148-0227    

DOI:10.1029/92JD01494

年代:1992

数据来源: WILEY

摘要:

Extended (180‐day) high resolution (T106) perpetual January and July integrations of the European Centre for Medium‐Range Weather Forecasts (ECMWF) model have been analyzed in terms of the spatial and temporal characteristics of the model's convective activity in the tropics. The model's outgoing longwave radiation (OLR) is used as a surrogate for convective activity, consistent with similar studies based on satellite observations. The 3 hourly temporal sampling is sufficient to allow diagnosis of intradiurnal and interdiurnal variability; the length of the integrations is adequate for identifying lower‐frequency, intraseasonal phenomena. Wherever possible, use is made of results from surface or satellite observations of the temporal characteristics of convection to verify the model results. At intradiurnal time scales the model captures the amplitude and phase of the diurnal harmonic over both land and sea. The largest amplitudes occur over the summer continents, with contrasting phases of maximum OLR depending on the presence of convective activity. Over the oceans the model shows a coherent structure to the diurnal cycle associated with regions of convection. Analysis of synoptic (2 to 10 days) and low‐frequency (greater than 10 days) variability shows that in many instances the model agrees well with observations. For both seasons the model simulates westward moving phenomena over the oceans, whose phase speed is reasonable. In July these easterly waves display well‐defined periodicities, in agreement with observations, while in January they are more episodic. Low‐frequency variability is more prevalent in January, particularly over the convectively active regions of the eastern hemisphere. In general, this variability has a larger spatial scale than the synoptic variability; its periodicities, some in excess of 30 days, are typical of intraseasonal

ISSN:0148-0227    

DOI:10.1029/92JD01408

年代:1992

数据来源: WILEY

摘要:

Satellite measurements of the effect of clouds on the top of atmosphere radiative energy budget are used to validate model simulations from the National Center for Atmospheric Research Community Climate Model (NCAR CCM). The ability of the NCAR CCM to reproduce the monthly mean global distribution and temporal variability on both daily and seasonal time scales is assessed. The comparison reveals several deficiencies in the CCM cloud representation. Most notable are the difficulties in properly simulating the effect of clouds on the planetary albedo. This problem arises from discrepancies in the model's portrayal of low‐level cloudiness and leads to significant errors in the absorbed solar radiation simulated by the model. The CCM performs much better in simulating the effect of clouds on the longwave radiation emitted to space, indicating its relative success in capturing the vertical distribution of cloudiness. The daily variability of the radiative effects of clouds in both the shortwave and longwave spectral regions is systematically overestimated. Analysis of the seasonal variations illustrates a distinct lack of coupling in the seasonal changes in the radiative effects of cloudiness between the tropics and mid‐latitudes and between the northern and southern hemisphere. Much of this problem also arises from difficulties in simulating low‐level cloudiness, placing further emphasis on the need for better model parameterizations of boundary layer c

ISSN:0148-0227    

DOI:10.1029/92JD01495

年代:1992

数据来源: WILEY