摘要:

In this paper we describe fundamental properties of an “off‐line” three‐dimensional transport model, that is, a model which uses prescribed rather than predicted winds. The model is currently used primarily for studying problems of the middle atmosphere because we have not (yet) incorporated a formulation for the convective transport of trace species, a prerequisite for many tropospheric problems. The off‐line model is simpler and less expensive than a model which predicts the wind and mass evolution (an “on‐line” model), but it is more complex than the two‐dimensional (2‐D) zonally averaged transport models often used in the study of chemistry and transport in the middle atmosphere. It thus serves as a model of intermediate complexity and can fill a useful niche for the study of transport and chemistry. We compare simulations of four tracers, released in the lower stratosphere, in both the on‐ and off‐line models to document the difference resulting from differences in modeling the same problem with this intermediate model. These differences identify the price to be paid in going to a cheaper and simpler calculation. The off‐line model transports a tracer in three dimensions. For this reason, it requires fewer approximations than 2‐D transport model, which must parameterize the effects of mixing by transient and zonally asymmetric wind features. We compare simulations of the off‐line model with simulations of a 2‐D model for two problems. First, we compare 2‐D and three‐dimensional (3‐D) models by simulating the emission of an NOx‐like tracer by a fleet of high‐speed aircraft. The off‐line model is then used to simulate the transport of14C and to contrast its simulation properties to that of the host of 2‐D models which participated in an identical simulation in a recent NASA model intercomparison. The off‐line model is shown to be somewhat sensitive to the sampling strategy for off‐line winds. Simulations with daily averaged winds are in very good qualitative agreement but are less diffusive than when driven with instantaneous winds sampled at half‐hour intervals. Simulations with the off‐line and 2‐D models are quite similar in the middle and upper stratosphere but behave quite differently in the lower stratosphere, where the 3‐D model has a substantially more vigorous circulation. The off‐line model is quite realistic in its simulation of14C. While there are still systematic differences between the 3‐D calculation and the observations, the differences seem to be substantially reduced when compared with the body of 2‐D simulations documented in

ISSN:0148-0227    

DOI:10.1029/93JD02694

年代:1994

数据来源: WILEY

摘要:

A particle grid air quality modeling approach that can incorporate chemistry is proposed as an alternative to the conventional partial differential equation (PDE) grid air quality modeling approach. In this approach, each particle is tagged with different species masses and particles in the same grid participate in chemical reactions. The approach is flexible and removes the advection and point source problems encountered in the PDE approach. For a typical grid size of 5 km × 5 km × 50 m used in the lowest layer of an urban air quality model, use of 2000–3000 particles of unequal masses per grid cell will yield a highly accurate grid‐averaged instantaneous concentration field that undergoes eddy diffusion for a period of about 1 day. Use of an hourly averaged concentration reduces the demand of particle per cell to about 500. Increasing the grid size also reduces the demand on the number of particles per cell. For the choice of our Lagrangian integral time scales, the time step must be small (10 s) for vertical dispersion simulation but can be large (200 s) for horizontal dispersion simulation. To reduce computation time, a time‐splitting scheme is, proposed to simulate the horizontal and vertical dispersion simulations in an alternating sequence. The present study also shows that the oft‐used second‐order‐accurate finite difference scheme for solving the diffusion equation tends to overpredict the peak of a sharply peaked

ISSN:0148-0227    

DOI:10.1029/93JD02795

年代:1994

数据来源: WILEY

摘要:

The particle grid method is applied to a system of 10 reacting chemical species in a two‐dimensional rotating flow field with and without diffusion. Two types of chemistry grids are used to describe the chemical reactions: a fixed coarse grid and a moving (the advection case) or stationary (the advection plus diffusion case) fine grid. Two particle number densities are also used: 256 and 576 particles per fixed coarse grid cell. The species mass redistributed back to the particle after each reaction step is assumed to be proportional to the species mass in the particle before the reaction. The simulation results are very accurate, especially in the advection chemistry case. Accuracy improves with the use of a fine grid. A higher particle number density also reduces the concentration fluctuation in the cases involving diffusion. We also show by examples that chemistry can lead to significantly different results from numerical methods for the diffusion equation (let alone the advection equation) which otherwise yield almost identical solutions. The absence of this difficulty in the particle grid method further enhances its attractivenes

ISSN:0148-0227    

DOI:10.1029/93JD02796

年代:1994

数据来源: WILEY

摘要:

Trace gas concentrations and atmospheric state parameters were measured aboard the NOAA King Air research aircraft during flights on August 16, 1988, along the Appalachian Mountains from central Pennsylvania to northern Georgia. Stepwise profiles were flown over five surface sites where measurements of certain atmospheric parameters were being made. A stationary cold front lying across southern Virginia effectively divided the area into two weather regimes; to the north of the frontal zone the air was slightly cooler and much drier than that to the south. Considerable convective activity developed from early to midafternoon along and south of the front. The comparison between the aircraft and ground sites included measurements of the primary pollutants SO2and several hydrocarbons, NOy, the secondary pollutants H2O2and O3, and meteorological parameters. Continuity between the aircraft and surface meteorological and trace gas measurements was consistent at the northernmost site, which is situated in a relatively level valley. The agreement was poorer at the other four ground sites, which are located on or near mountaintops. Most of the meteorological and trace gas measurements, other than those made at Scotia, were found to differ by substantial margins, often by more than 10 times the resolution of the instruments. However, within a few hours after the flights, the surface measurements, particularly those of H2O2and O3, achieved values comparable to those measured with the aircraft, thus suggesting that air sampled at the canopy level did not mix readily with the bulk of the boundary layer.

ISSN:0148-0227    

DOI:10.1029/93JD02679

年代:1994

数据来源: WILEY

摘要:

Estimates of stratospheric age from observations of long‐lived trace gases with increasing tropospheric concentrations invoke the implicit assumption that an air parcel has been transported intact from the tropopical tropopause. However, because of rapid and irreversible mixing in the stratosphere, a particular air parcel cannot be identified with one that left the troposphere at some prior time. The parcel contains a mix of air with a range of transit times, and the mean value over this range is the most appropriate definition of age. The measured tracer concentration is also a mean over the parcel, but its value depends both on the transit time distribution and the past history of the tracer in the troposphere. In principle, only if the tropospheric concentration is increasing linearly can the age be directly inferred. We illustrate these points by employing both a one‐dimensional diffusive analog of stratospheric transport, and the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS). Within the limits of the GCM, we estimate the time over which tropospheric tracer concentrations must be approximately linear in order to determine stratospheric age unambiguously; the concentration of an exponentially increasing tracer is a function only of age if the growth time constant is greater than about 7 years, which is true for all the chlorofluorocarbons. More rapid source variations (for example, the annual cycle in CO2) have no such direct relationship with

ISSN:0148-0227    

DOI:10.1029/93JD03192

年代:1994

数据来源: WILEY

摘要:

The fine‐scale structure in lower stratospheric tracer transport during the period of the two Arctic Airborne Stratospheric Expeditions (January and February 1989; December 1991 to March 1992) is investigated using contour advection with surgery calculations. These calculations show that Rossby wave breaking is an ongoing occurrence during these periods and that air is ejected from the polar vortex in the form of long filamentary structures. There is good qualitative agreement between these filaments and measurements of chemical tracers taken aboard the NASA ER‐2 aircraft. The ejected air generally remains filamentary and is stretched and mixed with midlatitude air as it is wrapped around the vortex. This process transfers vortex air into midlatitudes and also produces a narrow region of fine‐scale filaments surrounding the polar vortex. Among other things, this makes it difficult to define a vortex edge. The calculations also show that strong stirring can occur inside as well as outside the v

ISSN:0148-0227    

DOI:10.1029/93JD02556

年代:1994

数据来源: WILEY

摘要:

Investigations of the kinematics of the lower stratospheric Arctic vortex during the winter of 1991–1992 using the contour advection with surgery technique reveal three distinct events in which there was substantial intrusion of midlatitude air into the vortex, in apparent contradiction of the view that the polar vortex constitutes an isolated air mass. Two of these events, in late January and mid‐February, were well documented. They were predicted in high‐resolution forecasts by the European Centre for Medium‐Range Weather Forecasts, most clearly in experimental forecasts with reduced diffusion. Direct confirmation of the presence of the intrusions and of their calculated locations was provided by aerosol observations from the airborne differential absorption laser lidar aboard the NASA DC‐8, taken as part of the second Airborne Arctic Stratospheric Expedition campaign; aerosol‐rich air of midlatitude origin was seen in the expected position of the intrusions. The reality of the February event was also confirmed by in situ measurements from the NASA ER‐2. Such events may be significant for the chemical processes taking place within the winter vortex. The intrusions were evidently related to the meteorology of the northern stratosphereduring this winter and in particular to persistent tropospheric blocking over the northeastern Atlantic Ocean and western Europe and concomitant ridging into the lower stratospheric vortex in this region. Nevertheless, preliminary investigations have indicated that such events are not uncommon in other northern hemisphere winters, although no such events were found in the southern hemisphere during the Antarctic w

ISSN:0148-0227    

DOI:10.1029/93JD02557

年代:1994

数据来源: WILEY

摘要:

Thirteen years of Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment CCl3F and CCl2F2measurements at five remote, surface, globally distributed sites are analyzed. Comparisons are made against shipboard measurements by the Scripps Institution of Oceanography group and archived air samples collected at Cape Grim, Tasmania, since 1978. CCl3F in the lower troposphere was increasing at an average rate of 9.2 ppt/yr over the period July 1978 to June 1988. CCl2F2was increasing at an average 17.3 ppt/yr in the lower troposphere over the same period. However, between July 1988 and June 1991 the increases of CCl3F and CCl2F2in this region have averaged just 7.0 ppt/yr and 15.7 ppt/yr, respectively. The rate of increase has been decreasing 2.4 ppt/yr2and 2.9 ppt/yr2over this 3‐year period. Based on a recent scenario of the global releases of these compounds and using the new calibration scale SIO 1993, the equilibrium lifetimes are estimated to be and years for CCl3F and CCl2F2, respectively. Using these lifetime estimates and a two‐dimensional model, it is estimated that global releases of these two chlorofluorocarbons in 1990 were 249±28×106kg for CCl3F and 366±30×106kg for CCl2F2. It is also estimated that combined releases of these chlorofluorocarbons in 1990 were 21±5% less than those

ISSN:0148-0227    

DOI:10.1029/93JD02715

年代:1994

数据来源: WILEY

摘要:

Satellite data are presented showing the timing of sudden warmings in the lower stratosphere during the winters 1979–1992. A three‐dimensional dynamical‐radiative‐photochemical model is used to establish how Arctic ozone depletion will respond to a doubling of CO2according to the timing of the warmings. In a series of idealized experiments the timing of the warmings is varied by specifying different geopotential wave amplitudes at the 316‐mbar model lower boundary. Results from a “transient climate change experiment” show that the chosen wave amplitudes are appropriate for both the current and the doubled CO2atmosphere. For doubled CO2the experiments show that any significant risk of an Arctic ozone hole will be confined to those years with only a late stratospheric warming. In all other years the results suggest that springtime total ozone over the Arctic is more likely to increase by a small amount due to a combination of slower homogeneous chemistry and changes in transport. The predictions obtained from the idealized studies are then tested by prescribing at the model lower boundary the observed geopotential wave amplitudes from two specific years with late winter warmings. Doubling CO2amounts produced no significant increase in ozone depletion with the 1989 wave amplitudes, but with 1990 wave amplitudes, an Arctic ozone hole occurred with minimum column of 187 Dobson Units. This contrasting response is attributed to the large midwinter pulse in the 1989 wave amplitudes compared to the less dramatic and shorter timescale fluctuations in the 1990 wave amplitudes. It is concluded that under doubled CO2conditions an Arctic ozone hole is likely to occur in years with late stratospheric warmings following winters in which there were no significant pulses in the upper tropospheric planetary wa

ISSN:0148-0227    

DOI:10.1029/93JD02831

年代:1994

数据来源: WILEY

摘要:

The concentration of methanesulfonic acid (MSA) was determined in a shallow south central Greenland ice core (20D). This study provides a high‐resolution record of the DMS‐derived biogenic sulfur in Greenland precipitation over the past 200 years. The mean concentration of MSA is 3.30 ppb (σ = 2.38 ppb, n = 1134). The general trend of MSA is an increase from 3.01 to 4.10 ppb between 1767 and 1900, followed by a steady decrease to 2.34 ppb at the present time. This trend is in marked contrast to that of non‐sea‐salt sulfate (nss SO42−), which increases dramatically after 1900 due to the input of anthropogenic sulfur. The MSA fraction ((MSA/(MSA + nss SO42−))* 100) ranges from a mean of 15% in preindustrial ice to less than 5% in recent ice. These MSA fractions suggest that approximately 15 to 40% of the sulfur in recent Greenland ice is of biological origin. It is suggested that there is a significant low‐latitude component to the biogenic sulfur in the core and that variations in the MSA fraction reflect changes in the relative strengths of low‐ and high‐latitude inputs. The data show no evidence for a strong dependence of dimethyl sulfide (DMS) emissions on sea surface temperature during the last century. There is also no indication that the yield of MSA from DMS oxidation has been altered by increased NOxlevels over the North Atlantic

ISSN:0148-0227    

DOI:10.1029/93JD02732

年代:1994

数据来源: WILEY