Mark G. Lawrence

The relationship between relative humidity and the dewpoint temperature in moist air - A simple conversion and applications

The relative humidity (RH) and the dewpoint temperature (td) are two widely used indicators of the amount of moisture in air. The exact conversion from RH to td, as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%), which does not appear to be widely known by the meteorological community: td decreases by about 1°C for every 5% decrease in RH (starting at td= t, the dry bulb temperature, when RH = 100%). This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as zLCL >> (20 + t/5)(100 – RH), where zLCL is in meters when t is in degrees Celcius and RH in percent. Finally, a his...

Fresh air in the 21st century

Ozone is an air quality problem today for much of the worlds population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.

Influence of NOx emissions from ships on tropospheric photochemistry and climate

Emissions of nitrogen oxides (NOx, the sum of NO and NO2) from fossil-fuel burning dominate the NOx burden of the lower troposphere in many regions. These emissions increase tropospheric ozone and hydroxyl-radical concentrations over their natural ‘background’ levels, thereby increasing the oxidizing power of the atmosphere. Fossil-fuel emissions of NOx (refs 3, 4) account for about half of the global NOx source to the atmosphere; other significant sources are from biomass burning, soil emissions, aircraft exhausts and lightning, all primarily continental. However, ocean-going ships burning fossil fuels may also contribute a significant fraction (>10%) to global NOx production. Here we use NOx emission data and a high-resolution chemistry–transport model to estimate that ship NOx emissions result in a more than 100-fold increase in surface NOx concentrations in heavily traversed ocean regions. This enhancement has a notable effect on modelled surface ozone and hydroxyl-radical concentrations. In particular, a predicted fivefold increase in the July hydroxyl-radical burden over the northern Atlantic and Pacific oceans would be expected to reduce the atmospheric lifetimes of reactive greenhouse gases—such as methane—as well as to increase aerosol production rates and cloud reflectivities, therefore exerting a cooling influence on the climate.

A model for studies of tropospheric photochemistry: Description, global distributions, and evaluation

A model of atmospheric photochemistry and transport has been developed and applied toward investigating global tropospheric chemistry. The Model of Atmospheric Transport and Chemistry - Max-Planck-Institute for Chemistry version (MATCH-MPIC) is described and key characteristics of its global simulation are presented and compared to available observations. MATCH-MPIC is an “offline” model which reads in gridded time-dependent values for the most basic meteorological parameters (e.g., temperature, surface pressure, horizontal winds), then uses these to compute further meteorological parameters required for atmospheric chemistry simulations (convective transport, cloud microphysics, etc.). The meteorology component of MATCH-MPIC simulates transport by advection, convection, and dry turbulent mixing, as well as the full tropospheric hydrological cycle (water vapor transport, condensation, evaporation, and precipitation). The photochemistry component of MATCH-MPIC represents the major known sources (e.g., industry, biomass burning), transformations (chemical reactions and photolysis), and sinks (e.g., wet and dry deposition) which affect the O3hyphen;HOx-NOy-CH4-CO photochemical framework of the “background” troposphere. The results of two versions of the model are considered, focusing on the more recent version. O3 is in relatively good agreement with observed soundings, although it is generally underestimated at low levels and overestimated at high levels, particularly for the more recent version of the model. We conclude that the simulated stratosphere-troposphere flux of O3 is too large, despite the fact that the total flux is 1100 Tg(O3)/yr, whereas the upper limit estimated in recent literature is over 1400 Tg(O3)/yr. The OH distribution yields a tropospheric CH4 lifetime of 10.1 years, in contrast to the lifetime of 7.8 years in the earlier model version, which nearly spans the range of current estimates in the literature (7.5–10.2 years). Surface CO mixing ratios are in good agreement with observations. NO is generally underestimated, a problem similar to what has also been found in several other recent model studies. HNO3 is also considerably underestimated. H2O2 and CH3OOH, on the other hand, are in relatively good agreement with available observations, though both tend to be underestimated at high concentrations and overestimated at low concentrations. Possible reasons for these differences are considered.

The impact of precipitation scavenging on the transport of trace gases : A 3-dimensional model sensitivity study

With the global Chemistry-Transport model MATCHsensitivity simulations were performed to determinethe degree to which especially upward transport ofgases from the earths surface is limited byconvective and large-scale precipitation scavenging.When only dissolution of species in the liquid phaseis taken into account, mixing ratio reductions in themiddle and upper troposphere by ≈10% arecalculated for gases with a Henrys Law constant H of103 mol/l/atm. The removal increases to ≈50% forH = 104 mol/l/atm, and to 90% for H =105 mol/l/atm. We also consider scavenging by theice phase, which is generally much less efficient thanby the aqueous phase. In fact, rejection of gases fromfreezing water droplets may be a source of trace gasat higher altitudes.H2O2 and the strong acids (H2SO4,HNO3, HCl, HBr, HI) have such large solubilitiesthat they become largely removed by precipitation.When significant concentrations of these gases andsulfate aerosol exist above the liquid water domain ofthe atmosphere, they have likely been produced thereor at higher altitudes, although some could have comefrom trace gas rejection from ice particles or fromevaporating hydrometeors. Several other gases areaffected by precipitation, but not strongly enough toprevent fractional transfer to the middle and uppertroposphere: e.g., HNO4, HNO2 at pH ≤5,CH2O, the organic acids at pH ≤6,CH3SOCH3, HOCl, HOBr, and HOI. NH3 islargely removed by liquid phase scavenging at pH ≤7 and SO2 atpH ≥7. At pH less thanabout 6, upward transport of SO2 should largelydepend on the efficiency of oxidation processes in thewater droplets by O3 and H2O2.Most gases have solubilities which are too low forsignificant precipitation scavenging and aqueous phaseoxidation to occur. This holds, e.g., for O3, CO,the hydrocarbons, NO, NO2, HCN, CH3CN,CH3SCH3, CH3O2H, CH3CHOandhigher aldehydes, CH3OH and higher alcohols,peroxyacetylnitrate (PAN), CH3COCH3 andother ketones (note that some of these are not listedin Table I because their solubilities are below 10mol/l/atm). Especially for the short-lived gases,transfer from the boundary layer to the middle andupper troposphere is actually promoted by the enhancedupward transport that occurs in clouds.

Distribution of reactive nitrogen species in the remote free troposphere: Data and model comparisons

The available reactive nitrogen measurements from the global free troposphere obtained during the period of 1985—1995 have been compiled and analyzed. The species of interest are NO, NO x (NO#NO 2 ), NO : , PAN, HNO 3 and O 3 . Data extending to 13 km have been gridded with a 5i]5i horizontal and 1 km vertical resolution. The data have been divided into two seasons, namely ‘‘Winter’’ and ‘‘Summer’’ depending upon the time and location of the observations. Data described here as well as additional analysis have also been archived and are accessible on-line through the World Wide Web at: http://george.arc.nasa.gov/&athakur. Global maps of the reactive nitrogen species distribution are produced in a form that would be most useful for the test and evaluation of models of tropospheric transport and chemistry. Limited comparisons of the observed reactive nitrogen species data with predictions by 3-D global models were performed using three selected models. Significant model to model as well as data to model di⁄erences were frequently observed. During summer, models tended to underpredict NO (!25 to !60%) while significantly overpredicting HNO 3 (#250 to #400%) especially in the upper troposphere. Similarly, the seasonal HNO 3 variations predicted by some models were opposite to those observed. PAN was generally overpredicted, especially in the upper troposphere, while NO : was underpredicted. Ozone on average was better simulated but significant deviations at specific locations were evident. By comparing model predictions with observations, an overall quantitative assessment of the accuracy with which these three models describe the global distribution of measured reactive nitrogen species is provided. No reliable trend information for any of the reactive nitrogen species was possible based on the presently available data set. The reactive nitrogen data currently o⁄er only a limited spatial and temporal coverage for the validation of global models. ( 1999 Elsevier Science Ltd. All rights reserved.

An empirical analysis of the strength of the phytoplankton‐dimethylsulfide‐cloud‐climate feedback cycle

The possible influence of the marine biogeochemical sulfur cycle on the global climate has been a topic of much recent research. Based on the hypothesis that phytoplankton could affect cloud albedo by producing dimethylsulfide, which is a precursor to aerosols and cloud condensation nuclei, and that cloud albedo could in turn affect the productivity of the phytoplankton, the presence of such a feedback cycle would have significant implications for models of global climate change. By considering available data on the relationships between individual components of the proposed feedback, an empirical model is developed of the cycle as a whole, allowing an assessment to be made of the degree to which the cycle could thermostatically regulate the climate. It is estimated that the feedback strength is about 20% (10% – 50%) of that which would be necessary to completely counteract a perturbation to the global climate, such as is anticipated due to accumulation of anthropogenic greenhouse gases.

Comparison between global chemistry transport model results and Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) data

Ozone distributions from state-of-the-art global three-dimensional chemistry transport models are compared to O 3 data collected on Airbus A340 passenger aircraft as part of the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) project. The model results are compared to monthly averaged data at cruise altitudes in the upper troposphere and lower stratosphere and monthly averaged vertical profiles collected over particular cities during takeoff and landing. The models generally show good agreement with the data in regions which have previously been well documented and where the meteorology is well understood/captured by meteorological models (e.g., over Europe). However, in the upper troposphere and lower stratosphere, models often fail to capture sharp gradients across the tropopause and from the subtropics to the tropics. In some models, this is related to deficiencies in model transport schemes and upper boundary conditions. Also, regions of the globe where our understanding of meteorology is poorer and emissions are less well known (e.g., tropics, continental Africa, Asia, and South America) are not simulated as well by all models. At particular measurement locations, it is apparent that emission inventories used by some global models underestimate emissions in certain regions (e.g., over southern Asia) or have incorrect seasonal variations (e.g., biomass burning over South America). Deficiencies in chemical schemes may also explain differences between models and the data.

3-D global simulations of tropospheric CO distributions - results of the GIM/IGAC intercomparison 1997 exercise

Abstract The objective of the Tropospheric Ozone (O 3 ) Global Model Intercomparison Exercise performed in 1997 was to systematically evaluate the capabilities of the current generation of 3-dimensional global models to simulate tropospheric ozone and their precursor gases, and to identify key areas of uncertainty in our understanding of the tropospheric O 3 budget. This exercise has been organised by Global Integration Modelling (GIM) Activity of the International Global Atmospheric Chemistry (IGAC) Project. The present paper focuses on the capability of the models to simulate carbon monoxide, which is an important pollutant in the troposphere. The intercomparison of twelve 3-dimensional global chemistry/transport models shows significant differences between the models although all of them capture the general patterns in the global distribution of CO.

Tracer transport in deep convective updrafts: Plume ensemble versus bulk formulations

Abstract Two widely used approaches for parameterizing tracer transport based on convective mass fluxes are the plume ensemble formulation (PEF) and the bulk formulation (BF). Here the behavior of these two is contrasted for the specific case in which the BF airmass fluxes are derived as a direct simplification of an explicit PEF. Relative to the PEF, the BF has a greater rate of entrainment of midtropospheric air into the parcels that reach the highest altitudes, and thus is expected to compute less efficient transport of surface-layer tracers to the upper troposphere. In this study, this difference is quantified using a new algorithm for computing mass conserving, monotonic tracer transport for both the BF and PEF, along with a technique for decomposing a bulk mass flux profile into a set of consistent, discrete plumes for use in the PEF. Runs with a 3D global chemistry transport model (MATCH) show that the BF is likely to be an adequate approximation for most tracers with lifetimes of a week or longer....