A. I. Jonsson

Separating the Dynamical Effects of Climate Change and Ozone Depletion. Part II: Southern Hemisphere Troposphere

Abstract The separate effects of ozone depleting substances (ODSs) and greenhouse gases (GHGs) on forcing circulation changes in the Southern Hemisphere extratropical troposphere are investigated using a version of the Canadian Middle Atmosphere Model (CMAM) that is coupled to an ocean. Circulation-related diagnostics include zonal wind, tropopause pressure, Hadley cell width, jet location, annular mode index, precipitation, wave drag, and eddy fluxes of momentum and heat. As expected, the tropospheric response to the ODS forcing occurs primarily in austral summer, with past (1960–99) and future (2000–99) trends of opposite sign, while the GHG forcing produces more seasonally uniform trends with the same sign in the past and future. In summer the ODS forcing dominates past trends in all diagnostics, while the two forcings contribute nearly equally but oppositely to future trends. The ODS forcing produces a past surface temperature response consisting of cooling over eastern Antarctica, and is the dominant...

Evaluation of radiation scheme performance within chemistry climate models

[1] This paper evaluates global mean radiatively important properties of chemistry climate models (CCMs). We evaluate stratospheric temperatures and their 1980–2000 trends, January clear sky irradiances, heating rates, and greenhouse gas radiative forcings from an offline comparison of CCM radiation codes with line‐by‐line models, and CCMs’ representation of the solar cycle. CCM global mean temperatures and their change can give an indication of errors in radiative transfer codes and/or atmospheric composition. Biases in the global temperature climatology are generally small, although five out of 18 CCMs show biases in their climatology that likely indicate problems with their radiative transfer codes. Temperature trends also generally agree well with observations, although one model shows significant discrepancies that appear to be due to radiation errors. Heating rates and estimated temperature changes from CO2, ozone, and water vapor changes are generally well modeled. Other gases (N2O, CH4, and CFCs) have only played a minor role in stratospheric temperature change, but their heating rates have large fractional errors in many models. Models that do not account for variations in the spectrum of solar irradiance cannot properly simulate solar‐induced variations in stratospheric temperature. The combined long‐lived greenhouse gas global annual mean instantaneous net radiative forcing at the tropopause is within 30% of line‐by‐line models for all CCM radiation codes tested. Problems remain in simulating radiative forcing for stratospheric water vapor and ozone changes with errors between 3% and 200% compared to line by line models. The paper makes recommendations for CCM radiation code developers and future intercomparisons.

A one‐dimensional simulation of the water vapor isotope HDO in the tropical stratosphere

An existing one-dimensional chemical model has been extended with dynamics and isotopic chemistry to simulate chemical production, vertical ascent. and diffusion of H 2 16 O and HDO in the tropical stratosphere. Less abundant isotopes of water vapor and methane have been added to the models original chemical scheme. This has led to 11 additional compounds being considered, and the methane oxidation chain has been extended by 47 new reactions. The dynamical model includes vertical diffusion and a vertical ascent rate that varies throughout the year. The results of the model show values of the isotopic ratio that are expected from theoretical calculations. The δD values range from around -550‰ at the tropopause to about -300‰ at 1 hPa. The model simulations are also in agreement with the few existing measurements of δD in the stratosphere. An annual variation of the isotopic ratio at the tropopause will cause a wave pattern in the vertical profile similar to the tape recorder effect for water vapor. The size and shape of this annual variation is not clear, but simulations show that the effect it has on the resulting δD profile is significant.

Validating the reported random errors of ACE-FTS measurements

In order to validate the reported precision of space-based atmospheric composition measurements, validation studies often focus on measurements in the tropical stratosphere, where natural variability is weak. The scatter in tropical measurements can then be used as an upper limit on single-profile measurement precision. Here we introduce a method of quantifying the scatter of tropical measurements which aims to minimize the effects of short-term atmospheric variability while maintaining large enough sample sizes that the results can be taken as representative of the full data set. We apply this technique to measurements of O(3), HNO(3), CO, H(2)O, NO, NO(2), N(2)O, CH(4), CCl(2)F(2), and CCl(3)F produced by the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS). Tropical scatter in the ACE-FTS retrievals is found to be consistent with the reported random errors (RREs) for H(2)O and CO at altitudes above 20 km, validating the RREs for these measurements. Tropical scatter in measurements of NO, NO(2), CCl(2)F(2), and CCl(3)F is roughly consistent with the RREs as long as the effect of outliers in the data set is reduced through the use of robust statistics. The scatter in measurements of O(3), HNO(3), CH(4), and N(2)O in the stratosphere, while larger than the RREs, is shown to be consistent with the variability simulated in the Canadian Middle Atmosphere Model. This result implies that, for these species, stratospheric measurement scatter is dominated by natural variability, not random error, which provides added confidence in the scientific value of single-profile measurements.

The NOy budget above Eureka, Nunavut from ground-based FTIR measurements, space-based ACE-FTS measurements, and the CMAM-DAS, GEM-BACH, and SLIMCAT models

Reactive nitrogen species, NOy, play an important role in stratospheric chemistry. Using a Bruker 125HR FTIR installed at Eureka, Nunavut, ACE-FTS satellite data, and model simulations, we study the NOy budget for this Arctic site.