Mark A. Kritz

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

Radon measurements in the lower tropical Stratosphere: Evidence for rapid vertical transport and dehydration of tropospheric air

During the tropical experiment of NASAs Stratosphere-Troposphere Exchange Program (STEP), in situ radon and other trace constituent measurements were made aboard a NASA ER-2 high-altitude research aircraft to investigate the mechanisms of irreversible transfers from the troposphere into the tropical stratosphere. Observations made in and downwind of the cirrus shields of three large tropical cyclones and downwind of the cirrus anvil of a large cumulonimbus cloud cluster showed several clear instances of elevated radon activity occurring simultaneously with low total water mixing ratios. These observations are unambiguous evidence of an effective dehydration process, capable of reducing total water vapor mixing ratios to less than 2.5 ppmv, occurring in conjunction with troposphere-to-stratosphere transport and indicate that rapid localized convection, rather than slow regional mean motions, was responsible for the observed transports and associated with the accompanying dehydration. Radon activities measured in regions of active or recent troposphere-to-stratosphere transport were consistent with the 17 pCi/scm mean value needed to support the observed abundance of stratospheric 210Pb.

Validation of an off‐line three‐dimensional chemical transport model using observed radon profiles: 1. Observations

We present in Table 1 of this paper a series of free tropospheric radon profiles, obtained in the summer of 1994 on flights originating from Moffett Field, California (37.4°N, 122.0°W). These data were collected with the intent of acquiring a high-quality, statistically significant set of observations, in one season and at one location, suitable for use in the development and validation of three-dimensional global circulation and chemical transport models, as demonstrated in the companion paper [Stockwell, et al., this issue]. The profiles, which extend from the surface to 11.5 kilometers, were obtained on 11 flights flown in the period June 3 to August 16. A series of high-pressure whole-air samples (typically 11 or 12 per profile) were collected as the plane ascended to altitude, and analyzed immediately after each flight in a ground-based laboratory. A thorough discussion of sampling, analysis, calibration, and quality control procedures is provided.

Air mass origins and troposphere-to-stratosphere exchange associated with mid-latitude cyclogenesis and tropopause folding inferred from 7Be measurements

The 1984 extratropical mission of NASAs Stratosphere-Troposphere Exchange Project (STEP) studied cross-jet transports in regions of cyclogenesis and tropopause folding. Correlations of 7Be, ozone, water vapor, and potential vorticity measured on a NASA U-2 research aircraft flying in high shear regions above the jet core are indicative of mixing between the cyclonic and the anticyclonic sides of the jet and are consistent with the hypothesis that small-scale entrainments of upper tropospheric air into the lower stratosphere during cyclogenesis are important in maintaining the vertical gradients of 7Be, ozone, water vapor and other trace constituents in the lower few kilometers of the mid-latitude stratosphere. Correlations between 7Be and ozone suggest a lower tropical stratospheric origin for the ozone-poor lamina observed above the jet core.

Validation of an off‐line three‐dimensional chemical transport model using observed radon profiles: 2. Model results

We parameterize radon emissions in the TOMCAT global off-line three-dimensional chemical transport model (CTM) and compare modeled radon profiles with spatially and temporally matched observations obtained near Moffett Field, California, in June 1994. The CTM was forced using European Centre for Medium-Range Weather Forecasts analyses for April-August 1994. To identify the origin of modeled radon, we divided the radon sources into three regions. We performed CTM sensitivity experiments at horizontal resolutions of 2.8° × 2.8° (latitude × longitude) and 7.5° × 7.5°, and with and without moist convection and/or vertical diffusion. At the higher resolution the full CTM (i.e., including convection and vertical diffusion) generally agrees well with the observations in the free troposphere. The observations exhibit free-tropospheric radon peaks at altitudes where zonal wind speeds in the Pacific jet stream are greatest (generally at ≥7 km). In this region the modeled radon originates from Asia, and the observed variability in the radon concentration is reproduced. Thus, the model reproduces the position and strength of the jet. We identify observed radon peaks that may originate from convective lifting, as they do in the model results. If these observed peaks do originate from convection, then the full CTM captures the correct temporal variability in convection over Asia and the Pacific for the observation period. Quantitative comparison shows that the model-data agreement is degraded if we decrease the model resolution to 7.5° × 7.5°, or remove the parameterizations of convection and/or vertical diffusion. This suggests a resolution of near 2.8° × 2.8° is needed in global models for realistic simulations of short-lived species. We have identified certain shortcomings with TOMCAT: the model underestimates the amount of convective cloud, the convective cloud top height, and the amount of vertical diffusion. We identify improvements to ameliorate these problems.