CGD's Dr. Andrew Gettelman

Abstract

Ozonesondes launched from Beijing, China, over a 3 year time period (September 2002 to July 2005) are used to evaluate the performance of ozone profile retrievals in the upper troposphere and lower stratosphere (UTLS) from two new spaceborne instruments, the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua satellite and the Microwave Limb Sounder (MLS) on the NASA Aura satellite. Since the Global Positioning System ozone sensors (GPSO3) used in Beijing ozonesondes are new, comparisons with simultaneously launched Vaisala ECC sensors, and comparisons with an ozonesonde climatology from Sapporo, Japan, are presented. The results show that although the new GPSO3 sensor has a positive bias (about 20-30%) below 200 hPa and a negative bias (about 5-10%) above 60 hPa relative to known sensors, the measured ozone variability is consistent with Vaisala ECC ozonesondes, particularly in the UTLS region. The GPSO3 ozonesonde profiles over Beijing are then used to evaluate coincident ozone profiles from AIRS version 4 retrieval and MLS version 1.5 retrieval. Qualitatively, both satellite data sets can reproduce the gradients and variability of ozone in the UTLS region. Quantitatively, the agreement between the AIRS and ozonesonde ozone profiles is largely within 10% in the UTLS region (from 400 to 70 hPa). The statistical difference between the retrieval and ozonesonde data is minimum in the vicinity of the tropopause. The MLS ozone profiles also show good quality in the UTLS region with the best performance between 147 and 46 hPa.

Figure caption: Differences between AIRS and GPSO3 at altitude relative to the tropopause height. Gray crosses are individual differences between sonde data binned to AIRS levels. Dashed lines are layer average differences. Solid lines and boxes mark the median, upper and lower quartiles for each layer.

Support: This work is supported by the CAS Knowledge Innovation Project (grant KZCX3-SW-217), the National Natural Science Foundation of China (grants 40375013, 40333034, and 40675021), and NASA (grant EOS/03-0594-0572 to NCAR). The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Eyring, V., N. Butchart, D. W. Waugh, H. Akiyoshi, J. Austin, S. Bekki, G. E. Bodeker, B. A. Boville, C. Brühl, M. P. Chipperfield, E. Cordero, M. Dameris, M. Deushi, V. E. Fioletov, S. M. Frith, R. R. Garcia, A. Gettelman, M. A. Giorgetta, V. Grewe, L. Jourdain, D. E. Kinnison, E. Mancini, E. Manzini, M. Marchand, D. R. Marsh, T. Nagashima, P. A. Newman, J. E. Nielsen, S. Pawson, G. Pitari, D. A. Plummer, E. Rozanov, M. Schraner, T. G. Shepherd, K. Shibata, R. S. Stolarski, H. Struthers, W. Tian, and M. Yoshiki, 2006: Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past. Journal of Geophysical Research, 111, D22308, doi:10.1029/2006JD007327.

Abstract

Simulations of the stratosphere from thirteen coupled chemistry-climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long-term trends are compared for an extended period (1960-2004). Comparisons of polar high-latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long-term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Figure caption: (a) Seasonal (February to April) total column ozone anomalies for the Arctic (60-90°N), (b) seasonal (September to November) total column ozone anomalies for the Antarctic (90-60°S), and (c) annual total column ozone anomalies sets for the whole globe (90°S-90°N) from CCMs and four observational data sets. The CCM results are shown with colored lines while the mean and range of the four observational data sets are shown as a thick black line and grey shaded area respectively. The seasonal anomaly time series shown in Figures 15a and 15b have been smoothed by applying a 1:2:1 filter iteratively five times. The filter width is reduced to one at the ends of the time series. The annual global anomalies shown in Figure 15c are unsmoothed. The plots on the right show detrended mean annual cycles for each model (1980 to 1989) and the mean and range of the observations.

Support: Coordination of this study was supported by the Chemistry-Climate Model Validation Activity (CCMVal) for WCRP's (World Climate Research Programme) SPARC (Stratospheric Processes and their Role in Climate) project. We thank the British Atmospheric Data Centre, the SPARC data center, and the UK Met Office for providing the facilities for central data archives. Additional research was supported by the Visiting Scientist Program at the NOAA Geophysical Fluid Dynamics Laboratory, administered by the University Corporation for Atmospheric Research, and by the Global Environmental Research Fund (GERF) of the Ministry of the Environment (MOE) of Japan (A-1). CMAM is supported by CFCAS and NSERC. The European groups acknowledge support of the SCOUT-O3 Integrated Project which is funded by the European Commission. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Eyring, V., N. Butchart, D. W. Waugh, H. Akiyoshi, J. Austin, S. Bekki, G. E. Bodeker, B. A. Boville, C. Brühl, M. P. Chipperfield, E. Cordero, M. Dameris, M. Deushi, V. E. Fioletov, S. M. Frith, R. R. Garcia, A. Gettelman, M. A. Giorgetta, V. Grewe, L. Jourdain, D. E. Kinnison, E. Mancini, E. Manzini, M. Marchand, D. R. Marsh, T. Nagashima, P. A. Newman, J. E. Nielsen, S. Pawson, G. Pitari, D. A. Plummer, E. Rozanov, M. Schraner, T. G. Shepherd, K. Shibata, R. S. Stolarski, H. Struthers, W. Tian, and M. Yoshiki, 2007: Multi-model projections of stratospheric ozone in the 21st century. Journal of Geophysical Research, 112, D16303, doi:10.1029/2006JD008332.

Abstract

Simulations from eleven coupled chemistry-climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model-to-model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHG-induced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lower-stratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Cly near 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease to 1980 values and before the Antarctic. None of the CCMs predict future large decreases in the Arctic column ozone. By 2100, total column ozone is projected to be substantially above 1980 values in all regions except in the tropics.

Figure caption: (a) March Arctic (60°N to 90°N) total column ozone anomalies from CCMs (colored lines) and the mean from four observational data sets (thick black line for smoothed curve and black dots for individual years). (b) As for Figure 7a, but October Antarctic (90°S to 60°S) total column ozone anomalies. Time series have been smoothed as in Figure 1 and anomalies have been calculated by subtracting the 1980-1984 mean from the smoothed time series. Light gray shading between 2060 and 2070 shows the period when stratospheric concentrations of halogens in the polar lower stratosphere are expected to return to their 1980 values.

Support: Coordination of this study was supported by the Chemistry-Climate Model Validation activity (CCMVal) for World Climate Research Programme (WCRP)'s Stratospheric Processes and their Role in Climate (SPARC) project. We thank the British Atmospheric Data Centre, the SPARC data center, and the U.K. Met Office for providing the facilities for central data archives. Additional research was partially supported by a NASA grant, by the Visiting Scientist Program at the NOAA Geophysical Fluid Dynamics Laboratory, administered by the University Corporation for Atmospheric Research, and by the Global Environmental Research Fund (GERF) of the Ministry of the Environment (MOE) of Japan (A-1). CMAM is supported by CFCAS and NSERC. The Leeds work was supported by the U.K. Natural Environment Research Council (NERC). Computational resources for running the GEOSCCM were provided by NASA's high-performance computing project. The SOCOL group was supported by the Swiss Federal Institute of Technology and in part by the Swiss National Science Foundation (grant SCOPES IB7320-110884). The European groups acknowledge support of the SCOUT-O3 Integrated Project which is funded by the European Commission. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Gettelman, A., W. D. Collins, E. J. Fetzer, A. Eldering, F. W. Irion, P. B. Duffy, and Govindasamy Bala, 2006: Climatology of Upper-Tropospheric Relative Humidity from the Atmospheric Infrared Sounder and Implications for Climate. Journal of Climate, 19, doi: 10.1175/JCLI3956.1.

Abstract

Recently available satellite observations from the Atmospheric Infrared Sounder (AIRS) are used to calculate relative humidity in the troposphere. The observations illustrate many scales of variability in the atmosphere from the seasonal overturning Hadley-Walker circulation to high-frequency transient variability associated with baroclinic storms with high vertical resolution. The Asian monsoon circulation has a strong impact on upper-tropospheric humidity, with large humidity gradients to the west of the monsoon. The vertical structure of humidity is generally bimodal, with high humidity in the upper and lower troposphere, and a dry middle troposphere. The highest variances in humidity are seen around the midlatitude tropopause. AIRS data are compared to a simulation from a state-of-the-art climate model. The model does a good job of reproducing the mean humidity distribution but is slightly moister than the observations in the middle and upper troposphere. The model has difficultly reproducing many scales of observed variability, particularly in the Tropics. Differences in humidity imply global differences in the top of atmosphere fluxes of 1 W m-2.

Figure caption: Differences between CAM humidity sorted by cloud fraction (CAM) and AIRS humidity between 600 and 200 hPa (AIRS). Longwave radiation at the top of the atmosphere for (a) Jan and (b) Jul. Contour interval: 5 W m-2. Zonal mean longwave radiation at the top (solid) and surface (dotted-dashed) and shortwave radiation at the top (dash) and surface (dotted) for (c) Jan and (d) Jul. Latitude height difference in net heating rate for (e) Jan and (f) Jul. Contour interval: ±0.1 K day-1.

Support: The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Gettelman, A., E. J. Fetzer, A. Eldering, and F. W. Irion, 2006: The Global Distribution of Supersaturation in the Upper Troposphere from the Atmospheric Infrared Sounder. Journal of Climate, 19, doi:10.1175/JCLI3955.1.

Abstract

Satellite data from the Atmospheric Infrared Sounder (AIRS) is analyzed to examine regions of the upper troposphere that are supersaturated: where the relative humidity (RH) is greater than 100%. AIRS data compare well to other in situ and satellite observations of RH and provide daily global coverage up to 200 hPa, though satellite observations of supersaturation are highly uncertain. The climatology of supersaturation is analyzed statistically to understand where supersaturation occurs and how frequently. Supersaturation occurs in humid regions of the upper tropical tropopause near convection 10%-20% of the time at 200 hPa. Supersaturation is very frequent in the extratropical upper troposphere, occurring 20%-40% of the time, and over 50% of the time in storm track regions below the tropopause. The annual cycle of supersaturation is consistent for the 2.5 yr of data analyzed. More supersaturation is seen in the Southern Hemisphere midlatitudes, which may be attributed to higher temperature variance.

Figure caption: Frequency of supersaturation at 200 hPa for four seasons: (a) January-March, (b) April-June, (c) July-September, and (d) October-December. Red line is the seasonal mean tropopause from NCEP-NCAR reanalysis data.

Support: The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Gettelman, A. and D. E. Kinnison, 2007: The global impact of supersaturation in a coupled chemistry-climate model. Atmospheric Chemistry & Physics, 7, 1629-1643.

Abstract

Ice supersaturation is important for understanding condensation in the upper troposphere. Many general circulation models however do not permit supersaturation. In this study, a coupled chemistry climate model, the Whole Atmosphere Community Climate Model (WACCM), is modified to include supersaturation for the ice phase. Rather than a study of a detailed parameterization of supersaturation, the study is intended as a sensitivity experiment, to understand the potential impact of supersaturation, and of expected changes to stratospheric water vapor, on climate and chemistry. High clouds decrease and water vapor in the stratosphere increases at a similar rate to the prescribed supersaturation (20% supersaturation increases water vapor by nearly 20%). The stratospheric Brewer-Dobson circulation slows at high southern latitudes, consistent with slight changes in temperature likely induced by changes to cloud radiative forcing. The cloud changes also cause an increase in the seasonal cycle of near tropopause temperatures, increasing them in boreal summer over boreal winter. There are also impacts on chemistry, with small increases in ozone in the tropical lower stratosphere driven by enhanced production. The radiative impact of changing water vapor is dominated by the reduction in cloud forcing associated with fewer clouds (~+0.6 Wm-2) with a small component likely from the radiative effect (greenhouse trapping) of the extra water vapor (~+0.2 Wm-2), consistent with previous work. Representing supersaturation is thus important, and changes to supersaturation resulting from changes in aerosol loading for example, might have a modest impact on global radiative forcing, mostly through changes to clouds. There is no evidence of a strong impact of water vapor on tropical tropopause temperatures.

Figure caption: Zonal mean tropical (10 S-10 N) monthly water vapor on the equator for (A) Base case (Top) and (B) difference between supersaturation and base cases (SSAT - Base) in ppmv (bottom). Mean annual cycle is repeated twice.

Support: The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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Park, M., W. J. Randel, A. Gettelman S. T. Massie, and J. H. Jiang, 2007: Transport above the Asian Summer Monsoon Anticyclone inferred from Aura MLS Tracers. Journal of Geophysical Research, 112, D16309, doi:10.1029/2006JD008294.

Abstract

Tracer variability above the Asian summer monsoon anticyclone is investigated using Aura Microwave Limb Sounder (MLS) measurements of carbon monoxide, ozone, water vapor, and temperature during Northern Hemisphere summer (June to August) of 2005. Observations show persistent maxima in carbon monoxide and minima in ozone within the anticyclone in the upper troposphere-lower stratosphere (UTLS) throughout summer, and variations in these tracers are closely related to the intensity of underlying deep convection. Temperatures in the UTLS are also closely coupled to deep convection (cold anomalies are linked with enhanced convection), and the three-dimensional temperature patterns are consistent with a dynamical response to near- equatorial convection. Upper tropospheric water vapor in the monsoon region is strongly coherent with deep convection, both spatially and temporally. However, at the altitude of the tropopause, maximum water vapor is centered within the anticyclone, distant from the deepest convection, and is also less temporally correlated with convective intensity. Because the main outflow of deep convection occurs near ~12 km, well below the tropopause level (~16 km), we investigate the large-scale vertical transport within the anticyclone. The mean vertical circulation obtained from the ERA40 reanalysis data set and a free-running general circulation model is upward across the tropopause on the eastern end of the anticyclone, as part of the balanced three-dimensional monsoon circulation. In addition to deep transport from the most intense convection, this large-scale circulation may help explain the transport of constituents to tropopause level.

Figure caption: (a) Scatter plot of MLS carbon monoxide (ppbv) and ozone (ppbv) at 100 hPa over 60°S-60°N for 1-5 July 2005. Yellow dots represent stratospheric air (O3 = 300 and CO < 60 ppbv), and red dots denote tropospheric air (O3 < 300 and CO = 60 ppbv), respectively. Blue dots located in between denote the troposphere and stratosphere transition layer. (b) Map of air mass statistics diagnosed from CO-O3 relationship in Figure 9a for July-August 2005. Colors represent statistical air mass characteristics defined in this study, e.g., yellow: stratosphere, red: troposphere, blue: transition layer, green: stratosphere or transition layer, purple: troposphere or transition layer, orange: stratosphere or troposphere.

Support: This work was partially supported under the NASA ACMAP Aura Validation Project, and by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.