K. Minschwaner

Radiative forcings and global warming potentials of 39 greenhouse gases

The radiative forcings and global warming potentials for 39 greenhouse gases are evaluated using narrowband and broadband radiative transfer models. Unlike many previous studies, latitudinal and seasonal variations are considered explicitly, using distributions of major greenhouse gases from a combination of chemical-transport model results and Upper Atmosphere Research Satellite (UARS) measurements and cloud statistics from the International Satellite Cloud Climatology Project. The gases examined include CO 2 , CH 4 , N 2 O, plus a number of chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, hydrochlorocarbons, bromocarbons, iodocarbons, and perfluorocarbons (PFCs). The model calculations are performed on a 5° latitude grid from 82.5°S to 82.5°N. The radiative forcings determined by the model are then used to derive global warming potential for each of the compounds, which are compared with prior analyses. In addition, the latitudinal and seasonal dependence of radiative forcing since preindustrial time is calculated. The vertical profiles of the gases are found to be important in determining the radiative forcings; the use of height-independent vertical distributions of greenhouse gases, as used in many previous studies, produce errors of several percent in estimated radiative forcings for gases studied here; the errors for the short-lived compounds are relatively higher. Errors in evaluated radiative forcings caused by neglecting both the seasonal and the latitudinal distributions of greenhouse gases and atmospheres are generally smaller than those due to height-independent vertical distributions. Our total radiative forcing due to increase in major greenhouse gas concentrations for the period 1765-1992 is 2.32 Wm -2 , only 2% higher than other recent estimates; however, the differences for individual gases are as large as 23%.

Bulk properties of isentropic mixing into the tropics in the lower stratosphere

Timescales for mixing of midlatitude air into the tropical lower stratosphere are deduced from observations of long-lived tracers N 2 O and CCl 3 F. Bulk mixing between tropical and midlatitude regions is assumed to be isentropic and relatively slow compared with local mixing within each region. The mean value of the mixing timescale ranges from 12 to 18 months near 20 km. There is a tendency for shorter mixing times at higher and lower altitudes, although vertical profiles of mixing cannot be definitively established by the data. A more robust quantity is given by the fraction of midlatitude air entrained into the tropical upwelling region. Implied mixing fractions exceed 50% above 22 km.

Infrared radiative forcing and atmospheric lifetimes of trace species based on observations from UARS

Observations from instruments on the Upper Atmosphere Research Satellite (UARS) have been used to constrain calculations of infrared radiative forcing by CH4, CCl2F2, and N2O and to determine lifetimes of CCl2F2 and N2O. Radiative forcing is calculated as a change in net infrared flux at the tropopause that results from an increase in trace gas amount from preindustrial (1750) to contemporary (1992) times. Latitudinal and seasonal variations are considered explicitly, using distributions of trace gases and temperature in the stratosphere from UARS measurements and seasonally averaged cloud statistics from the International Satellite Cloud Climatology Project. Top-of-atmosphere fluxes calculated for the contemporary period are in good agreement with satellite measurements from the Earth Radiation Budget Experiment. Globally averaged values of the radiative forcing are 0.550, 0.132, and 0.111 W m−2 for CH4, CCl2F2, and N2O, respectively. The largest forcing occurs near subtropical latitudes during summer, predominantly as a result of the combination of cloud-free skies and a high, cold tropopause. Clouds are found to play a significant role in regulating infrared forcing, reducing the magnitude of the forcing by 30–40% compared with the case of clear skies. The vertical profile of CCl2F2 is important in determining its radiative forcing; use of a height-independent mixing ratio in the stratosphere leads to an overprediction of the forcing by 10%. The impact of stratospheric profiles on radiative forcing by CH4 and N2O is 2% or less. UARS-based distributions of CCl2F2 and N2O are used also to determine global destruction rates and instantaneous lifetimes of these gases. Rates of photolytic destruction in the stratosphere are calculated using solar ultraviolet irradiances measured on UARS and a line-by-line model of absorption in the oxygen Schumann-Runge bands. Lifetimes are 114±22 and 118±25 years for CCl2F2 and N2O, respectively.

An assessment of upper‐troposphere and lower‐stratosphere water vapor in MERRA, MERRA2 and ECMWF reanalyses using Aura MLS observations

Global water vapor (H2O) measurements from Microwave Limb Sounder (MLS) are used to evaluate upper tropospheric (UT) and lower stratospheric (LS) H2O products produced by NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA), its newest release MERRA2, and European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalyses. Focusing on the H2O amount and transport from UT to LS, we show that all reanalyses overestimate annual global mean UT H2O by up to ~150% compared to MLS observations. Substantial differences in H2O transport are also found between the observations and reanalyses. Vertically, H2O transport across the tropical tropopause (16–20 km) in the reanalyses is faster by up to ~86% compared to MLS observations. In the tropical LS (21–25 km), the mean vertical transport from ECMWF is 168% faster than the MLS estimate, while MERRA and MERRA2 have vertical transport velocities within 10% of MLS values. Horizontally at 100 hPa, both observation and reanalyses show faster poleward transport in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Compared to MLS observations, the H2O horizontal transport for both MERRA and MERRA2 is 106% faster in the NH but about 42–45% slower in the SH. ECMWF horizontal transport is 16% faster than MLS observations in both hemispheres. The ratio of northward to southward transport velocities for ECMWF is 1.4, which agrees with MLS observation, while the corresponding ratios for MERRA and MERRA2 are about 3.5 times larger.

The effects of tropical cirrus clouds on the abundance of lower stratospheric ozone

The distribution of many chemical constituents of the atmosphere (e.g., ozone) is at least partially determined by the distribution of net radiative heating in the atmosphere. In this paper, we demonstrate the significant effect of high cirrus clouds on the net radiative heating of the tropical lower stratosphere. A model of tropical lower stratospheric ozone is then used to demonstrate the sensitivity of calculated ozone to the varying cloud cover used in the model. We conclude that calculated ozone is sensitive to the inclusion of clouds in models and that models of the atmosphere should include a realistic description of tropical cirrus clouds in order to accurately simulate the chemical composition of the atmosphere.

Middle and upper thermospheric odd nitrogen: 1. A new analysis of rocket data

We evaluate the NRLMSISE-00 model for calculations of odd nitrogen (NO, N(4S)) in the middle to upper thermosphere (z = 140-250 km). NRLMSISE-00 incorporates new data on O 2 that improves the agreement between odd nitrogen models and data significantly. In particular, the photochemical calculation that uses NRLMSISE-00 predicts a NO solar cycle variation that is significantly less than previous calculations and that agrees well with the NO observations. This agreement is consistent with the inference from FUV solar occultation data that the O 2 abundance above 140 km varies weakly with solar activity and that the O 2 vertical profile at solar maximum is sensitive to other factors besides molecular diffusion. Residual discrepancies remain with the comparison of calculated to observed N( 4 S), which may be due to a combination of theoretical deficiencies and uncertainties in the observations.

Characterization of sensitivity degradation seen from the UV to NIR by RAIDS on the International Space Station

This paper presents an analysis of the sensitivity changes experienced by three of the eight sensors that comprise the Remote Atmospheric and Ionospheric Detection System (RAIDS) after more than a year operating on board the International Space Station (ISS). These sensors are the Extreme Ultraviolet Spectrograph (EUVS) that covers 550-1100 Å, the Middle Ultraviolet (MUV) spectrometer that covers 1900-3100Å, and the Near Infrared Spectrometer (NIRS) that covers 7220-8740 Å. The scientific goal for RAIDS is comprehensive remote sensing of the temperature, composition, and structure of the lower thermosphere and ionosphere from 85-200 km. RAIDS was installed on the ISS Japanese Expansion Module External Facility (JEM-EF) in September of 2009. After initial checkout the sensors began routine operations that are only interrupted for sensor safety by occasional ISS maneuvers as well as a few days per month when the orbit imparts a risk from exposure to the Sun. This history of measurements has been used to evaluate the rate of degradation of the RAIDS sensors exposed to an environment with significant sources of particulate and molecular contamination. The RAIDS EUVS, including both contamination and detector gain sag, has shown an overall signal loss rate of 0.2% per day since the start of the mission, with an upper boundary of 0.13% per day attributed solely to contamination effects. This upper boundary is driven by uncertainty in the change in the emission field due to changing solar conditions, and there is strong evidence that the true loss due to contamination is significantly smaller. The MUV and NIRS have shown stability to within 1% over the first year of operations.

Hydroxyl column abundance measurements: PEPSIOS instrumentation at the Fritz Peak Observatory and data analysis techniques

Abstract We review the observations and spectral analysis for daytime measurements of the vertical column abundance of hydroxyl (OH) in the earths atmosphere from the Fritz Peak Observatory in Colorado (40°N) and from a concurrent series of observations from New Mexico Institute of Mining and Technology (34°N). These are high-resolution measurements of solar ultraviolet absorption by atmospheric OH in the P 1 (1) 2 Π→ 2 Σ electronic transition at 3081.7 A . The Fritz Peak OH database, initiated in 1977, consists of over 19,000 measurements and shows distinct diurnal, seasonal, and long-term variations. New Mexico OH observations began in 1996 using an instrument of comparable design and operation. Data from both locations are in conflict with OH abundances calculated by photochemical models for solar zenith angles less than about 60°. In addition, OH variations observed with respect to latitude, season, and long-term changes are not explained using current models. We present a critical examination of instrument characteristics and spectral analysis, one which indicates no tendency for systematic or interference effects that could contaminate observed OH abundances or their variations. This includes results of sensitivity analyses on synthetic spectra, taking into account temperature and pressure variations along the atmospheric optical path, and effects of absorption by SO 2 and CH 2 O.

Signature of a tropical Pacific cyclone in the composition of the upper troposphere over Socorro, NM†

We present a case study based on balloon-borne ozone measurements during SEACIONS (SouthEast American Consortium for Intensive Ozonesonde Network Study) in August-September 2013. Data from Socorro, NM (34oN, 107oW) show a layer of anomalously low ozone in the upper troposphere (UT) during 8-14 August. Back trajectories, UT jet analyses, and data from the Microwave Limb Sounder (MLS) on the Aura satellite indicate that this feature originated from the marine boundary layer in the eastern/central tropical Pacific, where several disturbances and one hurricane (Henriette) formed within an active region of the Intertropical Convergence Zone in early August 2013. The hurricane and nearby convection pumped boundary layer air with low ozone (20-30 ppbv) into the UT. This outflow was advected to North America 3-5 days later by a strong subtropical jet, forming a tongue of low ozone observed in MLS fields and a corresponding layer of low ozone in Socorro vertical profiles.

A Radiative–Convective Equilibrium Perspective of Weakening of the Tropical Walker Circulation in Response to Global Warming

AbstractBoth observational analysis and GCM simulations indicate that the tropical Walker circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (<300 hPa) and is negative for the rest of the troposphere. As a result, both the change of clear-sky cooling rate and the change of tropospheric lapse rat...