Philippe Keckhut

An update of observed stratospheric temperature trends

An updated analysis of observed stratospheric temperature variability and trends is presented on the basis of satellite, radiosonde, and lidar observations. Satellite data include measurements from the series of NOAA operational instruments, including the Microwave Sounding Unit covering 1979–2007 and the Stratospheric Sounding Unit (SSU) covering 1979–2005. Radiosonde results are compared for six different data sets, incorporating a variety of homogeneity adjustments to account for changes in instrumentation and observational practices. Temperature changes in the lower stratosphere show cooling of ∼0.5 K/decade over much of the globe for 1979–2007, with some differences in detail among the different radiosonde and satellite data sets. Substantially larger cooling trends are observed in the Antarctic lower stratosphere during spring and summer, in association with development of the Antarctic ozone hole. Trends in the lower stratosphere derived from radiosonde data are also analyzed for a longer record (back to 1958); trends for the presatellite era (1958–1978) have a large range among the different homogenized data sets, implying large trend uncertainties. Trends in the middle and upper stratosphere have been derived from updated SSU data, taking into account changes in the SSU weighting functions due to observed atmospheric CO2 increases. The results show mean cooling of 0.5–1.5 K/decade during 1979–2005, with the greatest cooling in the upper stratosphere near 40–50 km. Temperature anomalies throughout the stratosphere were relatively constant during the decade 1995–2005. Long records of lidar temperature measurements at a few locations show reasonable agreement with SSU trends, although sampling uncertainties are large in the localized lidar measurements. Updated estimates of the solar cycle influence on stratospheric temperatures show a statistically significant signal in the tropics (∼30°N–S), with an amplitude (solar maximum minus solar minimum) of ∼0.5 K (lower stratosphere) to ∼1.0 K (upper stratosphere).

The SPARC Intercomparison of Middle-Atmosphere Climatologies

An updated assessment of uncertainties in ‘‘observed’’ climatological winds and temperatures in the middle atmosphere (over altitudes ;10‐80 km) is provided by detailed intercomparisons of contemporary and historic datasets. These datasets include global meteorological analyses and assimilations, climatologies derived from research satellite measurements, historical reference atmosphere circulation statistics, rocketsonde wind and temperature data, and lidar temperature measurements. The comparisons focus on a few basic circulation statistics (temperatures and zonal winds), with special attention given to tropical variability. Notable differences are found between analyses for temperatures near the tropical tropopause and polar lower stratosphere, temperatures near the global stratopause, and zonal winds throughout the Tropics. Comparisons of historical reference atmosphere and rocketsonde temperatures with more recent global analyses show the influence of decadal-scale cooling of the stratosphere and mesosphere. Detailed comparisons of the tropical semiannual oscillation (SAO) and quasibiennial oscillation (QBO) show large differences in amplitude between analyses; recent data assimilation schemes show the best agreement with equatorial radiosonde, rocket, and satellite data.

Climatology and trends of the middle atmospheric temperature (33-87 KM) as seen by Rayleigh lidar over the south of France

The technique of the Rayleigh lidar provides temperature profiles with a good temporal and vertical resolution in the middle atmosphere. Data obtained by 2 Rayleigh lidars set up at the Observatory of Haute-Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) from 1978 to 1989 led to a unique set of data of 1200 night-mean temperature profiles from 37 to 87 km. A climatology of the temperature over the south of France has been established from this data base and new results concerning both the short-term and long-term variability of the middle atmosphere are presented in this paper. The observed temperature around 44°N, 0°W is in general colder than the mean temperature given in the COSPAR International Reference Atmosphere (1986) near 75 km and warmer above 80 km. A clear semiannual variation is observed near 65 km with maxima occurring just after the equinoxes. The study of the short-term variability indicates clearly two frequency domains: long periods below 65 km with a maximum in December–January which relate to planetary waves and shorter periods above 65 km induced by the breaking of gravity waves. A correlation with the 11-year solar cycle, negative in the stratosphere and positive in the mesosphere, is found to be well above the 95% confidence level with an amplitude larger in winter than in summer. However, with only 11 years of data it is obviously difficult to conclude that this atmospheric perturbation is induced by the solar forcing. A cooling of the mesosphere of about −4 K/decade at 60–70 km is found but is at the limit of the 95% confidence level, whereas we do not observe any significant trend in the stratosphere.

Midlatitude long-term variability of the middle atmosphere : trends and cyclic and episodic changes

A study of variability and long-term trend detection in the middle atmosphere is performed using the temperature database obtained since 1979 by French Rayleigh lidars at 44°N. A multivariable analysis is used to consider natural variability, including solar activity, quasi-biennial oscillation (QBO), and volcanic eruption effects. As the solar proxy has an important role in this analysis, its choice is first discussed. Changes induced by solar activity reported here, mainly large negative correlation observed in the winter upper stratosphere, are not reproduced by numerical models without an unrealistic solar UV change. A significant warming of 6 K in the mesosphere is observed in summer 1992 and 1993. This large change could be due to aerosols injected in the stratosphere by the Mount Pinatubo eruption. Observed volcanic aerosol effects are compared with some numerical simulations and are shown to present some agreements. A long-term cooling of 0.4 K/year is observed in the mesosphere that is statistically significant but is larger than anthropogenic effects simulated in numerical models. In the upper stratosphere the small trend observed is not yet significant but in better agreement with model predictions.

A Critical Review of the Database Acquired for the Long-Term Surveillance of the Middle Atmosphere by the French Rayleigh Lidars

Abstract The database obtained by Rayleigh lidars over the south of France is now used for monitoring the middle-atmosphere structure and to validate satellite data. For these reasons it is crucial to ensure the quality of the data. The purpose of this paper is to review all possible sources of errors that could induce random or systematic bias in the temperature measurements. The characteristics of the lidars, the procedures used, as well as the data reduction software are then reviewed. Comparisons made between the lidar and other available techniques and between lidars of different characteristics lead to the conclusion that an accuracy of 1 K can be attained between 30 and about 70 km depending on the lidar power. The method itself is not affected by drift with time and provides absolute temperature data without any need of calibration and therefore is one of the best instruments for long-term monitoring.

Temperature climatology of the middle atmosphere from long-term lidar measurements at middle and low latitudes

Long-term measurements from several lidar instruments (Rayleigh and sodium) located at 44.0°N, 40.6°N, 34.4°N, and 19.5°N have been used to develop a new climatology of the middle atmosphere temperature. For each instrument, the measurements on every individual day of the year over the entire long-term record were averaged to build a composite year of temperature profiles. These profiles were then interpolated to provide temperature values at 1-km altitude intervals so that the climatology comprises daily temperature values at integer altitudes between 15 and 110 km, depending on the instrument. The climatologies for each lidar were then compared to the CIRA-86 model and to each other. Large differences between the lidar temperatures and the CIRA-86 temperatures are identified and explained. When compared to all instruments, CIRA-86 appears systematically much too cold between 90 and 95 km, by 20 K or more, and possibly 6–8 K too warm around 80 km, making its use as a reference atmosphere model questionable at these altitudes. The annual and semiannual components of the seasonal variability and the 2- to 33-day period variability were investigated. An annual cycle with 6–7 K amplitude in the upper stratosphere, increasing to 15–20 K at 80 km, is observed at midlatitudes. This cycle is in phase with the solar flux in the stratosphere and in opposite phase in the mesosphere with a very cold summer mesopause at 85 km, in good agreement with previous climatologies. At lower latitudes, a semiannual oscillation propagates downward from 85 to 30 km and is characterized by a stronger first cycle than the second (4 and 2 K amplitude respectively). The 2- to 33-day variability at midlatitudes shows a maximum during winter around 40 km and in the mesosphere. The first peak is associated with planetary wave activity and stratospheric warmings and the second to the occurrence of mesospheric temperature inversions. Finally, sudden seasonal transitions, highly consistent between all instruments, have been observed. In particular, in the early winter midlatitudes a two-step warming of the winter mesosphere between 65 and 85 km as well as a cooling of the lower mesosphere appear to be real climatological events rather than some short-term geophysical or instrumental random variability.

Cirrus climatological results from lidar measurements at OHP (44°N, 6°E)

A climatology of cirrus clouds over the Observatoire de Haute Provence in France has been constructed from the analysis of ground-based lidar measurements taken from 1997 to 1999. During this period the high-resolution Rayleigh/Mie lidar collected 384 nights of measurements and cirrus profiles are observed in about half of these cases. We find subvisible cirrus (tau < 0.03) constitute ∼20% of cirrus cloud occurrences and that the mean thickness of a subvisible cirrus cloud layer is less than 1 km. A discussion of the error associated with these determinations is also presented.

Validation of temperature measurements from the Halogen Occultation Experiment

The Halogen Occultation Experiment (HALOE) onboard UARS measures profiles of limb path solar attenuation in eight infrared bands. These measurements are used to infer profiles of temperature, gas mixing ratios of seven species, and aerosol extinction at five wavelengths. The objective of this paper is to validate profiles of temperature retrieved from atmospheric transmission measurements in the 2.80-μm CO2 band. Temperatures are retrieved for levels above where aerosol affects the signals (35 km) to altitudes where the signal-to-noise decreases to unity (∽85 km). At altitudes from 45 to 35 km the profile undergoes a gradual transition from retrieved to National Meteorological Center (NMC) temperatures and below 35 km the profile is strictly from the NMC. This validation covers the uncertainty analysis, internal validations, and comparisons with independent measurements. Monte Carlo calculations using all known random and systematic errors determine typical measurement uncertainties of 5 K for altitudes below 80 km. Comparisons of coincident HALOE sunrise and sunset measurements are an indicator of the upper limit of measurement uncertainty. The sunrise-sunset comparisons have random and systematic differences which are less than 10 K for altitudes below 80 km. Comparisons of HALOE to lidar and rocket measurements typically have random differences of ∽5 K for altitudes below 65 km. The mean differences for the correlative comparisons indicate that HALOE temperatures have a cold bias (2 to 5 K) in the upper stratosphere and stratopause.

Ozone and temperature trends in the upper stratosphere at five stations of the Network for the Detection of Atmospheric Composition Change

Upper stratospheric ozone anomalies from the satellite-borne Solar Backscatter Ultra-Violet (SBUV), Stratospheric Aerosol and Gas Experiment II (SAGE II), Halogen Occultation Experiment (HALOE), Global Ozone Monitoring by Occultation of Stars (GOMOS), and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments agree within 5% or better with ground-based data from lidars and microwave radiometers at five stations of the Network for the Detection of Atmospheric Composition Change (NDACC), from 45°S to 48°N. From 1979 until the late 1990s, all available data show a clear decline of ozone near 40 km, by 10%–15%. This decline has not continued in the last 10 years. At some sites, ozone at 40 km appears to have increased since 2000, consistent with the beginning decline of stratospheric chlorine. The phaseout of chlorofluorocarbons after the International Montreal Protocol in 1987 has been successful, and is now showing positive effects on ozone in the upper stratosphere. Temperature anomalies near 40 km altitude from European Centre for Medium Range Weather Forecast reanalyses (ERA-40), from National Centers for Environmental Prediction (NCEP) operational analyses, and from HALOE and lidar measurements show good consistency at the five stations, within about 3 K. Since about 1985, upper stratospheric temperatures have been fluctuating around a constant level at all five NDACC stations. This non-decline of upper stratospheric temperatures is a significant change from the more or less linear cooling of the upper stratosphere up until the mid-1990s, reported in previous trend assessments. It is also at odds with the almost linear 1 K per decade cooling simulated over the entire 1979–2010 period by chemistry–climate models (CCMs). The same CCM simulations, however, track the historical ozone anomalies quite well, including the change of ozone tendency in the late 1990s.

Subtropical tropopause break as a possible stratospheric source of ozone in the tropical troposphere

Abstract The understanding of the transport of trace chemical species between the stratosphere and the troposphere is necessary for global change prediction. Until recently it was believed that stratospheric inputs, through jet streaks and tropopause folding, should occur only at extratropical latitudes. A case study of a tropopause fold was reported at Pointe-a-Pitre by Gouget et al . (1996). We presently corroborate this first case study by new observations in the Indian Ocean suggesting that stratosphere-troposphere exchanges, induced by the subtropical jet, are actually occurring near the edge of the tropics. Key to these exchanges is the crucial region of the junction between the lowermost stratosphere and the tropical and extratropical tropospheres, defined by Holton (1996) as the intersection zone of the 2 PVU potential vorticity surface with the 380 K potential temperature level. In this paper, the wind and ozone climatological context is given using vertical radiosounding data from Reunion Island (France, 21 °S, 55 °E) and Irene (South Africa, 25 °S, 28 °E). A case of subtropical tropopause fold occurring between Madagascar and Reunion Islands is presented using ECMWF and TOMS data. Observations are found to be fairly accordant with a dynamical jet analysis and suggest that stratospheric air intrusions are possible during winter in the sub-tropics edges.