I. S. McDermid

Ground‐based microwave monitoring of stratospheric ozone

A microwave instrument developed for operational measurements of ozone for the Network for Detection of Stratospheric Change is discussed. The instrument observes two spectral lines near 3-mm wavelength with a bandwidth of 630 MHz, allowing profile retrieval from 20 to 70 km. The observing technique and calibration procedures are described. The measurement forward model and retrieval algorithm are formulated. Preliminary comparisons with a colocated ground-based lidar and the SAGE II instrument are presented. The measurements are shown to typically agree to within 5 to 10 percent.

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.

Comparison of STOIC 1989 ground‐based lidar, microwave spectrometer, and Dobson spectrophotometer Umkehr ozone profiles with ozone profiles from balloon‐borne electrochemical concentration cell ozonesondes

Ground-based measurements of stratospheric ozone using a Jet Propulsion Laboratory (JPL) lidar, a NASA Goddard Space Flight Center (GSFC) lidar, a Millitech Corporation/NASA Langley Research Center (Millitech/LaRC) microwave spectrometer, and a NOAA Dobson ozone spectrophotometer were compared with in situ measurements made quasi-simultaneously with balloon-borne electrochemical concentration cell (ECC) ozonesondes during 10 days of the Stratospheric Ozone Intercomparison Campaign (STOIC). The campaign was conducted at Table Mountain Observatory, California, during the summer of 1989. ECC ozonesondes were flown by NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) personnel as well as by personnel from the NASA Wallops Island Flight Facility (WFF). Within the altitude range of 20–32 km, ozone measurement precisions were estimated to be ±0.6 to ±1.2% for the JPL lidar, ±0.7% for the GSFC lidar, ±4% for the microwave spectrometer, and ±3% for the NOAA ECC ozonesonde instruments. These precisions decreased in the 32 to 38.6-km altitude range to ±1.3, ±1.5, and ±3% to ±10% for the JPL lidar, GSFC lidar, and the ECC sondes, respectively, but remained at ±4% for the microwave instrument. Ozone measurement accuracies in the 20 to 32 km altitude range were estimated to be ±1.2 to ±2.4% for the JPL lidar, ±1.4% for the GSFC lidar, ±6% for the microwave radiometer, and ±5% for the ECC ozonesondes. The accuracies decreased in the 32 to 38.6-km altitude range to ±2.6, ±3.0, ±7, and 1 ± 4% to −4 ± 10% for the JPL lidar, the GSFC lidar, the microwave spectrometer, and the ECC ozonesondes, respectively. While accuracy estimates for the ECC sondes were obtained by combining random and estimated bias errors, the accuracies for the lidar instruments were obtained by doubling the measurement precision figures, with the assumption that such doubling accounts for systematic errors. Within the altitude range of 20–36 km the mean ozone profiles produced by the JPL, GSFC, and the Millitech/LaRC groups did not differ from the mean ECC sonde ozone profile by more than about 2, 4, and 5%, respectively. Six morning Dobson instrument Umkehr observations yielded mean ozone amounts in layers 3 and 5–7 that agreed with comparison ECC ozonesonde data to within ±4%. In layer 4 the difference was 7.8%. (Less favorable comparison data were obtained for six afternoon Umkehr observations made in highly polluted near-surface air.) This good agreement in overall results obtained lends credence to the reliability of the ozone measurements made at Table Mountain Observatory during STOIC 1989.

OPAL: Network for the Detection of Stratospheric Change ozone profiler assessment at Lauder, New Zealand 2. Intercomparison of revised results

Following a blind intercomparison of ozone profiling instruments in the Network for the Detection of Stratospheric Change at Lauder, New Zealand, revisions to the analyses were made resulting in a new data set. This paper compares the revised results from two differential absorption lidars (RIVM and GSFC), a microwave radiometer (Millitech/LaRC), and electrochemical concentration cell (ECC) balloon sondes (NIWA). In general, the results are substantially improved compared to the earlier blind intercomparison. The level of agreement was similar both for single profiles and for the campaign average profile and was approximately 5% for the lidars and the sondes over the altitude range from 15 to 42 km (32 km for sondes). The revised microwave data show a bias of 5–10% high in the region from 22 to 42 km. Starting at 42 km, the lidar errors increase significantly, and comparisons of the microwave results were not possible above this altitude.

OPAL: Network for the Detection of Stratospheric Change Ozone Profiler Assessment at Lauder, New Zealand. 1. Blind Intercomparison.

An intercomparison of ozone-profiling instruments, two differential absorption lidars, a microwave radiometer, electrochemical concentration sondes, and the SAGE II satellite instrument is presented. The ground-based instruments were located at the Network for the Detection of Stratospheric Change (NDSC) primary station at Lauder, New Zealand. The campaign, which took place between April 15 and 29, 1995, strictly followed the NDSC guidelines for a blind intercomparison. Agreement between the measurements was within 15% for single profiles and within 10% for the campaign average, in the region from 20 to 40 km altitude. Outside of this region the differences were greater but can generally be ascribed to the limits of a particular instrument.

Microwave ozone and lidar aerosol profile observations at Table Mountain, California, following the Pinatubo eruption

Ozone profiles measured with a ground-based microwave instrument in years from 1989 through 1992 at latitude 34°N. were searched for effects arising from the eruption of Mount Pinatubo in 1991. Between 20 and 24 km, ozone values after November 1991 were systematically less than in earlier years, and the deviation generally grew with time through May 1992. The minimum was 11 to 15% below values in earlier years, and occurred later than minima observed with ozonesondes at higher latitudes or predicted by some models. Although there was a steady increase in ozone levels in the 18–10 hPa range between February and May 1992, the highest values were not significantly above normal. Effects of the quasi-biennial oscillation on ozone values are much smaller than the observed changes. Colocated lidar observations show that stratospheric aerosol levels were steadily elevated from November 1991 through at least March 1992, with some intermittent activity in late August through October 1991. The main aerosol cloud thus arrived at the same time as depletion first appeared.

Global validation of ENVISAT ozone profiles using lidar measurements

Satellite sensors provide global measurements of ozone concentration that can be used to study the effects of the implementation of the Montreal Protocol. However, a key issue in deriving long-term ozone trends from successive satellite instruments is inter-comparability. Ground-based measurements offer continuous time series, but only at a few locations. The combination of ground-based measurements with satellite data is therefore an effective means to evaluate satellite instrument inter-comparability. In this study, we present validation results of ozone profiles from three atmospheric sensors onboard ENVISAT by comparison with lidar measurements. Results for the SCIAMACHY ozone profiles (version 3.01) show reasonable agreement with ground-based measurements (0 to −20%). The MIPAS full-resolution (version 4.61) dataset has good agreement with lidar (0 to 10%), whereas a small positive bias (up to 20%) was found for the MIPAS reduced-resolution prototype data. GOMOS dark-limb data (version 5.00) agree very well (0 ± 5%) with the correlative data, but underestimate ozone concentration at the polar regions.

Lidar observations of the middle-atmosphere thermal tides at Mauna Loa (19.5°N): comparison with HRDI and GSWM

The tidal signature in the middle atmospheric thermal structure (15 - 95 km) at Mauna Loa, Hawaii, (19.5 degrees N) is investigated using more than 145 hours of nighttime lidar measurements obtained during October 3 - 16, 1996 and October 2 - 11, 1997. The daytime HRDI temperatures taken in September and October 1993 - 1997 and zonally averaged at the same latitude are also used. The nighttime lidar and daytime HRDI temperature evolution and tidal signatures are compared to the predictions of the GSWM tidal model. Agreement is found between lidar and GSWM below 60 km, and between HRDI and GSWM above 85 km. Some significant disagreement is found between 60 and 80 km altitude. In particular, a strong semidiurnal signature is observed by lidar and not predicted by GSWM. It appears that the tidal structure observed by lidar is more representative of that predicted by GSWM at 24 degrees N, suggesting a latitudinal shift between theory and observation. It is not clear whether this shift is related to an indetermination of the tidal source and/or propagation or if the observed differences are simply due to local/regional Local-Solar-Time-related oscillations obscuring the tidal signature.

Mesospheric temperature inversions observed from long-term lidar measurements at mid and low latitudes

Results from new observations of mesospheric temperature inversion layers using long-term lidar measurements at mid- and low-latitudes are presented. Observations of inversions above Table Mountain, California, (34.4 degrees N) and Mauna Loa, Hawaii, (19.5 degrees N) are in very good agreement with previous lidar and satellite observations. At least two distinct types of events have been observed. The winter inversions occur near 70 km altitude at midlatitudes in December-January and about 1 - 2 months laster at subtropical latitudes. The tidal signature in the middle atmospheric thermal structure has been investigated using more than 140 hours of nighttime lidar measurements at TMF during January 1997 and February 1998. The temperature profiles (30 - 85 km) revealed the presence of persistent mesospheric inversions around 65 - 70 km altitude with a clear Local-Solar-Time (LST) dependence. Also, some higher altitude inversions (80 - 85 km) have been observed at lower latitudes around the equinoxes and 1 - 2 months later at mid-latitudes. In particular the temperature minimum systematically observed at the altitude of approximately 80 km and propagating downward throughout the night might also suggest the important role played by the tides.

Climatology of the middle atmosphere temperature from long-term lidar measurements at mid- and low-latitudes

Long term measurements from several lidar instruments, located at 44.0 degrees N, 40.6 degrees N, 34.4 degrees N, and 19.5 degrees N, were used to develop a new climatology of the middle atmosphere temperature. For each instrument, the measurements on every day of the year over the entire record were averaged to build a composite year of temperature profiles. The lidar climatologies were compared to the CIRA-86 model which appears to be systematically too cold between 90 and 95 km, by greater than or equal to 20 K, and possibly 6 - 8 K too warm around 80 km, making its use as a reference questionable at these altitudes. The annual and semi-annual components of the seasonal variability and the 2- to 33-day period variability were also investigated. An annual cycle with 6 - 7 K amplitude in the upper stratosphere, increasing to 15 - 20 K at 80 km, is observed at mid-latitudes. At lower latitudes, a semiannual oscillation (SAO) propagates downward from 85 to 30 km and is characterized by a stronger first cycle than the second (4 K and 2 K amplitude). The 2- to 33- day variability at mid-latitudes shows a maximum during winter around 40 km and in the mesosphere. Finally, sudden seasonal transitions, highly consistent between all instruments, have been observed, in particular in the early winter mid-latitudes with a two-step warming of the mesosphere between 65 and 85 km.