Umkehr Layers Research Articles

Annual and semiannual waves in ozone as derived from SBUV vertical global ozone profiles

Global averages of monthly ozone profile data obtained using the solar backscatter ultraviolet (SBUV) instrument, are analyzed for the period from November 1978 to September 1985. Differences between ozone profile data for Umkehr layers 4 to 10 ‐ from 19 km to 53 km, approximately ‐ and the deseasonalized ozone data are first estimated for the entire time period at all Umkehr layers, and are then Fourier analyzed to obtain the amplitude, phase and percentage contribution to the total variance of the first and second harmonics. It is found that the annual wave in global ozone is strong in the high and low stratosphere and weak in the middle stratosphere, whereas the semiannual wave in global ozone is strong in the middle stratosphere. Global ozone deviations at Umkehr layer 7 are compared to the 10.7 cm solar flux to yield a strong positive correlation for the entire period, which implies that the quasi‐semiannual wave in global ozone depends on the quasi‐semiannual wave of the 10.7 cm solar flux.

Lidar measurements and Umkehr observations of the ozone vertical distribution at the Observatoire de Haute-Provence

Lidar measurements and Umkehr observations of the ozone vertical distribution are performed routinely at the Observatoire de Haute-Provence (44 N, 5 E), since respectively 1985 and 1983. as part of the newly implemented Automated Dobson Network and Network for the Detection of Stratospheric Changes. From 1985 to 1988, the corresponding data base includes more than 1000 Umkehr observations and 95 lidar profiles, allowing for a comparison between the two methods over 82 coincidences. Such a comparison has been performed with respect to the three Umkehr retrieval methods presently available : conventional Umkehr, short Umkehr and the newly developed ‘new-conventional’ Umkehr methods. The analysis shows that the ozone values, as retrieved by the new-conventional Umkehr method, which takes into account more accurately the natural variability of the ozone vertical distribution and the temperature influence on the ozone absorption coefficients, are in rather good agreement with the lidar measurements. No statistically significant bias can be observed in the Umkehr layers 4 to 7 between the two methods of measurement. In the Umkehr layer 8, a positive bias between the Umkehr retrievals and the lidar measurements can be observed, mainly during the winter months, which can be accounted for by signal induced noise effects leading to an underestimate of the ozone concentration from the lidar measurements, and by the large variability in the daily temperature profiles as observed during the same period of the year.

Trend analysis of aerosol‐corrected Umkehr ozone profile data through 1987

Trend analysis of stratospheric Umkehr ozone profile data from 10 stations over the period 1977–1987 is considered. Two different correction methods to adjust the Umkehr data measurements for errors caused by volcanic aerosols, the theoretical model‐based corrections method of DeLuisi et al. (1989) and an empirical method based on use of a composite optical thickness series, are examined and compared. Linear trend models which also include the F10.7 solar flux term to account for solar cycle variations in the Umkehr data were estimated for the Umkehr data at each station using both aerosol error correction methods. The trend and solar flux effect estimates are generally similar for both methods. The results indicate a significant overall negative trend, exclusive of trend variations associated with solar flux variations, of the order of −0.5% per year in Umkehr layers 7–9 over the period 1977–1987, and a significant positive solar cycle association in all layers 4–9, with the maximum solar effect estimated to occur in layer 9. A comparison between the solar backscattered ultraviolet (SBUV) monthly average ozone profile data near the 10 Umkehr stations and the corresponding Umkehr data corrected for aerosol errors is performed to investigate possible drifts, and linear drifts are estimated for the differences between SBUV and corrected Umkehr data at each station. The results show a substantial overall negative linear drift in SBUV data relative to corrected Umkehr data in layers 7–9, with estimated values of the drift of the order of −1.0% per year for layers 8 and 9.

Comparisons of satellite ozone data in the lower stratosphere for 1978/1979

Lower stratospheric zonal mean ozone distributions are compared in both hemispheres from the Nimbus 7 Solar Backscattered Ultraviolet (SBUV) (version 5) and Limb Infrared Monitor of the Stratosphere (LIMS) data sets. Generally, there is very good agreement with regard to individual profile shape and magnitude and with their longitudinal variability about the zonal mean. When the data are considered in terms of Umkehr layer amounts, there are subtle but significant differences in the respective meridional ozone gradients and their variations with time. These differences do not seem to be related to SBUV diffuser plate drift effects because differences are not apparent in the upper stratosphere and because they are pronounced in late spring to summer of both hemispheres of the lower stratosphere. It appears more likely that the differences are related to a changing ozone optical depth (LIMS) and constraints to an assumed profile climatology in the retrieval (SBUV). Stratospheric Aerosol and Gas Experiment (SAGE) satellite ozone results are also presented for April 1979 at 10, 16, and 30 mbar, and they tend to split the differences between SBUV and LIMS. Additional comparisons with electrochemical concentration cell (ECC) sonde and Mastsonde ozone profiles are presented in an attempt to sort out the more noticeable differences. At tropical latitudes, LIMS ozone is too large in layer 4; no measurements were available for layer 3. SBUV is too high in layer 3. Layer 5 results are not conclusive, but LIMS appears high by about 10% while SBUV appears low by a similar percentage. At higher latitudes (38°–55°N) of the northern hemisphere, ECC and Mastsonde comparison results are not accurate enough to differentiate between SBUV and LIMS in layer 5, even though there are persistent differences in the satellite data of the order of 10–15%. LIMS ozone is about 10% larger than balloon values in layer 4 during northern hemisphere spring. Both SBUV and LIMS are too high in layer 3. The sign of the SBUV/LIMS satellite differences changes with latitude in layers 4 and 5, and the differences in layer 4 approach 20% at mid‐latitudes during late spring and summer. The May SBUV/LIMS differences at 30°N are similar to the SBUV/ECC differences found at Palestine, Texas (32°N) in the June 1983 Balloon Ozone Intercomparison Campaign (BOIC) I ozone intercomparison. All of this means that annual variations in the SBUV and LIMS satellite ozone data and their meridional gradients in the middle to lower stratosphere must be judged as only qualitative at this time.

Ground-based microwave radiometry to determine stratospheric and mesospheric ozone profiles

From April 1984 to April 1985 a microwave radiometer was operated at Bern (Switzerland, latitude 47°N) measuring the thermal emission of the rotational ozone transition at 142.2 GHz to determine stratospheric and mesospheric ozone abundances in the range ~25–~75 km altitude. From a total of 334 retrieved day-time profiles, monthly mean ozone partial pressures for Umkehr layers 6–10 were calculated. On this basis ozone variations compare favorably with Umkehr data from the nearby Arosa (Switzerland, 150 km east of Bern) station and with a monthly zonal mean model compiled from satellite data by Keating and Young. From the microwave data an annual mean ozone distribution was determined. The method retrieves somewhat larger ozone volume mixing ratios between 25 and 30 km altitude. For the rest of the measurement range of the sensor there is good agreement with 20 year annual mean ozone values from Arosa, with the Krueger and Minzner profile and with the respective annual mean data given by Keating and Young. The microwave ozone sensor can easily be adapted for operational use, where it can supplement and expand the measurement range of the traditional Umkehr network.

A statistical trend analysis of ozonesonde data

A detailed statistical analysis of monthly averages of ozonesonde readings is performed to assess trends in ozone in the troposphere and the lower to mid‐stratosphere. Regression time series models, which include seasonal and trend factors, are estimated for 13 stations located mainly in the mid‐latitudes of the northern hemisphere. At each station, trend estimates are calculated for 14 “fractional” Umkehr layers covering the altitude range from 0 to 33 km. For the 1970–1982 period, our main findings indicate an overall negative trend in ozonesonde data in the lower stratosphere (15–21 km) of about −0.5% per year, and some evidence of a positive trend in the troposphere (0–5 km) of about 0.8% per year. An in‐depth sensitivity study of the trend estimates is performed with respect to various correction procedures used to normalize ozonesonde readings to Dobson total ozone measurements. The main results indicate that the negative trend findings in the 15 to 21‐km altitude region are robust to the normalization procedures considered.

Average ozone profiles for 1979 from the NIMBUS 7 SBUV instrument

Monthly average ozone profiles from the first year of operation of the solar backscattered ultraviolet (SBUV) instrument on the NIMBUS 7 satellite (November 1978 through October 1979) are tabulated for 10° latitude bands from 80°S to 80°N. For each month and latitude zone we list layer ozone amounts and standard deviations for 12 Umkehr layers (approximately 0–60 km) and the average total ozone for each zone. The ozone mixing ratio and number density at the center of each layer are also given. We find that the average SBUV profiles agree with average Umkehr profiles measured at Boulder to within 10% over most of the range. The Krueger‐Minizner model is found to be a good representative ozone model for 45°N, agreeing with an annual average of our measured profiles to better than 10% between 20 and 60 km.

Analysis of upper stratospheric Umkehr ozone profile data for trends and the effects of stratospheric aerosols

A statistical analysis of stratospheric ozone profile data from the Umkehr method is considered for the detection of trends which may be associated with the release of chlorofluoromethanes (CFMs), where possible effects of atmospheric aerosols on the Umkehr measurements are also taken into account. In the statistical trend analysis, time series models have been estimated using monthly averages of Umkehr data over the past 15 to 20 years through 1980 at each of 13 Umkehr stations and at each of the five highest Umkehr layers, 5–9, which cover an altitude range of approximately 24–48 km. The time series regression models incorporate seasonal, trend, and noise factors and an additional factor to explicitly account for the effects of atmospheric aerosols on the Umkehr measurements. At each Umkehr station, the explanatory series used in the statistical model to account for the aerosol effect is a 5 month running average of the monthly atmospheric transmission data at Mauna Loa, Hawaii, the only long running aerosol data available. A random effects model is used to combine the 13 individual station trend estimates from the time series models to form an overall estimate of trend for each Umkehr layer. The analysis indicates statistically significant trends in the upper Umkehr layers 7 and 8 of the order of −0.2 to −0.3% per year over the period 1970–1980, with little trend in the lower layers 5 and 6. The results of the estimation of trends as well as aerosol effects for the Umkehr data are compared with recent corresponding theoretical predictions.

Bayesian probability calculations for stratospheric ozone modifications

A methodology is described for combining current information on ozone perturbations estimated from chemical model calculations and ozone trend analyses. This approach, known as Bayes analysis, yields probabilities for different amounts of change in total column ozone and ozone in Umkehr layer 8 (near 40 km). An examination is made of the sensitivity of the results to two different chemical models, and uncertainties involved in chemical model and trend analysis estimates.

Analysis of upper stratospheric ozone profile data from the ground‐based umkehr method and the Nimbus‐4 BUV satellite experiment

We present a statistical trend analysis of stratospheric ozone profile data in the five highest Umkehr layers (layers 5–9), which cover approximately the 24 to 48 km region of the stratosphere, from the ground‐based Umkehr measurements and from the Nimbus‐4 BUV satellite experiment. Time series modeling of monthly Umkehr profile ozone data in Umkehr layers 5–9 from a collection of 13 ground‐based Dobson stations over the period 1958–1980 is considered for the detection of trends that may be associated with the release of CFMs. Using a random effects model to account for various sources of variation among individual station trend estimates in each Umkehr layer, our findings show little evidence of any significant trend in ozone in any of the Umkehr layers over the period 1970–1980. These trend results depend to some extent, of course, on the particular choice of explanatory factors employed in the time series models to describe variations in the Umkehr data. A possibly important source of variation on the Umkehr data, especially in the uppermost two or three layers, which has not been explicitly accounted for in these models is the presence of stratospheric aerosols, caused primarily by volcanic activity. The possible impact of such aerosol effects on long‐term trend estimates obtained from Umkehr data is currently under investigation, and results of such an analysis will be presented in a future report. We also analyze BUV ozone profile data from the Nimbus‐4 satellite over the period April 1970 to April 1977. These BUV profile data have first been interpolated to yield ozone amounts corresponding to the Umkehr layers 5–9. The globe has then been made into a grid with blocks of 10° latitude by 20° longitude, and monthly averages of ozone have been computed for the layers 5–9 for each block. Linear regression models that contain a linear trend term are estimated from the monthly BUV time series data for each of the five Umkehr layers in each block over the 7 year period April 1970 to April 1977. The results show a large negative overall trend on the order of 1.5 to 2.0% per year in the BUV data at Umkehr layers 8 and 9, with little or no trend indicated in layers 5 and 6. However, a comparison of monthly BUV satellite data with the corresponding Umkehr ground profile ozone data is also performed, which indicates a substantial downward drift in the BUV data at layers 8 and 9, relative to the collection of Umkehr stations, of about the same magnitude as the estimated trend in BUV data in those layers.

Results of a comparison between Umkehr and ozone‐sonde data

AbstractResults of an intercomparison of 26 pairs of vertical distributions of ozone obtained at Aspendale (38°S) using the Umkehr technique and Mast‐Brewer ozone‐sondes on the same days are presented. Correlation coefficients between the integrated ozone amounts found by the two methods for Umkehr layers 2, 3 and 4 are found to be between 0.82 and 0.88, while the correlations for layers 1 and 5 are 0.47 and 0.37 respectively. On the average the Umkehr method indicates substantially more ozone in layers 1 and 2 than do the ozone‐sondes, but slightly less in layer 4. The Umkehr method thus tends to smooth out the vertical distribution of ozone relative to that observed by the ozone‐sondes. Results of comparisons at different times and places suggest a variation in the response of the Umkehr method relative to sonde methods.