Backscatter Ultraviolet Instrument Research Articles

Apportionment of long-term trends in different sections of total ozone column over tropical region.

Theadditive time-series decomposition analysis was performed on National Oceanic and Atmospheric Administration Solar Backscatter Ultraviolet Instrument Merge satellite dataset version 8.6 for the period January 1979 to December 2019 with an objective to detect and apportion long-term trends present in the total ozone column (TOC) and the long-term trends exist in the respective ozone contents present in the vertical sub-columns constituting the TOC viz. upper, middle and lower stratosphere as well as near-surface for the tropical region. Linear regression analysis was performed on the deseasonalized monthly mean time series of TOC and corresponding ozone contents present in each partitioned layer for three different time spans, viz. 1979-2019 (complete time series), 1979-1998 (pre-inflection years), and 1999-2019 (post-inflection years), where 1998 was taken as inflection year. For the complete time-series, statistically significant negative trends were observed in TOC and corresponding ozone contents in the sub-columns over most of the tropical region. Expectedly, during pre-inflection years, strong negative trends were noted for TOC and ozone contents in the partitioned vertical layers. In contrast, during the post-inflection year time span, long-term trends in TOC were statistically insignificant over two-third of the tropical region, but one-third of the subtropical region exhibited negative trends in TOC. During this time span, positive trends were observed in the ozone contents present in the upper stratospheric sub-column. However, negative trends in ozone contents persisted in the middle and the lower stratosphere. It wasinterestingto note that the ozone contents confined in near-surface layer manifested strong negative trends during pre-inflection years and the same reversed into strong positive trends that in post-inflection span. The observed, contrasting, long-term trends and variability in the respective partitioned layer of the TOC confounded any clear sign of recovery in the TOC over the tropical region. The continuation of declining trends in the middle stratosphere and increasing trends in the near-surface layer of ozone contents is a matter of concern.

Homogenizing and estimating the uncertainty in NOAA's long-term vertical ozone profile records measured with the electrochemical concentration cell ozonesonde

Abstract. NOAA's program of long-term monitoring of the vertical distribution of ozone with electrochemical concentration cell (ECC) ozonesondes has undergone a number of changes over the 50-year record. In order to produce a homogenous data set, these changes must be documented and, where necessary, appropriate corrections applied. This is the first comprehensive and consistent reprocessing of NOAA's ozonesonde data records that corrects for these changes using the rawest form of the data (cell current and pump temperature) in native resolution as well as a point-by-point uncertainty calculation that is unique to each sounding. The reprocessing is carried out uniformly at all eight ozonesonde sites in NOAA's network with differences in sensing solution and ozonesonde types accounted for in the same way at all sites. The corrections used to homogenize the NOAA ozonesonde data records greatly improve the ozonesonde measurements with an average one sigma uncertainty of ±4–6 % in the stratosphere and ±5–20 % in the troposphere. A comparison of the integrated column ozone from the ozonesonde profile with co-located Dobson spectrophotometers total column ozone measurements shows agreement within ±5 % for > 70 % of the profiles. Very good agreement is also found in the stratosphere between ozonesonde profiles and profiles retrieved from the Solar Backscatter Ultraviolet (SBUV) instruments.

The representation of solar cycle signals in stratospheric ozone – Part 1: A comparison of recently updated satellite observations

Abstract. Changes in incoming solar ultraviolet radiation over the 11-year solar cycle affect stratospheric ozone abundances. It is important to quantify the magnitude, structure, and seasonality of the associated solar-ozone response (SOR) to understand the impact of the 11-year solar cycle on climate. Part 1 of this two-part study uses multiple linear regression analysis to extract the SOR in a number of recently updated satellite ozone datasets covering different periods within the epoch 1970 to 2013. The annual mean SOR in the updated version 7.0 (v7.0) Stratospheric Aerosol and Gas Experiment (SAGE) II number density dataset (1984–2004) is very consistent with that found in the previous v6.2. In contrast, we find a substantial decrease in the magnitude of the SOR in the tropical upper stratosphere in the SAGE II v7.0 mixing ratio dataset (∼ 1 %) compared to the v6.2 (∼ 4 %). This difference is shown to be largely attributable to the change in the independent stratospheric temperature dataset used to convert SAGE II ozone number densities to mixing ratios. Since these temperature records contain substantial uncertainties, we suggest that datasets based on SAGE II number densities are currently most reliable for evaluating the SOR. We further analyse three extended ozone datasets that combine SAGE II v7.0 number densities with more recent GOMOS (Global Ozone Monitoring by Occultation of Stars) or OSIRIS (Optical Spectrograph and Infrared Imager System) measurements. The extended SAGE–OSIRIS dataset (1984–2013) shows a smaller and less statistically significant SOR across much of the tropical upper stratosphere compared to the SAGE II data alone. In contrast, the two SAGE–GOMOS datasets (1984–2011) show SORs that are in closer agreement with the original SAGE II data and therefore appear to provide a more reliable estimate of the SOR. We also analyse the SOR in the recent Solar Backscatter Ultraviolet Instrument (SBUV) Merged Ozone Dataset (SBUVMOD) version 8.6 (VN8.6) (1970–2012) and SBUV Merged Cohesive VN8.6 (1978–2012) datasets and compare them to the previous SBUVMOD VN8.0 (1970–2009). Over their full lengths, the three records generally agree in terms of the broad magnitude and structure of the annual mean SOR. The main difference is that SBUVMOD VN8.6 shows a smaller and less significant SOR in the tropical upper stratosphere and therefore more closely resembles the SAGE II v7.0 mixing ratio data than does the SBUV Merged Cohesive VN8.6, which has a more continuous SOR of ∼ 2 % in this region. The sparse spatial and temporal sampling of limb satellite instruments prohibits the extraction of sub-annual variations in the SOR from SAGE-based datasets. However, the SBUVMOD VN8.6 dataset suggests substantial month-to-month variations in the SOR, particularly in the winter extratropics, which may be important for the proposed high-latitude dynamical response to the solar cycle. Overall, the results highlight substantial uncertainties in the magnitude and structure of the observed SOR from different satellite records. The implications of these uncertainties for understanding and modelling the effects of solar variability on climate should be explored.

The influence of PMCs on water vapor and drivers behind PMC variability from SOFIE observations

Observations from the Solar Occultation For Ice Experiment (SOFIE) are used to quantify relationships between polar mesospheric clouds (PMC) and their environment. Dehydration due to ice growth is found to be greatest ∼1.8km above the height of peak ice mass density on average, and H2O enhancement due to sublimation is greatest near the bottom of the PMC layer. The dehydration and hydration layers contain a similar amount of H2O, although less than is found in ice layers, a difference that may be due to meridional transport. Because PMCs modify the surrounding water vapor, PMC–H2O relationships can be misleading and recommendations are made for dealing with this issue. The dependence of PMCs on water vapor and temperature was quantified, accounting for the effects of ice on water vapor. The approach examined inter-annual variations and considered the subset of PMCs detected by the Solar Backscatter Ultraviolet (SBUV) instruments, which are less sensitive than SOFIE. Results in the Northern Hemisphere indicate that PMC variations are dominated by temperature, but that a combination of temperature and water vapor provides the best explanation of the observations. In the Southern Hemisphere PMC variability is attributed primarily to temperature, with water vapor playing a minor role. The subset of SBUV PMCs are found to be one third as sensitive to changing temperature as the entire PMC population observed by SOFIE. Finally, an approach is presented which allows temperature and water vapor anomalies to be estimated from various PMC data sets such as SBUV. Using recently reported SBUV PMC trends at 64–74°N latitude with the results of this study indicates a cooling trend of −0.27±0.14Kdecade−1 and a water vapor increase of +0.66±0.34%decade−1 (both at 80–84km). This cooling trend agrees with reports based on observations in the middle atmosphere at similar latitudes. The water vapor increase is lower than expected due to increasing methane, although this difference may be consistent with H2O loss due to photolysis at PMC altitudes.

Updated PMC trends derived from SBUV data

AbstractPrevious analysis of polar mesospheric clouds (PMCs) observed by Solar Backscatter Ultraviolet (SBUV) instruments found that long‐term variations in PMC brightness and occurrence frequency were anticorrelated with solar activity and that an increasing secular trend was present at most latitudes. In this paper, long‐term PMC variations are presented in terms of ice water content (IWC), a physically based variable which is easier to interpret than previously reported UV albedo values. This model‐based conversion from albedo to IWC removes most scattering angle effects. The derived long‐term PMC variations in the SBUV data set are qualitatively the same using either an empirically derived adjustment for local time effects or no adjustment (i.e., assuming cancelation of interannual variations in tidally induced amplitude and/or phase). When we use stratospheric ozone variations as a proxy for mesospheric temperature changes, as suggested by recent model studies, we can explain more of the long‐term IWC variability than if we use a linear trend. These results show that PMC ice water content in bright clouds increased rapidly from 1979 through the late 1990s and has been approximately constant from the late 1990s through 2013. The numerical value and sign of this trend during the last 15 years depend on the choice of end points and latitude band. Simultaneously, the solar response of IWC observed by SBUV has weakened during the most recent cycle in the Northern Hemisphere, but increased in the Southern Hemisphere.

Comparison of spectral radiance responsivity calibration techniques used for backscatter ultraviolet satellite instruments

Methods of absolute radiometric calibration of backscatter ultraviolet (BUV) satellite instruments are compared as part of an effort to minimize pre-launch calibration uncertainties. An internally illuminated integrating sphere source has been used for the Shuttle Solar BUV, Total Ozone Mapping Spectrometer, Ozone Mapping Instrument, and Global Ozone Monitoring Experiment 2 using standardized procedures traceable to national standards. These sphere-based spectral responsivities agree to within the derived combined standard uncertainty of 1.87% relative to calibrations performed using an external diffuser illuminated by standard irradiance sources, the customary spectral radiance responsivity calibration method for BUV instruments. The combined standard uncertainty for these calibration techniques as implemented at the NASA Goddard Space Flight Center’s Radiometric Calibration and Development Laboratory is shown to less than 2% at 250 nm when using a single traceable calibration standard.

Interpreting the 35 year SBUV PMC record with SOFIE observations

AbstractObservations of polar mesospheric clouds (PMC) from the solar backscatter ultraviolet (SBUV) instruments are compared to the Solar Occultation For Ice Experiment (SOFIE). SBUV measurements are used to determine the PMC vertical column ice mass abundance (or ice water content, IWC) for comparison with SOFIE. Improved SBUV simulations are employed that accounted for nonspherical particles, and the multiwavelength measurements are used to retrieve particle size as the median radius (rm) of a Gaussian distribution. SBUV IWC retrievals incorporated rm as temporally smoothed values, to reduce propagation of rm errors but capture the effects of variable rm in IWC. For seasonal averages the SBUV‐SOFIE rm differences are 22 ± 6% in the Northern Hemisphere (NH) and 22 ± 12% in the Southern Hemisphere (SH), and IWC differences were 3 ± 6% in the NH and 7 ± 12% in the SH. The SBUV record was used to derive IWC and particle size for 1979–2013 for 64–74°N. The results indicate a secular increase in rm of 0.23 ± 0.16 nm decade−1 and a linear IWC trend of 2.6 ± 1.2% decade−1. Multiple regression of IWC to solar and trend terms reveals that 1.6% decade−1 of the IWC trend can be attributed to a decline in solar Lyman α since 1979. The residual IWC trend from multiple regression (1.0% decade−1), presumably due to other external forcing such as changing atmospheric composition, is a factor of six less than PMC trends previously reported from a shorter time series of SBUV data.

Comparison of profile total ozone from SBUV (v8.6) with GOME-type and ground-based total ozone for a 16-year period (1996 to 2011)

Abstract. This paper describes the comparison of the variability of total column ozone inferred from the three independent multi-year data records, namely, (i) Solar Backscatter Ultraviolet Instrument (SBUV) v8.6 profile total ozone, (ii) GTO (GOME-type total ozone), and (iii) ground-based total ozone data records covering the 16-year overlap period (March 1996 through June 2011). Analyses are conducted based on area-weighted zonal means for 0–30° S, 0–30° N, 50–30° S, and 30–60° N. It has been found that, on average, the differences in monthly zonal mean total ozone vary between −0.3 and 0.8 % and are well within 1%. For GTO minus SBUV, the standard deviations and ranges (maximum minus minimum) of the differences regarding monthly zonal mean total ozone vary between 0.6–0.7% and 2.8–3.8% respectively, depending on the latitude band. The corresponding standard deviations and ranges regarding the differences in monthly zonal mean anomalies show values between 0.4–0.6% and 2.2–3.5%. The standard deviations and ranges of the differences ground-based minus SBUV regarding both monthly zonal means and anomalies are larger by a factor of 1.4–2.9 in comparison to GTO minus SBUV. The ground-based zonal means demonstrate larger scattering of monthly data compared to satellite-based records. The differences in the scattering are significantly reduced if seasonal zonal averages are analyzed. The trends of the differences GTO minus SBUV and ground-based minus SBUV are found to vary between −0.04 and 0.1% yr−1 (−0.1 and 0.3 DU yr−1). These negligibly small trends have provided strong evidence that there are no significant time-dependent differences among these multi-year total ozone data records. Analyses of the annual deviations from pre-1980 level indicate that, for the 15-year period of 1996 to 2010, all three data records show a gradual increase at 30–60° N from −5% in 1996 to −2% in 2010. In contrast, at 50–30° S and 30° S–30° N there has been a levelling off in the 15 years after 1996. The deviations inferred from GTO and SBUV show agreement within 1%, but a slight increase has been found in the differences during the period 1996–2010.

Performance Monitoring of the S-NPP Ozone Mapping and Profiler Suite’s Sensor Data Records

Launched on October 28, 2011, the S-NPP satellite carried an ozone mapping and profiler suite (OMPS) of sensors. OMPS opened its aperture door on January 26, 2012 to begin its Earth observation mission. After an early orbit checkout of the sensors for a couple of months and an intensive evaluation of the sensor data for several more months, an initial on-orbit calibration for OMPS was established using data acquired during these periods. To date in 2013, this sensor system calibration has been applied to produce OMPS nadir sensor data records (SDRs) and the resulting ozone environment data records (EDRs). This paper provides an evaluation of the combined performance of the orbital OMPS nadir sensors coupled with the ground data processing system for the current SDR's provisional status, and offers lessons learned during the first 1.5 years of operation. Examples of the sensors' short-term and limited long-term responses are provided, including cross-comparisons of OMPS EDRs with concurrent solar backscatter ultraviolet instrument (SBUV2) data from other NOAA orbital sensors, to illustrate the on-orbit stability of the data products despite some secular changes to the calibration and sensor.

Long-term changes in the upper stratospheric ozone at Syowa, Antarctica

Abstract. Analyses of stratospheric ozone data determined from Dobson–Umkehr measurements since 1977 at the Syowa (69.0° S, 39.6° E), Antarctica, station show a significant decrease in ozone at altitudes higher than that of the 4 hPa pressure level during the 1980s and 1990s. Ozone values over Syowa have remained low since 2001. The time series of upper stratospheric ozone from the homogenized NOAA SBUV (Solar Backscatter Ultraviolet Instrument)(/2) 8.6 overpass data (±4°, 24 h) are in qualitative agreement with those from the Syowa station data. Ozone recovery during the austral spring over the Syowa station appears to be slower than predicted by the equivalent effective stratospheric chlorine (EESC) curve. The long-term changes in the station's equivalent latitude (indicative of vortex size/position in winter and spring) are derived from MERRA (Modern Era Retrospective-analysis for Research and Applications) reanalyses at ~ 2 and ~ 50 hPa. These data are used to attribute some of the upper and middle stratospheric ozone changes to the changes in vortex position relative to the station's location. In addition, high correlation of the Southern Hemisphere annular mode (SAM) with polar upper stratospheric ozone during years of maximum solar activity points toward a strong relationship between the strength of the Brewer–Dobson circulation and the polar stratospheric ozone recovery. In the lower stratosphere, ozone recovery attributable to CFCs (chlorofluorocarbons) is still not definitive, whereas the recovery of the upper stratosphere is slower than predicted. Further research indicates that dynamical and other chemical changes in the atmosphere are delaying detection of recovery over this station.

Characterization of in band stray light in SBUV/2 instruments

Abstract. Significant in-band stray light (IBSL) error at solar zenith angle (SZA) values larger than 77° near sunset in 4 SBUV/2 (Solar Backscattered Ultraviolet) instruments, on board the NOAA-14, 17, 18 and 19 satellites, has been characterized. The IBSL error is caused by large surface reflection and scattering of the air-gapped depolarizer in front of the instrument's monochromator aperture. The source of the IBSL error is direct solar illumination of instrument components near the aperture rather than from earth shine. The IBSL contamination at 273 nm can reach 40% of earth radiance near sunset, which results in as much as a 50% error in the retrieved ozone from the upper stratosphere. We have analyzed SBUV/2 albedo measurements on both the dayside and nightside to develop an empirical model for the IBSL error. This error has been corrected in the V8.6 SBUV/2 ozone retrieval.

The version 8.6 SBUV ozone data record: An overview

AbstractUnder a NASA program to produce long‐term data records from instruments on multiple satellites, data from a series of nine Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments have been reprocessed to create a coherent ozone time series. Data from the BUV instrument on Nimbus 4, SBUV on Nimbus 7, and SBUV/2 instruments on NOAA 9, 11, 14, 16, 17, 18, and 19 covering the period 1970–1972 and 1979–2011 were used to create a long‐term data set. The goal is an ozone Earth Science Data Record—a consistent, calibrated ozone time series that can be used for trend analyses and other studies. In order to create this ozone data set, the radiances were adjusted and used to reprocess the entire data records for each of the nine instruments. Interinstrument comparisons during periods of overlap as well as comparisons with data from other satellite and ground‐based instruments were used to evaluate the consistency of the record and make calibration adjustments as needed. Additional improvements in this version 8.6 processing included the use of the Brion, Daumont, and Malicet ozone cross sections, and a cloud‐height climatology derived from Aura OMI measurements. Validation of the reprocessed ozone shows that total column ozone is consistent with the Brewer/Dobson network to within about 1% for the new time series. Comparisons with MLS, SAGE, sondes, and lidar show that ozone at individual levels in the stratosphere is generally consistent to within 5%.

Validation of ozone monthly zonal mean profiles obtained from the version 8.6 Solar Backscatter Ultraviolet algorithm

Abstract. We present the validation of ozone profiles from a number of Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments that were recently reprocessed using an updated (version 8.6) algorithm. The SBUV data record spans a 41 yr period from 1970 to 2011 with a 5 yr gap in the 1970s. The ultimate goal is to create a consistent, well-calibrated data set of ozone profiles that can be used for climate studies and trend analyses. SBUV ozone profiles have been intensively validated against satellite profile measurements from the Microwave Limb Sounders (MLS) (on board the UARS and Aura satellites) and the Stratospheric Aerosol and Gas Experiment (SAGE II) and ground-based observations from the microwave spectrometers, lidars, Umkehr instruments and balloon-borne ozonesondes. In the stratosphere between 25 and 1 hPa the mean biases and standard deviations are mostly within 5% for monthly zonal mean ozone profiles. Above and below this layer the vertical resolution of the SBUV algorithm decreases. We combine several layers of data in the troposphere/lower stratosphere to account for the lower resolution. The bias in the SBUV tropospheric/lower stratospheric combined layer relative to similarly integrated columns from Aura MLS, ozonesonde and Umkehr instruments varies within 5%. We also estimate the drift of the SBUV instruments and their potential effect on the long-term stability of the combined data record. Data from the SBUV instruments that collectively cover the 1980s and 2000s are very stable, with drifts mostly less than 0.5% per year. The features of individual SBUV(/2) instruments are discussed and recommendations for creating a merged SBUV data set are provided.

A comparison of 40 years of SBUV measurements of column ozone with data from the Dobson/Brewer network

AbstractTotal column ozone data from the Nimbus‐4 Backscatter UltraViolet (BUV) instrument, Nimbus‐7 Solar Backscatter Ultraviolet (SBUV) instrument, as well as from seven NOAA SBUV/2 instruments have been newly reprocessed with the Version 8.6 ozone retrieval algorithm. This yields a coherent data set that, unlike the Total Ozone Mapping Spectrometer ozone record, has no data gaps or significant time periods with large uncertainties due to calibration issues from 1979 to the present. The column ozone data from the first 3 years of the BUV record (1970–1972) is of high quality and can be used to extend the satellite ozone record back over 40 years. With the new algorithm, an improved total column ozone value is calculated by summing up the profile information as opposed to a single direct column measurement, and the algorithm is optimized for the detection of long‐term trends. The results from this processing of these data have been systematically compared to total ozone data from Brewer and Dobson spectrophotometers for many individual ground stations as a function of time, satellite solar zenith angle, and latitude. The time series comparisons show an agreement within ± 1% over the past 40 years with the bias approaching zero over the last decade. The aerosols associated with the eruption of Mt Pinatubo in 1991 produced an underestimation of ozone for our retrievals at high slant columns while the near‐nadir values were relatively unaffected. There is very little systematic offset between the satellite and ground‐based measurements as a function of latitude with the Nimbus‐4 BUV data (1970–1976) showing the largest offsets. The comparisons as a function of satellite solar zenith angle show consistent behavior for all instruments. Comparisons with ozonesonde data show good agreement in the integrated column up to 25 hPa with differences of no more than 5%.

Solar response in tropical stratospheric ozone: a 3-D chemical transport model study using ERA reanalyses

Abstract. We have used an off-line 3-D chemical transport model (CTM) to investigate the 11-yr solar cycle response in tropical stratospheric ozone. The model is forced with European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analysis (ERA-40/operational and ERA-Interim) data for the 1979–2005 time period. We have compared the modelled solar response in ozone to observation-based data sets that are constructed using satellite instruments such as Total Ozone Mapping Spectrometer (TOMS), Solar Backscatter UltraViolet instrument (SBUV), Stratospheric Aerosol and Gas Experiment (SAGE) and Halogen Occultation Experiment (HALOE). A significant difference is seen between simulated and observed ozone during the 1980s, which is probably due to inhomogeneities in the ERA-40 reanalyses. In general, the model with ERA-Interim dynamics shows better agreement with the observations from 1990 onwards than with ERA-40. Overall both standard model simulations are partially able to simulate a "double peak"-structured ozone solar response with a minimum around 30 km, and these are in better agreement with HALOE than SAGE-corrected SBUV (SBUV/SAGE) or SAGE-based data sets. In the tropical lower stratosphere (TLS), the modelled solar response with time-varying aerosols is amplified through aliasing with a volcanic signal, as the model overestimates ozone loss during high aerosol loading years. However, the modelled solar response with fixed dynamics and constant aerosols shows a positive signal which is in better agreement with SBUV/SAGE and SAGE-based data sets in the TLS. Our model simulations suggests that photochemistry contributes to the ozone solar response in this region. The largest model-observation differences occur in the upper stratosphere where SBUV/SAGE and SAGE-based data show a significant (up to 4%) solar response whereas the standard model and HALOE do not. This is partly due to a positive solar response in the ECMWF upper stratospheric temperatures which reduces the modelled ozone signal. The large positive upper stratospheric solar response seen in SBUV/SAGE and SAGE-based data can be reproduced in model runs with fixed dynamical fields (i.e. no inter-annual meteorological changes). As these runs effectively assume no long-term temperature changes (solar-induced or otherwise), it should provide an upper limit of the ozone solar response. Overall, full quantification of the solar response in stratospheric ozone is limited by differences in the observed data sets and by uncertainties in the solar response in stratospheric temperatures.

Evaluation of AIM CIPS measurements of Polar Mesospheric Clouds by comparison with SBUV data

Polar Mesospheric Cloud (PMC) observations from the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) experiment are compared to results from the Solar Backscatter UltraViolet (SBUV/2) instrument for two PMC seasons in each hemisphere, from 2007–2009. For these comparisons, a CIPS cloud retrieval algorithm based on a technique similar to that used in the operational SBUV PMC retrievals was developed. CIPS nadir data analyzed this way agree with SBUV data to better than 1% (2%) for PMC occurrence frequency (albedo). Non-nadir, forward scattering CIPS data are also analyzed with the SBUV-type algorithm, enabling an indirect verification of the CIPS operational retrievals. CIPS operational PMC frequencies agree with these CIPS–SBUV-type retrievals to within 0.6% at 80–85° latitude, and are lower by 9.8% at 70–75° latitude. Overall, the comparisons show that the v3.20 CIPS operational cloud retrievals are valid for scientific research.

Polar mesospheric clouds (PMCs) observed by the Ozone Monitoring Instrument (OMI) on Aura

Backscattered ultraviolet (BUV) instruments designed for measuring stratospheric ozone profiles have proven to be robust tools for observing polar mesospheric clouds (PMCs). These measurements are available for more than 30 years and have been used to demonstrate the existence of long‐term variations in PMC occurrence frequency and brightness. The Ozone Monitoring Instrument (OMI) on the EOS Aura satellite provides new and improved capabilities for PMC characterization. OMI uses smaller pixels than previous BUV instruments, which increases its ability to identify PMCs and discern more spatial structure, and its wide cross‐track viewing swath provides full polar coverage up to 90° latitude every day in both hemispheres. This cross‐track coverage allows the evolution of PMC regions to be followed over several consecutive orbits. Localized PMC variations determined from OMI measurements are consistent with coincident Solar Backscatter Ultraviolet model 2 (SBUV/2) measurements. Nine seasons of PMC observations from OMI are now available and clearly demonstrate the advantages of these measurements for PMC analysis.

SBUV observations of polar mesospheric clouds compared with MLS temperature and water vapor measurements

Earlier studies have shown that Polar Mesospheric Clouds (PMC) occur more frequently in the northern hemisphere (NH) than the SH, consistent with colder NH temperatures. Coincident PMC observations with the Solar Backscatter Ultraviolet instruments on the NOAA‐16 and NOAA‐18 satellite and temperature and water vapor measurements with the Microwave Limb Sounder on the Aura satellite support this result. These coincident measurements also show that, for similar temperatures and water vapor mixing ratios, PMCs occur more frequently and are brighter in the NH than the SH. Possible reasons for these hemispheric differences are discussed.

First evidence of a 27 day solar signature in noctilucent cloud occurrence frequency

This paper presents evidence of a connection between the 27 day modulation of the solar activity and noctilucent cloud (NLC) occurrence frequency as measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) and solar backscatter ultraviolet (SBUV) instruments. Observations show anticorrelations significant at the 90% confidence level between noctilucent cloud occurrence rate anomalies and Lyman‐α irradiance variation during several seasons in both hemispheres. A superposed epoch analysis confirms these results and also reveals a clear recurrence pattern in noctilucent clouds occurrence anomalies with a ∼27 day period. The superposed epoch analysis also shows that the maximum NLC response in the Northern Hemisphere is clearly localized at 0 day phase lag, while in the Southern Hemisphere the maximum response is broader and occurs at 0 ± 2 day phase lag. Microwave Limb Sounder mesospheric products suggest that the more likely driver for the variation in NLC occurrence is temperature instead of water vapor, but the mechanisms responsible for the observed variations are not yet fully understood.

Long term variations in the frequency of polar mesospheric clouds in the Northern Hemisphere from SBUV

Earlier studies have indicated that there is a secular increase in the occurrence frequency of polar mesospheric clouds (PMC), along with an anti‐correlation with the solar activity. The combined data records from the Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments provide the longest satellite record (28 years) of the PMC frequency of occurrence. This record has been analyzed to determine the long‐term variation in the PMC occurrence frequency in each of three latitude bands (54°–64°N, 64°–74°N, 74°–82°N). This analysis includes an adjustment for changes in the local time of measurement due to the satellite orbital drift, to take into account diurnal variations in the PMC frequency. Multiple linear regression fits using solar activity and time show that the occurrence frequency nearly doubles from solar maximum to solar minimum in all latitude bands. There is a long‐term increase in the occurrence frequency ranging from 7% per decade at 64°–74°N to 20% per decade at 74°–82°N. These secular increases are significant at the 95% level for the 74°–82°N and all latitudes combined. We find a time lag of half a year (with an uncertainty of one year) between the minimum solar activity and the maximum PMC activity, consistent with our previous findings for PMC albedo.