Solar Backscatter Ultraviolet Research Articles

Mesospheric Ice Clouds as Indicators of Upper Atmosphere Climate Change: Workshop on Modeling Polar Mesospheric Cloud Trends; Boulder, Colorado, 10–11 December 2009

The 20‐year‐old speculation that high‐altitude summertime ice clouds (polar mesospheric, or noctilucent, clouds, here denoted MC) are affected by anthropogenic activities has recently received support from a 30‐year time series of solar backscatter ultraviolet (SBUV) satellite measurements. SBUV data reveal a significant trend in bright MC properties. However, the robustness of the trend, extracted from interannual, local time, and solar cycle variability, and its underlying causes remains debatable.

A new ENSO index derived from satellite measurements of column ozone

Abstract. Column Ozone measured in tropical latitudes from Nimbus 7 total ozone mapping spectrometer (TOMS), Earth Probe TOMS, solar backscatter ultraviolet (SBUV), and Aura ozone monitoring instrument (OMI) are used to derive an El Nino-Southern Oscillation (ENSO) index. This index, which covers a time period from 1979 to the present, is defined as the "Ozone ENSO Index" (OEI) and is the first developed from atmospheric trace gas measurements. The OEI is constructed by first averaging monthly mean column ozone over two broad regions in the western and eastern Pacific and then taking their difference. This differencing yields a self-calibrating ENSO index which is independent of individual instrument calibration offsets and drifts in measurements over the long record. The combined Aura OMI and MLS ozone data confirm that zonal variability in total column ozone in the tropics caused by ENSO events lies almost entirely in the troposphere. As a result, the OEI can be derived directly from total column ozone instead of tropospheric column ozone. For clear-sky ozone measurements a +1 K change in Nino 3.4 index corresponds to +2.9 Dobson Unit (DU) change in the OEI, while a +1 hPa change in SOI coincides with a −1.7 DU change in the OEI. For ozone measurements under all cloud conditions these numbers are +2.4 DU and −1.4 DU, respectively. As an ENSO index based upon ozone, it is potentially useful in evaluating climate models predicting long term changes in ozone and other trace gases.

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.

An investigation of long-term variability of ozone 3-dimensional fields in the tropics connected with the 11-year solar cycle and peculiarities of the general atmospheric circulation

In this study, the long-term features of 3-dimensional ozone fields in the tropics (30° S– 30° N) over the 25-year period (1979–2003) based on the solar backscatter ultraviolet (SBUV) version 8 satellite observations were analysed. The application of the Empirical Orthogonal Function (EOF) decomposition allows characteristic spatial and temporal patterns to be identified in fields of partial ozone pressure anomalies, which are closely related to the natural causes such as the 11-year solar cycle, the Quasi Biennial Oscillation (QBO) and the Quasi Biennial Oscillation and an Annual Beat (QBO-AB). Also the vertical structure of the ozone QBO at the equator was investigated in comparison with the vertical structure of the zonal wind QBO. The analysis of the SBUV ozone data showed that the phase of the ozone QBO at the equator remains constant within the layer 10–50 hPa.

Technical Note: A SAGE-corrected SBUV zonal-mean ozone data set

Abstract. A stratospheric vertically resolved, monthly, zonal-mean ozone data set based on Satellite Aerosol and Gas Experiment (SAGE) and Solar Backscatter UltraViolet (SBUV) data spanning 1979–2005 is presented. Drifts in individual SBUV instruments and inter-SBUV biases are corrected using SAGE I and II by calculating differences between coincident SAGE-SBUV measurements. In this way the daily, near-global coverage of SBUV(/2) is combined with the stability and precision of SAGE to provide a homogeneous ozone record suitable for trend analysis. The resultant SAGE-corrected SBUV data set, shows, for example, a more realistic Quasi-Biennial Oscillation signal compared to the one derived from SBUV data alone. Furthermore, this methodology can be used to extend the present data set beyond the lifetime of SAGE II.

Evolution of stratospheric ozone and water vapour time series studied with satellite measurements

Abstract. The long term evolution of stratospheric ozone and water vapour has been investigated by extending satellite time series to April 2008. For ozone, we examine monthly average ozone values from various satellite data sets for nine latitude and altitude bins covering 60° S to 60° N and 20–45 km and covering the time period of 1979–2008. Data are from the Stratospheric Aerosol and Gas Experiment (SAGE I+II), the HALogen Occultation Experiment (HALOE), the Solar BackscatterUltraViolet-2 (SBUV/2) instrument, the Sub-Millimetre Radiometer (SMR), the Optical Spectrograph InfraRed Imager System (OSIRIS), and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartograpY (SCIAMACHY). Monthly ozone anomalies are calculated by utilising a linear regression model, which also models the solar, quasi-biennial oscillation (QBO), and seasonal cycle contributions. Individual instrument ozone anomalies are combined producing an all instrument average. Assuming a turning point of 1997 and that the all instrument average is represented by good instrumental long term stability, the largest statistically significant ozone declines (at two sigma) from 1979–1997 are seen at the mid-latitudes between 35 and 45 km, namely −7.2%±0.9%/decade in the Northern Hemisphere and −7.1%±0.9%/in the Southern Hemisphere. Furthermore, for the period 1997 to 2008 we find that the same locations show the largest ozone recovery (+1.4% and +0.8%/decade respectively) compared to other global regions, although the estimated trend model errors indicate that the trend estimates are not significantly different from a zero trend at the 2 sigma level. An all instrument average is also constructed from water vapour anomalies during 1991–2008, using the SAGE II, HALOE, SMR, and the Microwave Limb Sounder (Aura/MLS) measurements. We report that the decrease in water vapour values after 2001 slows down around 2004–2005 in the lower tropical stratosphere (20–25 km) and has even shown signs of increasing until present. We show that a similar correlation is also seen with the temperature measured at 100 hPa during this same period.

Modulation of natural variability on a trend analysis of the updated cohesive SBUV(/2) total ozone

Utilizing the series of Solar Backscattered Ultraviolet (SBUV) instruments, total ozone measurements from the NASA/Nimbus-7 and the operational NOAA polar orbiting satellites, the NOAA/Climate Prediction Center has compiled a long term ‘cohesive’ SBUV(/2) total ozone dataset. The compilations are aimed at achieving a level of quality suitable for trend analysis. The zonal dataset starts in 1979, and extends to 2007. This paper provides an update of the statistical analysis of the above dataset to examine the impact of the Montreal Protocol and its later revisions. The traditional statistical time series trend, or hockey-stick algorithm, is applied to this cohesive total ozone dataset. The algorithm allows for a change in trend in January 1996. Specifically, we examine the net slope post 1996 to see if it is both positive and statistically significant at the 95% confidence level. Our results indicate the following: (1) For 45°N the net slope after 1996, while positive, is not statistically significant at the 95% level of confidence. Thus, we cannot state that the ozone is actually increasing in this area. (2) For 45°S the net slope remains negative, but not statistically significant at the 95% level of confidence. For this region it appears that the best we can state is that the ozone may be levelling off. Our results do indicate, however, that the ozone variability in the southern hemisphere is impacted by the Antarctic oscillation much like the northern hemisphere is impacted by the Arctic oscillation, suggesting a real linkage of these terms. (3) For the integrated domain of 50°N–50°S, the averaging process diminishes greatly the standard errors of the terms. The result is that the net slope for the integrated domain is +1.24% per decade and is statistically significant at the 95% confidence level. Also, the solar coefficient is statistically significant indicating a tropical dominance of this effect.

Measurements and products from the Solar Backscatter Ultraviolet (SBUV/2) and Ozone Mapping and Profiler Suite (OMPS) instruments

This paper presents an overview of the state of the National Oceanic and Atmospheric Administration (NOAA) satellite ozone programme including assessments of the current Solar Backscatter Ultraviolet (SBUV/2) and the future Ozone Mapping and Profiler Suite (OMPS) instruments and products. It provides evaluation of the quality of the NOAA Polar-orbiting Operational Satellites (POES) SBUV/2 measurements and the Version 8 algorithm products, both for operational and reprocessed data records. The presentation summarizes work on the instrument calibration and characterization; the algorithm theory and implementation; and the information content, quality and validation of the ozone estimates. This is followed by similar information on the measurements and products expected from the OMPS on the National Polar Operational Environmental Satellite System (NPOESS) and NPOESS Preparatory Project (NPP) beginning in 2011.

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.

Vertical ozone profiles deduced from measurements of SBUS on FY-3 satellite

The FY-3 satellite is a second-generation Chinese polar-orbit meteorological satellite. The Solar Backscatter Ultraviolet Sounder (SBUS), one of the main payloads on the FY-3 satellite, is the first Chinese ozone-monitoring instrument on a meteorological satellite. As part of the in-orbit validation of FY-3, we carried out a retrieval trial using measurements from SBUS during the period of 17-30 July, 2008, and compared those data with measurements and retrieved profiles from the National Oceanic and Atmospheric Administration (NOAA) satellite SBUV/2. The results show that the precision of the measurements and retrieved profiles are quite good. The averaged relative difference percentages of the ozone profiles retrieved from SBUS and those from SBUV/2 are within ±7%.

Estimating the 27‐day and 11‐year solar cycle variations in tropical upper stratospheric ozone

Spectral analysis of the solar backscatter ultraviolet (SBUV) satellite instrument ozone and solar flux time series was used to estimate the ozone response in the tropical upper stratosphere to the solar flux harmonics of the 27‐day Sun rotation cycle with periods of 9, 13.5, and 27 days. Solar UV flux at 205 nm, Mg II index, composite solar Lyman alpha, and 10.7 cm solar flux data sets were tested as proxies for the solar signal. The Mg II index has the highest coherency with tropical ozone in the upper stratosphere among all solar proxies. The analysis shows that during the periods of high solar activity, about half of the ozone variance for periods of 13.5 and 27 days near 40 km can be attributed to the fluctuations of the Mg II index. During the periods of low solar activity, the 27‐day signal is below the 90% statistical significance level, while the 13.5‐day signal is usually significant. Also, a 9‐day period can be seen in ozone data during the time of low solar activity. The ozone response to solar variations with a 27‐day period was used to estimate the 11‐year solar cycle amplitude in tropical ozone. Biases in individual SBUV instrument data were included as a part of the statistical model. In that case, the estimate from the 27‐day period minimum to maximum range of the 11‐year cycle is about 2%. This agrees with the data in layers 8–9 (38–43 km) derived from the SBUV data set. Below these layers, the amplitude of the 11‐year cycle is only about one third of the one estimated from the 27‐day cycle. Above these layers, the amplitude of the 11‐year cycle is larger than that estimated from the 27‐day cycle, but it is primarily due to the high amplitude of the 11‐year cycle in the Nimbus 7 SBUV data.

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.

Changes in cloud and aerosol cover (1980–2006) from reflectivity time series using SeaWiFS, N7‐TOMS, EP‐TOMS, SBUV‐2, and OMI radiance data

The amount of solar radiation reflected back to space or reaching the Earth's surface is primarily governed by the amount of cloud cover and, to a much lesser extent, by Rayleigh scattering, aerosols, and various absorbing gases (e.g., O3, NO2, H2O). A useful measure of the effect of cloud plus aerosol cover is given by the amount that the 331 nm Lambert Equivalent Reflectivity (LER) of a scene exceeds the surface reflectivity for snow/ice‐free scenes after Rayleigh scattering has been removed. Twenty‐eight years of reflectivity data are available by overlapping data from several satellites: N7 (Nimbus 7, TOMS; 331 nm) from 1979 to 1992, SBUV‐2 series (Solar Backscatter Ultraviolet, NOAA; 331 nm) 1985 to 2007, EP (Earth‐Probe, TOMS; 331 nm) 1997 to 2006, SW (SeaWiFS; 412 nm) 1998 to 2006, and OMI (Ozone Measuring Instrument; 331 nm) 2004–2007. Only N7 and SW have a sufficiently long data record, Sun‐synchronous orbits, and are adequately calibrated for long‐term reflectivity trend estimation. Reflectivity data derived from these instruments and the SBUV‐2 series are compared during the overlapping years. Key issues in determining long‐term reflectivity changes that have occurred during the N7 and SW operating periods are discussed. The largest reflectivity changes in the 412 nm SW LER and 331 nm EP LER are found to occur near the equator and are associated with a large El Nino‐Southern Oscillation event. Most other changes that have occurred are regional, such as the apparent cloud decrease over northern Europe since 1998. The fractional occurrence (fraction of days) of high reflectivity values over Hudson Bay, Canada (snow/ice and clouds) appears to have decreased when comparing reflectivity data from 1980 to 1992 to 1997–2006, suggesting shorter duration of ice in Hudson Bay since 1980.

Evaluation of Atmospheric Infrared Sounder ozone profiles and total ozone retrievals with matched ozonesonde measurements, ECMWF ozone data, and Ozone Monitoring Instrument retrievals

An evaluation of the Atmospheric Infrared Sounder (AIRS) version 4 (V4) and version 5 (V5) retrieved ozone profiles and the total column ozone is performed using collocated ozonesonde (O3SND) profiles and total ozone measurements from the World Ozone and Ultraviolet Radiation Data Center (WOUDC) archives. Using O3SNDs and Brewer/Dobson Network (BD) measurements as the truth, bias and root‐mean‐squared (RMS) difference statistics are computed for the AIRS ozone profile retrievals and the derived total ozone. In addition, global monthly maps of total ozone generated for the AIRS retrievals are compared with the Ozone Monitoring Instrument (OMI) and the Solar Backscatter Ultra Violet (SBUV/2) instrument derived maps to evaluate the characteristic trends and seasonal cycle depicted by the AIRS retrieval. The results of the validation exercise reveal that the AIRS V5 algorithm significantly improves the ozone profile retrieval biases and RMS differences for the lower troposphere and, especially, over the tropical region where the V4 algorithm shows larger discrepancies with the O3SND measurements. The V5 retrieval biases with global O3SNDs are less than 5% for both the stratosphere and the troposphere. The RMS differences are less than 20% for the upper stratosphere and are close to 20% for the lower stratosphere and the troposphere. Total ozone amounts from both the V4 and V5 versions agree well with the global BD station measurements with a bias of less than 4% and an RMS difference of approximately 8%. Analysis of V5 total ozone monthly maps reveals that the V5 ozone retrievals depict seasonal trends and patterns in concurrence with OMI and SBUV/2 observations.

Four‐dimensional variational assimilation of ozone profiles from the Microwave Limb Sounder on the Aura satellite

Ozone profiles from the Microwave Limb Sounder (MLS) onboard the Aura satellite of the NASA's Earth Observing System (EOS) were experimentally added to the European Centre for Medium‐range Weather Forecasts (ECMWF) four‐dimensional variational (4D‐var) data assimilation system of version CY30R1, in which total ozone columns from Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) onboard the Envisat satellite and partial profiles from the Solar Backscatter Ultraviolet (SBUV/2) instrument onboard the NOAA‐16 satellite have been operationally assimilated. As shown by results for the autumn of 2005, additional constraints from MLS data significantly improved the agreement of the analyzed ozone fields with independent observations throughout most of the stratosphere, owing to the daily near‐global coverage and good vertical resolution of MLS observations. The largest impacts were seen in the middle and lower stratosphere, where model deficiencies could not be effectively corrected by the operational observations without the additional information on the ozone vertical distribution provided by MLS. Even in the upper stratosphere, where ozone concentrations are mainly determined by rapid chemical processes, dense and vertically resolved MLS data helped reduce the biases related to model deficiencies. These improvements resulted in a more realistic and consistent description of spatial and temporal variations in stratospheric ozone, as demonstrated by cases in the dynamically and chemically active regions. However, combined assimilation of the often discrepant ozone observations might lead to underestimation of tropospheric ozone. In addition, model deficiencies induced large biases in the upper stratosphere in the medium‐range (5‐day) ozone forecasts.

Ozone quasi‐biennial oscillations (QBO), semiannual oscillations (SAO), and correlations with temperature in the mesosphere, lower thermosphere, and stratosphere, based on measurements from SABER on TIMED and MLS on UARS

The ozone and temperature measurements from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on the Thermosphere‐Ionosphere‐Mesosphere‐Energetics and Dynamics (TIMED) satellite form a unique set. They provide global information over the range of local solar times, from the lower stratosphere into the lower thermosphere, going back to 2002, by one instrument. On the basis of zonal means of these data, we present new results from 20 to 100 km in altitude and from 48°S to 48°N in latitude of ozone and temperature semiannual oscillations (SAO) and quasi‐biennial oscillations (QBO). While some of the results are new for the stratosphere, such results for the mesosphere and lower thermosphere (MLT) have not been available before. The SAO and QBO components are mostly symmetric with respect to the Equator. The correlations of ozone and temperatures provide the opportunity to study the relative effects of dynamics and chemistry. Our results show that the ozone oscillations are largely and positively correlated to those of the temperature below about 30 km and above 80 km, and mostly anti‐correlated with temperature between about 30 and 80 km. We compare with measurements made about 10 years earlier by the MLS instrument on UARS, and with results by others from the solar mesosphere explorer (SME) satellite, the solar backscatter ultraviolet (SBUV), space shuttle experiments (CRISTA), and ground‐based measurements.

Long-term calibration monitoring of Spectralon diffusers BRDF in the air-ultraviolet

Long-term calibration monitoring of the bidirectional reflectance distribution function (BRDF) of Spectralon diffusers in the air-ultraviolet is presented. Four Spectralon diffusers were monitored in this study. Three of the diffusers, designated as H1, H2, and H3, were used in the prelaunch radiance calibration of the Solar Backscatter Ultraviolet/2 (SBUV/2) satellite instruments on National Oceanic and Atmospheric Administration (NOAA) 14 and 16. A fourth diffuser, designated as the 400 diffuser, was used in the prelaunch calibration of the Ozone Mapping and Profiler Suite (OMPS) instrument scheduled for initial flight in 2009 on the National Polar Orbiting Environmental Satellite System Preparatory Project. The BRDF data of this study were obtained between 1994 and 2005 using the scatterometer located in the National Aeronautics and Space Administration Goddard Space Flight Center Diffuser Calibration Laboratory. The diffusers were measured at 13 wavelengths between 230 and 425 nm at the incident and scatter angles used in the prelaunch calibrations of SBUV/2 and OMPS. Spectral features in the BRDF of Spectralon are also discussed. The comparison shows how the air-ultraviolet BRDF of these Spectralon samples changed over time under clean room deployment conditions.

The 11‐year solar cycle in stratospheric ozone: Comparison between Umkehr and SBUVv8 and effects on surface erythemal irradiance

Ozone profiles derived from ground‐based Umkehr measurements at five stations and from the merged data set of Solar Backscattered Ultra Violet (SBUVv8) satellite observations are used to estimate the seasonal influence of the 11‐year solar signal in the vertical distribution of stratospheric ozone. Both data sets show a strong response (2–3% of the annual mean) in the upper stratosphere, a very small response in the middle tropical stratosphere and a secondary maximum (∼2%) in the tropical lower stratosphere, in accordance to earlier reports. Thus a sequence of high‐low‐high response is seen in the vertical pattern of the solar cycle effect on ozone in the tropics, with its position sensitive to season, as it appears to move across the equator in direction opposite to the location of the maximum solar insolation. Results from Umkehr and satellite observations are in good agreement at levels up to 35 km, while discrepancies (of up to 2%) found at higher levels (around 40 km) are largest in summer. Finally, the estimated erythemal irradiance response to the 11‐year solar cycle, calculated for the observed ozone change during a solar cycle (minimum to maximum), showed an overall decrease in the irradiance at the surface which is maximum (−2% of annual mean) over the tropics.

Effects of model chemistry and data biases on stratospheric ozone assimilation

Abstract. The innovations or observation minus forecast (O–F) residuals produced by a data assimilation system provide a convenient metric of evaluating global analyses. In this study, O–F statistics from the Global Ozone Assimilation Testing System (GOATS) are used to examine how ozone assimilation products and their associated O–F statistics depend on input data biases and ozone photochemistry parameterizations (OPP). All the GOATS results shown are based on a 6-h forecast and analysis cycle using observations from SBUV/2 (Solar Backscatter UltraViolet instrument-2) during September–October 2002. Results show that zonal mean ozone analyses are more independent of observation biases and drifts when using an OPP, while the mean ozone O–Fs are more sensitive to observation drifts when using an OPP. In addition, SD O–Fs (standard deviations) are reduced in the upper stratosphere when using an OPP due to a reduction of forecast model noise and to increased covariance between the forecast model and the observations. Experiments that changed the OPP reference state to match the observations by using an "adaptive" OPP scheme reduced the mean ozone O–Fs at the expense of zonal mean ozone analyses being more susceptible to data biases and drifts. Additional experiments showed that the upper boundary of the ozone DAS can affect the quality of the ozone analysis and therefore should be placed well above (at least a scale height) the region of interest.

Latitude‐dependent long‐term variations in polar mesospheric clouds from SBUV version 3 PMC data

Previous studies have suggested that there should be secular trends in polar mesospheric cloud (PMC) occurrence frequency and brightness on decadal timescales and that those trends would be strongest at the lowest latitudes of the PMC existence region. We have analyzed the 27‐year PMC data set created from Solar Backscatter Ultraviolet (SBUV, SBUV/2) satellite instruments for long‐term variations in albedo using three latitude bands (50°–64°, 64°–74°, 74°–82°). The improved version 3 data set includes revisions to the PMC detection algorithm to produce more consistent results in all measurement conditions. A detailed error analysis yields an approximate uncertainty of 1–2% for seasonally averaged 252‐nm albedo values. Adjustments for local time variations in PMC brightness between different satellite data sets were derived to ensure accurate trend calculations. Multiple linear regression fits show that albedo variations are anticorrelated with solar activity in all latitude bands, with a stronger response at high latitudes. The albedo increase from solar maximum to solar minimum ranges from +2% at 50°–64°S to +17% at 74°–82°N. Secular trends in albedo are positive, with long‐term changes over 27 years ranging from +12% to +20% depending on hemisphere and latitude. The derived long‐term trend in PMC albedo at 50°–64° is smaller than that of higher latitudes. This result contradicts previous suggestions that PMC brightness changes might be most rapid at low latitudes. The albedo response to solar variations is larger in the Northern Hemisphere, while long‐term trends are approximately the same in both hemispheres.