Assessment of total columnar ozone climatological trends over the Indian sub-continent

In the present study, long term satellite and Dobson spectrophotometer Total Column Ozone (TCO) data have been used to study the interannual variability and also to assess climatological trends in TCO over different geographical locations of Indian sub-continent. TCO data were analyzed for the period 1957 to 2015 over New Delhi (28.63° N, 77.18° E), Varanasi (25.30° N, 83.02° E), Pune (18.53° N, 73.84° E) and Kodaikanal (10.0° N, 77.47° E). An extensive validation was performed for Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) retrieved TCO data independently with Dobson Spectrophotometer TCO measurements over New Delhi, Varanasi, Pune and Kodaikanal. The results of this exercise showed good correlation coefficient (r) of 0.87 (0.88), 0.84 (0.82), 0.91 (0.80) and 0.84 (Data not available) respectively. Climatological mean TCO over New Delhi, Varanasi, Pune and Kodaikanal are 275.02 ± 6.44 DU, 269.03 ± 7.34 DU, 260.78 ± 5.07 DU and 258.71 ± 6.36 DU respectively for the period 1957 to 2015. An increasing trend over New Delhi (0.20 DU year^–1), Pune (0.18 DU year^–1), Kodaikanal (0.14 DU year^–1) and decreasing trend over Varanasi (0.01 DU year^–1) were observed. High significance of TCO trend was found at New Delhi (p-value < 0.0001), Pune (p-value = 0.002) and Kodaikanal (p-value = 0.003) with negligible trend over Varanasi with p-value of 0.84. The TCO variations at different geographical locations associated with upper atmospheric meteorological parameters such as lower Stratospheric Temperature (ST) at 65 hPa and Tropopause Height (TH) were also addressed. Annual lower stratospheric temperature shows positive relationship with TCO and Stratospheric ozone over the study sites. Further, decadal variability in TCO with respect to solar activity at New Delhi was also analyzed.     

SCIAMACHY/Envisat,OMI/Aura,and ground-based total ozone measurements over Kyiv-Goloseyev station

Validation of satellite ozone measurements is important for data improvement due to instrumental long-term drifts and retrieval algorithm limitations. For satellite data quality estimation, we compare the total ozone content (TOC) derived from the satellite Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat and Ozone Monitoring Instrument (OMI)/Aura spectrometer overpass data and the ground-based measurements made with the Dobson spectrophotometer 040 at the GAW station No. 498 Kyiv-Goloseyev. The station was opened for Dobson ozone measurements in 2010. The results for Direct Sun, Zenith Blue, and Zenith Cloud observations are presented separately, in order to assess the influence of weather conditions (clear or cloudy sky) on the difference between satellite and ground-based measurements. Results from the SCIAMACHY–Dobson and OMI–Dobson difference analyses show small relative overestimation of TOC for satellite data. The ground-based Dobson 040 data are of high quality for Direct Sun and Zenith Blue from AD ((305.5 and 325.0 nm) and (317.5 and 339.9 nm)) pair measurements. Seasonal variations of the difference are seen with maximal satellite–Dobson data discrepancy near the winter solstice. Satellite TOC values are systematically higher than Dobson ones at solar zenith angles larger than 70°. This difference could be explained by seasonal non-uniformity in the satellite data.     

Total ozone observations at Athens,Greece by satellite-borne and ground-based instrumentation

A comparison is made of total ozone (TOZ) content observations conducted by the Dobson spectrophotometer No. 118, the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY), the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) over Athens, Greece, during 1991–2008. Spearman's and Wilcoxon's tests were used to determine the measure of the agreement between the ground-based and satellite column ozone data. The correlation coefficient between Dobson and Nimbus-7, ADEOS, Earth Probe, OMI and SCIAMACHY observations was found to be 0.95, 0.96, 0.94, 0.93 and 0.87, respectively, while the correlation coefficient between total ozone observations of SCIAMACHY and Earth Probe-TOMS and OMI is 0.85 and 0.93, respectively. SCIAMACHY overestimates the column ozone with respect to Dobson, Earth Probe-TOMS and OMI by 10, 15 and 3 DU, respectively, while Dobson underestimates the column ozone with respect to Nimbus-7, ADEOS and OMI by 5, 10 and 8 DU. The results obtained confirm that the Athens Dobson station may continue to be considered as a ground-truth total ozone station for the validation of the satellite column ozone observations. In addition, linear regression analysis of the deseasonalized monthly mean column ozone, as derived from Dobson measurements, gives an increase of +0.33 ± 0.07% per year during 1991–2000 and a decrease of –0.33 ± 0.07% per year for the period 2001–2008.     

Global validation of FY-3A total ozone unit (TOU) total ozone columns using ground-based Brewer and Dobson measurements

Analysis of the accuracy and variability of total ozone columns (TOC) has been conducted by many studies, while the TOC observations derived from the total ozone unit (TOU) on board the Chinese FengYun-3A (FY-3A) satellite platform are notably less well documented. Therefore, in this present study, we mainly focus on the global-scale validation of TOU-derived total ozone column data by comparing them with spatially and temporally co-located ground-based measurements from the well-established Brewer and Dobson spectrophotometer for the period July 2009 through December 2011. The results show that TOU-derived total ozone column data yields high accuracy, with the root mean square error less than 5% in comparison with ground-based measurements. Meanwhile, TOU underestimates Brewer measurements by 1.1% in the Northern Hemisphere and overestimates Dobson total ozone 0.3% globally. In addition, TOU-derived total ozone shows no significant dependence on latitude in comparison with either Brewer or Dobson total ozone measurements. Nevertheless, a significant dependence of TOU-derived total ozone is observed on the solar zenith angle (SZA) in comparison with both Brewer and Dobson, demonstrating that TOU underestimates at large SZA and overestimates at small SZA. Finally, the dependence of satellite – ground-based relative difference for total ozone values shows fair agreement when total ozone values are in the range 250–450 Dobson units (DU). Overall, the Chinese FY-3A/TOU performs well on total ozone retrieval with high accuracy, and the total ozone data derived from the TOU can be used as a reliable data source for ozone monitoring and other atmospheric applications.     

Impact of zonal wind on latitudinal variation of total columnar ozone over the Indian Peninsula

Latitudinal and seasonal variability of total columnar ozone from September 2007 to August 2008 across the Indian longitude sector within 10.5° N to 34.5° N and 70.5° E to 94.5° E using satellite data obtained from Aura Ozone Monitoring Instrument (OMI) of National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) is presented. The total column ozone (TCO) over the area of study shows a gradually varying pattern throughout the year. In the post-monsoon (autumn) and winter months, maximum TCO is observed in the north-western part of the subcontinent while the minimum is often observed towards the east at about the same latitudes. A west–east spatial gradient is clearly observed in autumn months. As winter approaches, a north–south spatial gradient becomes more prominent than the east–west gradient. It has been further observed that TCO does not vary significantly over the entire subcontinent in monsoon.     

Study of aerosol optical depth,ozone, and precipitable water vapour content over Sinhagad,a high-altitude station in the Western Ghats

This article reports the results of a study related to variations in columnar aerosol optical depth (AOD), total column ozone (TCO), and precipitable water content (PWC) over a high-altitude station, Sinhagad (18° 21′ N, 73° 45′ E, 1450 m above mean sea level (AMSL)), employing a microprocessor-based total ozone portable spectrometer, MICROTOPS-II, comprising both a sun photometer and ozonometer, during November 2009–April 2010. The aerosol optical depth at 500 nm (AOD_{500 nm}) portrayed seasonal variation with higher values (0.39) in summer and lower values (0.15) in winter. The TCO and PWC also exhibited lower values in winter and started increasing by the pre-monsoon season. The Ångström wavelength exponent, α, was found to be high (1.79) during February, indicating the relative dominance of accumulation-mode particles. During the summer season, the lower value (0.94) of the Ångström wavelength exponent indicates the relative dominance of coarse-mode particles. The ground-based observations from MICROTOPS-II revealed good correlation with satellite observations of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI). The observed short-wave solar flux at the bottom of the atmosphere decreased due to aerosol extinction and was found to be 19 and 78 W m^–2 for the winter and pre-monsoon seasons, respectively. This implies that greater concentrations of accumulation-mode particles – which are due to local anthropogenic sources – affected the down-welling radiation than those from natural sources – which are due to long-range transport processes – over the experimental location.     

Interannual variability of convective activity over the tropical Indian Ocean during the El Niño/La Niña events

Convection over the tropical Indian Ocean is important to the global and regional climate. This study presents the monthly climatology of convection, inferred from the outgoing longwave radiation (OLR), over the tropical Indian Ocean. We also examine the impact of El Niño/La Niña events on the convection pattern and how variations in convection over the domain influence the spatial rainfall distribution over India. We used 35 recent years (1974–2008) of satellite-derived OLR over the area, the occurrence of El Niño/La Niña events and high resolution grid point rainfall data over India. The most prominent feature of the annual cycle of OLR over the domain is the movements of convection from south-east to north and north-west during the winter to the summer monsoon season. This feature represents the movement of the inter-tropical convergence zone (ITCZ). The climatology of OLR during the winter months (December–February) over the domain is characterized by high subsidence over central India with a decrease of OLR values towards the north and south. Moderate convection is also seen over the Himalayan Range and the south-east Indian Ocean. In contrast, during the summer (June–September) the OLR pattern indicates deep convection along the monsoon trough and over central India, with subsidence over the extreme north-west desert region. The annual march of convection over the Arabian Sea and Bay of Bengal sector shows that the Arabian Sea has a limited role, compared to the Bay of Bengal, in the annual cycle of the convection over the tropical Indian Ocean. The composite OLR anomalies for the El Niño cases during the summer monsoon season show suppressed convection over all of India and moderate convection over the central equatorial Indian Ocean and over the northern part of the Bay of Bengal. Meanwhile in La Niña events the OLR pattern is nearly opposite to the El Niño case, with deep convection over entire Indian region and adjoining seas and subsidence over the northern Bay of Bengal and extreme north-west region. The spatial variability of the 1°?×?1° summer monsoon rainfall data over India is also examined during El Niño/La Niña events. The results show that rainfall of the summer monsoon season over the southern peninsular of India and some parts of central India are badly affected during El Niño cases, while the region lying along the monsoon trough and the west coast of India have received good amounts of rainfall. This spatial seasonal summer monsoon rainfall distribution pattern seems to average out the influence of El Niño events on total summer monsoon rainfall over India. It seems that, in El Niño events, the convection pattern over the Bay of Bengal remains unaffected during summer monsoon months and thus this region plays an important role in giving good summer monsoon rainfall over the northern part of India, which dilutes the influence of El Niño on seasonal scale summer monsoon rainfall over India. These results are also confirmed by using a monthly bias-corrected OLR dataset.     

Total ozone over Ankara and its forecasting using regression models

The present study is concerned with the total ozone variation in Ankara, Turkey (39°57′ N; 32°53′ E) and developing a total ozone prediction model by Total Ozone Mapping Spectrometer (TOMS) and single-site aerological data (1984–2006). The daily averages of the total column ozone over Ankara show a seasonal variation, with larger values in spring/winter and lower in autumn/summer and a large day-to-day variability. In this study, in order to forecast the total column ozone over Ankara, a multi-linear regression equation was performed. Predictors are selected by stepwise regression method. The ozone value on the previous day, temperature at the 100 hPa pressure level, previous day's temperature for the 700 hPa pressure level and temperature difference for 50 hPa and 500 hPa are the most important predictors. The success of monthly prognostic total column ozone models built up for Ankara is tested by using Brewer MK III Spectrophotometer data and satellite (OMI) data with the upper-air data in 2007. The Brewer MKIII has been in operation since November 2006 in Ankara. The ground-based one-year data show good agreement with the satellite OMI data.     

Improvement of the Umkehr ozone profile by the neural network method: analysis of the Belsk (51.80°N, 20.80°E) Umkehr data

Vertical profiles of atmospheric ozone by the neural network (NN) method are compared with those obtained by the standard Umkehr inversion algorithm – UMK92. Both methods used the same input, the so-called N values, derived from Umkehr measurements at Belsk (51.80°N, 20.80°E), Poland, by the Dobson spectrophotometer No 84. The vertical profiles of ozone from satellite observations, Microwave Limb Sounder (MLS) overpasses for the period 2004–2009, and from ozonesoundings performed at the nearby aerological station, Legionowo (52.4° N, 21.0° E), for the period 2000–2009 provide a reference data set for the NN model building. The NN methodology appears to be a promising tool for extracting information about the vertical ozone profile from ground-based Umkehr measurements, despite some limitations of the NN method itself, such as the results being limited to the analysed station, sensitivity to errors in the reference data sets, and lack of possibility to determine the actual retrieval errors. Accuracy of the NN ozone profiles is better for all Umkehr layers than that by the standard Umkehr inversion algorithm when NN and UMK92 profiles are compared with the reference profiles. It is especially pronounced for comparisons with the ozonesonde profiles for layers 4 and 1, where the absolute error changes from 10.6 Dobson units (DU) (UMK92) to 4.4 DU (NN) and from 6.6 DU (UMK92) to 3.5 DU (NN), respectively (1 Dobson unit is equal to 2.69 × 10²⁰ molecules/m²). The mean (over all Umkehr layers) correlation coefficient between NN-MLS, and NN-ozonesonde profiles is 0.75 and 0.85, respectively. The corresponding correlation coefficients for the comparison with UMK92 profiles are lower, i.e. 0.61 and 0.64, respectively.     

Validation of ground-based microwave radiometer data and its application in verifying atmospheric stability over Mahbubnagar during 2011 monsoon and post-monsoon seasons

Atmospheric instabilities, mainly convection, depend on temperature distribution and moisture availability. The development of convection can often lead to the formation of clouds and precipitation, release of latent heat, etc. The initiation or development of instabilities has to be studied in detail with high-resolution, ground-based instruments such as ground-based microwave radiometric measurements. In this study we evaluated ground-based microwave radiometer data (MWR)-retrieved temperature and relative humidity profiles and compared these to radiosonde observations. Analysis showed that MWR-measured temperature (specific humidity) has a warm (wet) bias below 3 km and cold (dry) bias above that altitude. Correlation of stability indices estimated from radiometer and radiosonde showed fairly good correlation, with a correlation coefficient greater than 0.5 with 95% significance. MWR was then utilized for the verification of atmospheric stability over Mahbubnagar (16° 44′ N, 77° 59′ E), India, during the second half of the monsoon and start of post-monsoon seasons. Radiometric observations showed strong day-to-day variation of atmospheric parameters as well as thermodynamic indices during the monsoon, which were weak during the post-monsoon season. The seasonal mean of thermodynamic indices and the associated seasonal difference showed that thunderstorm potential is higher during the post-monsoon season over the study site.     

Aerosol optical properties over Delhi and Manora Peak during a rare dust event in early April 2005

Dust storm events are annual phenomena observed over the Indo-Gangetic plain (IGP) during the pre-monsoon period (May–June). These dust storms affect the air quality, weather conditions and radiation budget of the region. In this paper we characterize the aerosol optical parameters associated with a rare dust storm event that hit the IGP during early April 2005. This event was considered rare as it occurred much earlier than the general occurrence of dust storms in India (May–June), and in the year 2005, the warmest year in the span of the previous hundred years.

In this study we considered the optical aerosol parameters for two places in the IGP: Delhi (28.5° N, 77.2° E, 325 m asl) and the high altitude station, Manora Peak (29.4° N, 79.5° E, 1958 m asl). Of the two selected stations, Delhi represents a highly populated and polluted location whereas Manora Peak represents a cleaner location in the central Himalayan region. During this dust storm event, the aerosol optical depth (AOD) was observed to increase considerably. The increment was 2.6–4.6 times over Delhi and 1.6–3.2 times over Manora Peak at wavelengths 380 and 1020 nm, respectively, with respect to the background values, whereas the Ångström exponent (α) for both the stations remained close to zero during the event. The effect shows a considerable increase in direct dust radiative forcing in terms of a reduction in the broadband global irradiance for Delhi as well as for Manora Peak stations. The direct aerosol radiative forcing thus obtained was about 34% in the 400–1100 nm wavelength band at Manora Peak.     

Aerosol characteristics during the coolest June month over New Delhi,northern India

June 2008, which is also the transition month between two major seasons for Indo-Gangetic Basin (IGB), has been identified the coolest June over New Delhi during the past century, showing mean temperature of 31.6 ± 1.7°C, which was found to be ～2°C less than its climatological mean (33.9°C). Aerosol optical properties for this month and thus obtained physical parameters have been studied using data from the CIMEL sun/sky radiometer, installed in New Delhi under the Aerosol Robotic Network (AERONET) programme. Results reveal bimodal aerosol volume size distribution. The monthly mean values for aerosol optical depth (AOD) at 500 nm (0.96 ± 0.31) and Ångström exponent at the wavelength pair of 440–870 nm (0.79 ± 0.42) show significant lower values whereas single scattering albedo at 675 nm shows a significantly larger value (0.94 ± 0.04) compared with previous measurements over the station. Results suggest dominance of scattering-type particles such as water-soluble aerosols from anthropogenic sources and dust aerosols from natural sources with higher relative humidity over the station. Radiative forcing caused due to the aerosols for the month of June 2008, which have been computed using the radiative-transfer model, informs low forcing at the top of atmosphere (TOA,?+14 W m^?2) as well as at surface (?33 W m^?2). The resultant atmospheric forcing (+47 W m^?2) indicates warming effect that caused heating of lower atmosphere at the rate of 0.89 K day^?1.     

Remote sensing ‘ground-based automatic UV / visible spectrometer’ for the study of atmospheric trace gases

An advance remote sensing instrument, the ‘ground-based automatic UV / visible spectrometer’, has been developed indigenously at Pune (18° 31′ N, 73° 55′ E) to cover the spectra (462–498 nm) of zenith sky scattered light. A spectrometry technique is used to find out the vertical column density (VCD) of many atmospheric trace gases, such as NO₂, O₃, H₂O and O₄. The VCDs of these gases are extracted from observed spectra by comparing the magnitude of the differential optical depth (DOD) of each species in the 462–498 nm spectral range. Slant column densities (SCDs) of each species are found to increase with solar zenith angle (SZA), due to the approaching higher path length of sunlight. The VCDs of O₃ and NO₂ derived by the UV / visible spectrometer are compared with the ozone monitoring instrument (OMI) Aura satellite and ground-based Brewer spectrometer data. The compared VCD values are found to be close to satellite and ground-based measurements.     

Zenith scattered light measurement: observations of BrO and OClO

Stratospheric BrO and OClO observations have been made for the first time over a tropical station, Pune (18° 31′ N, 73° 55′ E) using a Differential Optical Absorption Spectroscopy (DOAS) technique by measuring zenith sky scattered light spectra in the wavelength range of 346–358 nm by ultraviolet (UV)/visible spectrometer. The Differential Optical Density (DOD) fitting technique is applied for the right selection of a suitable spectral region for the analysis to minimize interference and poorly fitting absorption features, and also to minimize the residual of the fit. Observed DODs of O₃, NO_2, BrO, OClO, O₄, Rayleigh and Ring are well fitted with the calculated DODs and the percentage DODs are found to vary up to 0.5%, 0.8%, 0.15%, 0.13%, 1.5%, 1.2% and 1.3% respectively. Chlorine and bromine species play an important role in the ozone depletion, hence O₃, NO₂, BrO and OClO Slant Column Densities (SCDs) are derived between 76° and 94° Solar Zenith Angles (SZAs). The SCDs of O₃ are found to be decreased in the twilight period (i.e. between 90° and 94° SZA) in the presence of sufficient BrO and OClO. Total Column Densities (TCDs) of O₃, NO₂, BrO and OClO are derived by UV/visible spectrometry, Brewer spectrometry and satellite-based Scanning Imaging Absorption spectrometer for Atmospheric Cartography (SCIAMACHY) for Pune and the higher latitude station Kanpur (26° 28′ N, 80° 24′ E) during the period 1 April–31 June 2008. The day-to-day variations in O₃ and NO₂ TCDs over Pune are found to be more than over Kanpur. BrO TCDs vary between 1.9?×?10¹³ and 4?×?10¹³ molecules cm^?2 over Pune, which are derived by UV/visible spectrometry, while they vary for the high-altitude station Kanpur between 0.5?×?10¹³ and 3.5?×?10¹³ molecules cm^?2 derived by SCIAMACHY. The OClO TCDs are found to have an increasing trend with variations between 2?×?10¹³ and 4.5?×?10¹³ molecules cm^?2 during the above period.     

Aerosol characteristics over Delhi national capital region: a satellite view

Multi-sensor aerosol data sets are analysed to examine the aerosol characteristics over the Delhi national capital region. Both the Multiple-angle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) capture the seasonal cycle of aerosol optical depth (AOD) as observed by ground-based measurements. However, AOD from MISR shows a low bias relative to AOD from MODIS, which increases linearly at high AOD conditions. A large difference (by >25 W m^–2 per unit AOD) in the top-of-atmosphere direct radiative forcing efficiency derived from MODIS and MISR-retrieved AOD is observed during the winter and pre-monsoon seasons relative to the other seasons. The ubiquitous presence of dust (as indicated by non-spherical particle fraction to AOD and linear depolarization ratio values) is observed throughout the year. The aerosol layer is mostly confined to within 2 km of surface in the winter and post-monsoon seasons, while it expands beyond 6 km in the pre-monsoon and monsoon seasons. Columnar AOD is found to be highly sensitive to aerosol vertical distribution. The applicability of multi-sensor data sets and climatic implications are discussed.     

Cover Application of the NOAA-AVHRR images to the study of the large forest fires in Spain in the summer of 1994.

Ozone vertical profiles derived from Umkehr observations by the Dobson spectrophotometer at Belsk (52.50° N, 20.47° E) and from ozone soundings carried out at the nearest aerological station, Legionowo (52.24° N, 20.58° E), have been compared with those measured by the MLS instrument on board the Aura spacecraft during the sites' overpasses for the period 2004–2005. It is assumed that the satellite station distance should be less than 2° and less than 4° for the latitudinal and longitudinal difference, respectively. The bias, RMS error, and the correlation coefficients between the ozone content in the Umkehr layers have been calculated using Dobson/sonde/MLS data. The ozone mixing ratio at selected levels in the lower and mid‐stratosphere (from 215 hPa up to 6.8 hPa) have been compared using the sonde/MLS data. The number of analysed daily values was ～40 (Dobson/MLS), 60 (sonde/MLS), and 60 (Dobson/sonde) since August 2004. The comparisons show a good correspondence (bias ～±5%, RMS <10%, correlation coefficient >0.5) between the ozone content in Umkehr layers 4–8 and ozone mixing ratio at pressures <50 hPa. At lower stratosphere (Umkehr layer 3) and upper stratosphere (Umkehr layer 9), there is also statistically significant relationship between the data, but the biases and RMS are ～2 times larger, while the correlation coefficients are still high (>0.7).     

Comparison of total ozone column observations from space-borne Ozone Monitoring Instrument with ground-based Dobson Ozone Spectrophotometer measurements at an urban location in Indo-Gangetic Basin

This article presents a comparison analysis of OMIT (Ozone Monitoring Instrument retrieved overpass total ozone column (TOC)), and DOST (Dobson Ozone Spectrophotometer observed TOC) over Delhi during a period from October 2004 to June 2011. Megacity Delhi, located in Indo-Gangetic Basin, is an important site for comparison of ground-based and satellite retrieved TOCs due to significant anthropogenic emissions of ozone precursors, large shift in seasons, and large-scale crop residue burning in the region. DOST and OMIT data show an overall bias of 3.07% and significant correlation with coefficient of determination R² = 0.73. Large seasonal fluctuations in the biases and correlations have been observed ranging from 2.46% (winter) to 3.82% (spring), and R² = 0.84 (winter) to R² = 0.09 (summer), respectively. The large biases are attributed to changes in temperature, cloud cover, pollutants emissions from urban area, and crop-residue burning events. We also find notable variations in correlations between the datasets due to the varying burden of absorbing aerosols from open field crop-residue burning. The R² has changed from 0.67 (for aerosol optical depth, AOD 1.5–3.5) to 0.77 (for AOD 0–0.99). The dependence of the bias on solar zenith angle, cloud fraction, and satellite distance is also discussed. A simple linear regression analysis is applied to check the linkage between DOST and OMIT. The influence of atmospheric air temperature and relative humidity on OMIT at different pressure levels between 1000 and 20 hPa has been discussed.     

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.     

Validation of the MetOp-A total ozone data from GOME-2 and IASI using reference ground-based measurements at the Iberian Peninsula

One of the most important atmospheric composition products derived from the first EUMETSAT Meteorological Operational satellite (MetOp-A) is the total ozone column (TOC). For this purpose, MetOp-A has two instruments on board: the Global Ozone Monitoring Experiment 2 (GOME-2) that retrieves the TOC data from the backscattered solar ultraviolet-visible (UV-Vis) radiance, and the Infrared Atmospheric Sounding Interferometer (IASI) that uses the thermal infrared radiance to derive TOC data. This paper focuses on the simultaneous validation of the TOC data provided by these two MetOp-A instruments using the measurements recorded by five well-calibrated Brewer UV spectrophotometers located at the Iberian Peninsula during the complete 2009. The results show an excellent correlation between the ground-based data and the GOME-2 and IASI satellite observations (R² higher than 0.91). Differences between the ground-based and satellite TOC data show that the IASI instrument significantly overestimates the Brewer measurements (about 4.4% when all five ground-based stations are jointly used). In contrast, the GOME-2 instrument shows a slight underestimation (~ 1.6%). In addition, the absolute relative differences between the Brewer and GOME-2 data are quite smaller (about a factor higher than 2) than the Brewer-IASI absolute differences. The satellite viewing geometry (solar zenith angle and the view zenith angle) has no significant influence on the Brewer-satellite relative differences. Moreover, the analysis of these relative differences with respect to the ground-based TOC data indicates that GOME-2 instrument presents a slight underestimation for high TOC values. Finally, the IASI-GOME-2 correlation is high (R² ~ 0.92), but with a mean relative difference of about ± 6% which could be associated with the bias between UV-Vis and infrared spectroscopy used in the retrieval processes.     

Comparison of AMSR‐E and TMI sea surface temperature with Argo near‐surface temperature over the Indian Ocean

Sea surface temperature (SST) measurements from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) are compared with near‐surface temperature (foundation SST) in situ measurements obtained from Argo floats over the Indian Ocean. Spatial variation was compared for 2002–2006 and 11 floats were used for temporal variation collocated observations. The results show that TMI and AMSR‐E SST measurements are slightly overestimated during the pre‐ and post‐monsoon seasons and underestimated during the monsoon season. Statistical analysis shows that the SST from the AMSR‐E is better correlated with the Argo foundation SST compared to the TMI. The standard deviation (SD) and root mean square error (RMSE) for AMSR‐E SST are 0.58°C and 0.35°C, respectively, over the Equatorial Indian Ocean (EIO). The corresponding values for the TMI are 0.66°C and 0.47°C. Over the Arabian Sea the SD values are slightly higher compared to the EIO values, whereas RMSE values are less for both TMI and AMSR‐E SST. These retrieval accuracies are above the expected retrieval accuracy. The seasonal average spatial distribution of AMSR‐E SST shows a better match with the Argo foundation SST compared to TMI SST distributions. The robustness of the good spatial match during the monsoon season may be attributed to strong winds.