Search

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) version 4.10 (V4) level 2 aerosol data products, released in November 2016, include substantial improvements to the aerosol subtyping and lidar ratio selection algorithms. These improvements are described along with resulting changes in aerosol optical depth (AOD). The most fundamental change in the V4 level 2 aerosol products is a new algorithm to identify aerosol subtypes in the stratosphere. Four aerosol subtypes are introduced for stratospheric aerosols: polar stratospheric aerosol (PSA), volcanic ash, sulfate/other, and smoke. The tropospheric aerosol subtyping algorithm was also improved by adding the following enhancements: (1) all aerosol subtypes are now allowed over polar regions, whereas the version 3 (V3) algorithm allowed only clean continental and polluted continental aerosols; (2) a new dusty marine aerosol subtype is introduced, representing mixtures of dust and marine aerosols near the ocean surface; and (3) the polluted continental and smoke subtypes have been renamed polluted continental/smoke and elevated smoke, respectively. V4 also revises the lidar ratios for clean marine, dust, clean continental, and elevated smoke subtypes. As a consequence of the V4 updates, the mean 532 nm AOD retrieved by CALIOP has increased by 0.044 (0.036) or 52 % (40 %) for nighttime (daytime). Lidar ratio revisions are the most influential factor for AOD changes from V3 to V4, especially for cloud-free skies. Preliminary validation studies show that the AOD discrepancies between CALIOP and AERONET–MODIS (ocean) are reduced in V4 compared to V3.

Data products from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) were recently updated following the implementation of new (version 4.1) calibration algorithms for all of the level 1 attenuated backscatter measurements. In this work we present the motivation for and the implementation of the version 4.1 nighttime 532 nm parallel channel calibration. The nighttime 532 nm calibration is the most fundamental calibration of CALIOP data, since all of CALIOP’s other radiometric calibration procedures – i.e., the 532 nm daytime calibration and the 1064 nm calibrations during both nighttime and daytime – depend either directly or indirectly on the 532 nm nighttime calibration. The accuracy of the 532 nm nighttime calibration is significantly improved by raising the molecular normalization altitude from 30–34 km to 36–39 km to substantially reduce stratospheric aerosol contamination. Due to the greatly reduced molecular number density and consequently reduced signal-to-noise ratio at the higher altitudes used to avoid aerosols, the signal is averaged over a larger number of samples. The new calibration procedure is shown to eliminate biases introduced in earlier versions and consequently leads to an improved representation of stratospheric aerosols. Validation results using airborne lidar measurements are also presented. Biases relative to collocated measurements acquired by the Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) are reduced from 3.6 % ± 2.2 % in the version 3 data set to 1.6 % ± 2.4 % in the version 4.1 release.

The Solar Occultation For Ice Experiment (SOFIE) was launched onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite on 25 April 2007, and began science observations on 14 May 2007. SOFIE conducts solar occultation measurements in 16 spectral bands that are used to retrieve vertical profiles of temperature, O3, H2O, CO2, CH4, NO, and polar mesospheric cloud (PMC) extinction at wavelengths from 0.330 to 5.006 μm. SOFIE performs 15 sunset measurements at latitudes from 65° to 85°S and 15 sunrise measurements from 65° to 85°N each day. This work describes the SOFIE instrument, measurement approach, and retrieval results for the northern summer of 2007.

The Solar Occultation For Ice Experiment (SOFIE) was launched onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite on 25 April 2007, and began science observations on 14 May 2007. SOFIE conducts solar occultation measurements in 16 spectral bands that are used to retrieval vertical profiles of temperature, O 3 , H 2 O, CO 2 , CH 4 , NO, and polar mesospheric cloud (PMC) extinction at 11 wavelengths. SOFIE provides 15 sunrise and 15 sunset measurements each day at latitudes from 65° - 85°S and 65° - 85°N. This work describes the SOFIE experiment and shows preliminary retrieval results based on observations from the initial months on-orbit.

Accurate simultaneous retrievals of temperature and pressure are key to retrieving high quality mixing ratio profiles from occultation sensors. Equally important is accurate determination of the vertical separation between measurement points. Traditionally, these tasks are complicated by platform motion and CO 2 model errors. We present a new approach that is independent of platform motion and CO 2 concentration, using inexpensive modern 2D focal-plane arrays and an innovative refraction-angle measurement. This provides both accurate temperature retrievals and precise vertical separation of measurement samples, greatly improving the quality of mixing ratio retrievals. We show recent studies demonstrating the expected performance of the SOFIE instrument (Solar Occultation For Ice Experiment) to be launched as part of the AIM (Aeronomy of Ice Mission) in September 2006. This system will have the ability to retrieve accurate temperature, through mild particulate contamination (such as volcanic aerosol and cirrus) from cloud-top to stratopause, independent of mixing ratio knowledge. Additional CO 2 absorption channels will provide retrieved temperature and CO 2 mixing ratios through the mesosphere and into the lower thermosphere.

[1] Solar occultation measurement of atmospheric species with short photochemical lifetimes poses some difficulties if sharp gradients are present in the species concentrations near sunrise/sunset conditions. These photochemically induced variations introduce asymmetries in the species distribution along the line of sight that need to be taken into account in the retrieval process in order to prevent inaccuracies. Correction factors derived from photochemical model calculations have been routinely applied to the retrievals of stratospheric NO and NO2 by HALOE and ATMOS experiments. Mesospheric ozone, because of its twilight variations, also belongs to the group of species which require a correction procedure. Results from our mesospheric diurnal photochemical model indicate that proper accounting of diurnal variations leads to more than 20% increase in the ozone column along the line of sight for sunrise conditions near 0.1 hPA at the equator in January. Correspondingly, the retrieved ozone at the tangent point will be lower if these twilight variations are considered. The effects under sunset conditions are smaller. The current HALOE retrieval algorithm uses an approximate correction for mesospheric ozone. We have developed a new database of twilight gradients for the entire altitude range of HALOE measurements based on the results from a contemporary diurnal photochemical model. HALOE scans for January 29, 1992, have been reprocessed using the new diurnal correction factors. Sunrise ozone mixing ratios near 0.1 hPa are smaller by more than 20% compared to the HALOE V19 data. The differences between the corrections for sunrise and sunset retrievals suggest that, with the new diurnal corrections, the sunrise to sunset ozone ratios near 0.1 hPa obtained from approximately coincident HALOE data will be in much better agreement with the values derived from photochemical theory.

[1] We present a new retrieval technique for mesospheric measurements from the Halogen Occultation Experiment (HALOE). Previous HALOE retrievals did not account for polar mesospheric clouds (PMCs) and were biased whenever PMCs were in the sample volume. In the new algorithm we eliminate this bias by retrieving and correcting for PMC extinction, and we optimize the algorithm for this region of the atmosphere. We have reprocessed over ten years (1991–2002) of HALOE data with the improved algorithm, yielding nearly 75,000 vertical profiles of temperature, H2O, O3, NO and extinction at five infrared wavelengths. In this paper we describe the new algorithm in detail, present several examples of the results and discuss some initial validation and quality assessments.

The Halogen Occultation Experiment (HALOE) operated in orbit for over 14 years, providing high quality measurements from the upper troposphere into the lower thermosphere. Since the quality of this data set depended on the long-term stability of the instrument, a series of analysis tests were designed to routinely monitor instrument performance. These tests evaluated possible changes in the gas cells, electronic gains, optical performance, and signal temperature dependencies. The gas cell stability was determined from an analysis of the Doppler shift signature in retrieved mixing ratios. Electronic gain stability was determined by instrument scans of the solar disk, each with different balance settings. Optical and tracking performance was also determined from solar scan data. The only statistically significant changes detected were: 1. a small methane gas cell change, causing less than 0.5% per decade change in retrieved methane, and 2. a small optical alignment change in the HF channel that only affects HF results below 25 kilometers. These detailed analyses indicate that the HALOE instrument remained stable throughout the mission, adding confidence to the long-term atmospheric trends deduced from HALOE products.

The Halogen Occultation Experiment (HALOE) conducted satellite solar occultation measurements for 14 years ending on 21 November 2005. HALOE contained a calibration wheel, which included three neutral density filters that were used to examine response linearity through a combination of ground and on-orbit measurements. Although measurement uncertainties preclude a confident assessment of the true extent of nonlinearity, the on-orbit data lead to the conclusion that any existing response nonlinearity has changed by less than 2% over the mission lifetime. This conclusion eliminates a potentially significant uncertainty when using HALOE data for studies of long-term atmospheric trends.