Technical note: Reanalysis of Aura MLS Chemical Observations

Abstract

<p><strong>Abstract.</strong> This paper presents a reanalysis of the atmospheric chemical composition from the upper troposphere to the lower mesosphere from August 2004 to December 2017. This reanalysis is produced by the Belgian Assimilation System for Chemical ObsErvations (BASCOE) constrained by the chemical observations from the Microwave Limb Sounder (MLS) onboard the Aura satellite. BASCOE is based on the Ensemble Kalman Filter (EnKF) method and includes a chemical transport model driven by the winds and temperature from the ERA-Interim meteorological reanalysis. The model resolution is 3.75° in longitude, 2.5° in latitude and 37 vertical levels from the surface to 0.1 hPa with 25 levels above 100 hPa. The outputs are provided every 6 hours. This reanalysis is called BRAM2 for BASCOE Reanalysis of Aura MLS, version 2.</p> <p>Vertical profiles of eight species from MLS version 4 are assimilated and are evaluated in this paper: ozone (O<sub>3</sub>), water vapour (H<sub>2</sub>O), nitrous oxide (N<sub>2</sub>O), nitric acid (HNO<sub>3</sub>), hydrogen chloride (HCl), chlorine oxide (ClO), methyl chloride (CH<sub>3</sub>Cl) and carbon monoxide (CO). They are evaluated using independent observations from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACEFTS), the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES), N<sub>2</sub>O observations from another MLS radiometer than the one used to deliver the standard product and ozonesondes. The evaluation is done in four regions of interest where only selected species are evaluated. These regions are (1) the lower stratospheric polar vortex where O<sub>3</sub>, H<sub>2</sub>O, N<sub>2</sub>O, HNO<sub>3</sub>, HCl and ClO are evaluated, (2) the upper stratospheric lower mesospheric polar vortex where H<sub>2</sub>O, N<sub>2</sub>O, HNO<sub>3</sub> and CO are evaluated, (3) the tropical tropopause layer (TTL) where O<sub>3</sub>, H<sub>2</sub>O, CO and CH<sub>3</sub>Cl are evaluated and (4) the middle stratosphere where O<sub>3</sub>, H<sub>2</sub>O, N<sub>2</sub>O, HNO<sub>3</sub>, HCl, ClO and CH<sub>3</sub>Cl are evaluated.</p> <p>In general BRAM2 reproduces MLS observations within their uncertainties and agrees well with independent observations, with several limitations discussed in this paper (see the summary in Sect. 5.5). In particular, ozone is not assimilated at altitudes above (i.e. pressures lower than) 4 hPa due to a model bias that cannot be corrected by the assimilation. MLS ozone profiles display unphysical oscillations in the TTL which are corrected by the assimilation, allowing a good agreement with ozonesondes. Moreover, in the upper troposphere, comparison of BRAM2 with MLS and independent observations suggests a positive bias in MLS O<sub>3</sub> and a negative bias in MLS H<sub>2</sub>O. The reanalysis also reveals a drift in MLS N<sub>2</sub>O against independent observations which highlights the potential use of BRAM2 to estimate biases between instruments. BRAM2 is publicly available and will be extended to assimilate MLS observations post 2017.</p>

Toward a Reanalysis of Stratospheric Ozone for Trend Studies: Assimilation of the Aura Microwave Limb Sounder and Ozone Mapping and Profiler Suite Limb Profiler Data

“…ACE‐FTS is well‐suited for evaluation of stratospheric ozone because of its high accuracy, high vertical resolution, and stability. These data were recently used by Errera et al () for a comprehensive evaluation of their chemical reanalysis of the stratosphere.…”

Climatological impact of the Brewer–Dobson Circulation on the N<sub>2</sub>O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses

“…The BASCOE data assimilation system (Errera et al, 2019) is built on a Chemistry-Transport Model which consists in a kinematic transport module with the FFSL advection scheme (Lin and Rood, 1996) and an explicit solver for stratospheric chemistry, comprising 65 species and 243 reactions (Prignon et al, 2019). The transport module requires on input only the surface pressure and horizontal wind fields from reanalyses, as it relies on mass continuity to derive vertical mass fluxes.…”

“…The TEM diagnosis is also applied to N 2 O assimilated fields from the BASCOE Reanalysis of Aura MLS, version 2 (BRAM2, Errera et al, 2019) which covers the period August 2004-August 2019. For BRAM2, BASCOE is driven by dynamical fields from ERA-Interim, with a horizontal resolution of 3.75 • x2.5 • longitude-latitude.…”

“…is also compared to a chemical reanalysis of Aura Microwave Limb Sounder (MLS) using the BASCOE assimilation system driven by the ERA-Interim reananlysis (BRAM2, Errera et al, 2019). https://doi.org/10.5194/acp-2020-262 Preprint.…”

An inter-comparison of tropospheric ozone reanalysis products from CAMS, CAMS-Interim, TCR-1 and TCR-2

“…A second version of the EnKF-based reanalysis (TCR-2) has been recently produced using an updated model and satellite retrievals for the years 2005-2018 (Kanaya et al, 2019;Miyazaki et al, 2019a;Thompson et al, 2019). For stratospheric constituents, several studies have been conducted to produce and compare stratospheric chemical reanalysis products (Davis et al, 2017;Errera et al, 2019).…”