MIPAS Aerosol and Cloud Profiles

Introduction

This data repository provides access to Envisat Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aerosol and cloud detections. MIPAS was a passive infrared (~4.2 - 14.6 µm) emission sounder providing measurements between about 5 to 70 km. The vertical sampling was 3 km in the troposphere and stratosphere from 2002 to 2004 and 1.5 km in the UTLS and 2 km in the stratosphere from 2005 to 2012. The aerosol and cloud data provided here was derived from the geo-located and calibrated MIPAS level1b hyperspectral radiance data, available by ESA (ESA MIPAS data access).

For cloud and aerosol detection the cloud index (CI) and aerosol cloud index (ACI) were used. The CI is the ratio between the radiances around 792 cm^-1, which is around the centre of a CO₂ Q-branch, and 833 cm^-1, which is a window region (Spang et al., 2004). The region around 792 cm^-1 is nearly saturated and hence, mainly sensitive towards temperature with only little impact from cloud, whereas the window region around 833 cm^-1 is highly sensitive to clouds and also slightly sensitive to water vapour. This results in high CI values for clear air and lower CI values for clouds and aerosol. For the discrimination between clear air and aerosol/clouds threshold values were identified. Often constant values between 1.8 and 4.5 are used. Since the water vapour in the atmosphere increases with decreasing altitude and hence, has an increasing impact on the window region around 833 cm^-1, altitude and latitude variable threshold values, ranging from about 1.8 to 6.5, provide the highest sensitivity in the troposphere and stratosphere (Sembhi et al., 2012).

The ACI is the maximum of the CI and the aerosol index (AI). The AI is the ratio between 792 cm^-1 and 960 cm^-1. The 960 cm^-1 window is located between CO₂ hot bands. The AI has the advantage that it is not sensitive to water vapour and hence, allows for a better sensitivity in the troposphere, but it has the disadvantage that it is sensitive to temperature excitation of the CO₂ hot bands at around 50 km (semiannual temperature oscillation), which results in less sensitivity in the stratosphere. The ACI combines the strength of the CI in the stratosphere and the strength of the AI in the troposphere (Griessbach et al., 2016). It is also a continuous index, where large values indicate clear air and low values indicate aerosol/clouds. A fixed threshold value of 7 was found to provide a sensitivity compareable to the altitude and latitude variable thresholds (Griessbach et al., 2016). However, in contrast to the CI, using higher ACI values allows for the detection of optically thinner clouds and aerosol layers, such as e.g. the stratospheric aerosol layer.

The discrimination between ice and aerosol relies on the ACI. All aerosol/cloud spectra with ACI <= 7 were selected. Since the optical properties of ice clouds and aerosol, such as sulfate aerosol, volcanic ash, mineral dust, supercooled ternary solutions, nitric acid trihydrate, exhibit different spectral slopes between 830, 960, and 1224 cm^-1, these windows were used to calculate brightness temperatures. Using a brightness temperature difference correlation the spectra were assigned to the ice clouds/optically thick cloud group or the aerosol group (Griessbach et al., 2016). Here, the ice cloud group also includes optically thick spectra, because the spectral slope between the three windows does not differ.

For the volcanic ash/mineral dust detection a simple radiance correlation between the 825 and 950 cm-1 windows was used, as the optical properties of mineral material have a characteristic slope between these two windos (Griessbach et al., 2014).

Browse images

The browse images show global maps of MIPAS ice cloud top height, aerosol top height, and ash top height observations. Each figure shows all orbits and profiles of one day (black dots). The cloud top heights are indicated by coloured symbols.

Data access

The data is provided in one file per year. The files contain one line per scan for tangent altitudes below 30 km. For each measured spectrum information on the measurement orbit number, profile number, measurement time since 01 January 2000, tangent point latitude, longitude, and altitude, the CI (Spang et al., 2004), CI cloud flag using a modified (at polar latitudes) and extended (to altitudes below 10 km) version of the latitude and altitude variable thresholds by Sembhi et al. (2012) ( Reinhold Spang, personal communication, 2017), ACI (Griessbach et al., 2016) and flags for clear air, cloud, aerosol, and ash are provided (Griessbach et al., 2014, 2016).

The CI is the cloud index. The CI-flag set to 1 indicates the presence of clouds or aerosol. The ACI is the aerosol-cloud index. The clear flag set to 1 indicates clear air. The ice flag set to 1 indicates ice or optically thick clouds. The aerosol flag set to 1 indicates any type of aerosol including non-ice polar stratospheric clouds (STS and NAT). The ash flag indicates volcanic ash or mineral dust. The flags for clear air, ice, and aerosol are mutually exclusive, whereas the flag for ash is based on a second independent methods.

Legal notes

The data sets and browse images provided on this site are licensed under a Creative Commons Attribution 4.0 International License. You are free to share the material in any medium or format and adapt it for any purpose, even commercially. You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits. The licensor cannot revoke these freedoms as long as you follow the license terms. The data are distributed in the hope that it will be useful, but without any warranty. Please follow the link to see the terms and conditions of the license:

click here to see the license

References

Citation, if the data are used in publications:

Aerosol-Cloud-Index (ACI) and the aerosol-ice discrimination:

Griessbach, S., Hoffmann, L., Spang, R., von Hobe, M., Müller, R., and Riese, M.: Infrared limb emission measurements of aerosol in the troposphere and stratosphere, Atmos. Meas. Tech., 9, 4399–4423, https://doi.org/10.5194/amt-9-4399-2016, 2016.

Ash detection:

Griessbach, S., Hoffmann, L., Spang, R., and Riese, M.: Volcanic ash detection with infrared limb sounding: MIPAS observations and radiative transfer simulations, Atmos. Meas. Tech., 7, 1487–1507, https://doi.org/10.5194/amt-7-1487-2014, 2014.

Cloud-Index (CI) and altitude variable thresholds:

Spang, R., Remedios, J. J., and Barkley, M. P.: Colour indices for the detection and differentiation of cloud type in infra-red limb emission spectra, Adv. Space Res., 33, 1041–1047, 2004.

Sembhi, H., Remedios, J., Trent, T., Moore, D. P., Spang, R., Massie, S., and Vernier, J.-P.: MIPAS detection of cloud and aerosol particle occurrence in the UTLS with comparison to HIRDLS and CALIOP, Atmos. Meas. Tech., 5, 2537–2553,doi:10.5194/amt-5-2537-2012, 2012.

Contact

We encourage anyone interested in the data set to contact us for discussions on how you like to use the data. This might help to reduce the risk of mistaken applications and helps us to track the distribution and areas of applications of the data set.

Please do not hesitate to contact us if you have any further questions:

Dr. Sabine Griessbach

Forschungszentrum Jülich
Jülich Supercomputing Centre
52425 Jülich
Germany

e-mail: s.griessbach@fz-juelich.de