Probing the Atmospheric Precipitable Water Vapor with SOFIA, Part III. Atlas of Seasonal Median PWV Maps from ERA5, Implications for FIFI-LS and in situ Comparison Between the ERA5 and MERRA-2 Atmospheric Re-analyses

Abstract

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory for far-infrared astronomy stationed at the Armstrong Flight Research Center (AFRC) in Palmdale, CA, USA. Although SOFIA flies at altitudes of ∼41,000 ft, any far-infrared observations from within the Earth’s atmosphere are nevertheless hampered by water vapor absorbing the astronomical signal. The primary atmospheric parameter governing absorption in the far-infrared is the total upward precipitable water vapor, PWV. In this paper we present global PWV maps derived from re-analyses from the European Centre for Medium-Range Weather Forecasts, ECMWF, with a geographical resolution of 0.°5, for flight altitudes ranging from 37,000 ft to 45,000 ft and each meteorological season. These maps were validated with FIFI-LS PWV measurements on board SOFIA and allow an investigation of the global morphology and seasonal dependence of the total upward PWV in the stratosphere. We additionally investigate the observing conditions, in terms of PWV, at various locations, especially around SOFIA’s home base, Palmdale, but also around sites in the southern hemisphere like Tahiti, Santiago de Chile (Chile), Buenos Aires (Argentina), and Christchurch (New Zealand). From the southern sites investigated Christchurch provides the best conditions in terms of PWV (and efficiency), Tahiti the worst. Using total power sky measurements with FIFI-LS we also derive a mean emissivity of the telescope (primary, secondary and tertiary mirror) of ϵ Tel = 20.5 ± 1.6% around the astronomically significant [C ii] emission line. We finally compare atmospheric re-analyses from GEOS (MERRA-2) and ECMWF (ERA5) to our FIFI-LS PWV measurements. Both re-analyses correlate linearly with our FIFI-LS PWV measurements from all flight altitudes but with different scaling factors. However, MERRA-2 correlates significantly less well than ERA5 especially for flight altitudes below 41,000 ft.

Research on PWV calculation combining MODIS and ERA5

“…Although atmospheric water vapor accounts for only 0.1%~3% of the atmosphere, it can cause 75% of the greenhouse effect in the atmosphere [5] . Water vapor acquisition is difficult and difficult to achieve, because the spatial distribution of water vapor is very uneven and changes quickly, and the scale is more subtle than wind speed and air temperature [6] .…”

Probing the Atmospheric Precipitable Water Vapor with SOFIA, Part. IV. Water Vapor Estimates from FORCAST Grism Spectra

“…This comparison yielded a relative calibration of the latter set of values and established a method of determining and correcting for atmospheric absorption in FIFI-LS data at any point during a FIFI-LS/SOFIA flight using only the ERA5-ECMWF values. In paper III (Iserlohe et al 2022), we presented worldwide, seasonal, median PWV maps, based on the ERA5-ECMWF data set, which could have been used to substantially improve the SOFIA flight planning process, had the program continued. Additionally we discussed observing conditions, in terms of PWV, for various locations around the world.…”