Google [x] Project Loon Measurements of Stratospheric Temperature, Pressure and Wind as a Data Source for Reanalyses over Southern High Latitudes

Abstract:

Balloon-borne measurements are ideal candidates for assimilation into reanalyses as they provide data at a spatial and temporal scale that cannot be gained from satellite-based measurements. Google [x] Project Loon is flying hundreds of long duration super-pressure balloons at altitudes of around 20 km in the southern mid-latitudes. Onboard measurements of temperature and pressure, as well as wind data which can be derived from balloon location information, will significantly increase the availability of these data over southern middle and high latitudes.

We present an analysis of the scientific utility of Project Loon temperature, pressure and wind data. Project Loon temperature measurements from 79 balloon flights indicate a bias that depends on the solar zenith angle. This bias results from the daytime heating of the balloon by solar radiation and can be statistically modelled using a linear least squares regression model. Once the measurements are corrected using this model, differences between the balloon-borne temperature measurements and NCEP CFSR temperatures are reduced to less than 1K for all solar zenith angles smaller than 150°. This suggests that an empirically derived correction for Loon-based temperature measurements may make these data suitable for use in reanalyses. Loon pressure measurements agree well with reanalyses, with differences between balloon measurements and NCEP CFSR of 0.174±0.571 hPa and against NCEP NCAR 0.535±0.537 hPa. Wind fields derived from balloon location information compare favourably with MERRA and ERA-Interim reanalyses. Loon wind fields exhibit high-frequency variability which isn't observed in MERRA or ERA-Interim reanalyses. Project Loon provides long duration measurements with high temporal resolution that are valuable to reanalyses, particularly in a region that has a sparse measurement record such as the middle and high latitudes of the Southern Hemisphere.