Assessment of Stratospheric Balloon Observations towards Assimilation in NOAA's GSI-based Global Data Assimilation System

Stratospheric observations in the present earth observing system architecture are derived primarily from satellite data. Such observations are difficult to validate as few in situ measurements in the stratosphere exist. This study aims at exploiting existing in situ balloon observations provided by a private entity, Loon, to supplement the observation architecture toward improving forecasts and for validating satellite measurements. In recent years, Loon, a subsidiary of Google's parent company Alphabet, has launched over a thousand large superpressure balloons that carry communication instrumentation in the stratosphere for months at a time to provide continuous interconnectivity to internet-deficient regions around the world. In addition to the communication instrumentation, the balloon payload embarks a series of sensors that downlink information including 4-dimensional GPS data, solar altitude relative to the balloon, and atmospheric observations.

This study evaluates Loon stratospheric wind and temperature observations to quantify their quality towards assimilation in NOAA's Gridpoint Statistical Interpolation (GSI) based Global Data Assimilation System (GDAS). The balloon data are compared with GDAS and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, where the Numerical weather prediction (NWP) analyses are collocated to the balloon positions in space and time. Statistical diagnostics reveal small biases in wind speed and larger temperature errors, particularly during local daytime. Applications of quality control (QC) parameters that are identified are shown to reduce systematic errors likely related to the onboard sensor instrumentation. Bias corrections (BC) to the balloon observations result in improved data quality, in particular for nighttime temperatures. The Loon data are preprocessed using corresponding QC and BC metrics before assimilation. NWP observing system experiments (OSEs) are underway, and the potential impact of assimilating the balloon observations on NOAA global weather prediction will be discussed.