A Climatological Analysis of Upper-Level Velocity Potential using Global Weather Reanalysis, 1959-2020

Upper-level (200 hPa) velocity potential (VP200) is useful in identifying areas of
rising or sinking atmospheric motions on varying temporal scales (e.g., weekly, seasonal,
interannual) especially in the global tropics. These areas are associated with enhancement
(rising motion) or suppression (sinking motion) of tropical convection and subsequent
weather phenomena dependent on these processes (e.g., tropical cyclones). This study
employed commonly used global weather reanalysis datasets to calculate and compare VP200
on varying temporal scales and quantify any differences that existed between them from 1959
to 2020 over four key regions of tropical variability (Equatorial Africa, Amazon Basin,
Equatorial Central Pacific, and Equatorial Indonesia). To supplement this analysis, the highly
correlated variables to VP200 of outgoing longwave radiation (OLR) and daily precipitation
rate were used and directly compared with independent OLR and precipitation datasets to
determine the reanalysis’ level of agreement with the independent data. The ECMWF ERA5
held the highest agreement to these data over all regions examined and was reasoned to have
the highest confidence in accurately capturing the variability of VP200 fields for the study
period. Confidence was decreased in the usefulness of the NCEP/NCAR Reanalysis 1 as it
consistently performed poorly over much of the study domain. The results of this study also
emphasized the usefulness in ensemble-based approaches to assess climate variability and
understanding of potential biases and uncertainties that are inherent in these data sources.