An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers

February 2, 2018

CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph, recently published a paper in the Journal of Hydrometeorology, titled An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers (https://doi.org/10.1175/JHM-D-17-0114.1). The paper is included in the journal’s special collection on MERRA-2, Modern-Era Retrospective analysis for Research and Applications version 2.

Using airborne observations from various field campaigns over the northeastern Pacific along with two atmospheric reanalysis products (ERA-Interim and MERRA-2), the study validated key characteristics of atmospheric rivers (ARs) depicted by reanalyses against observations, as well as evaluating how well the 21 observed ARs represent the total of about 6000 ARs that occurred during the winters of 1979-2016 over the northeastern Pacific.

Results showed that the reanalysis products accurately depict the strength of the observed ARs in terms of the total water vapor flowing along an individual AR across its entire width, with a mean error of only +3% or -1% depending on the reanalysis product being evaluated. Additionally the 21 observed ARs well represent the mean strength of the total of about 6000 ARs identified in reanalysis products, with a mean difference of 5% or 14% depending on the reanalysis product being compared. Similar comparisons were also done for AR width, and for ARs in other regions and seasons. The study highlights the values of both dedicated observations of specific cases and spatiotemporally more complete global reanalysis products in understanding the characteristics and impacts of ARs.

Figure Caption: (left) Histogram of AR widths based on all ARs detected in ERA-Interim over the northeastern Pacific (AR centroids within 163.4–124.6°W, 23–46.4°N) during 15 January to 25 March of 1979–2016 (gray bars). Also shown are the mean AR width (km) based on all reanalysis ARs that contributed to the histogram (red solid), the subset of the reanalysis ARs that correspond to the 21 dropsonde transects (red dashed), and the observed value based on the 21 dropsonde transects as reported in Ralph et al. (2017b) (blue dashed for the mean, and blue circles for individual transects). The mean AR width value is also indicated in the figure legend for each sample. Red shading indicates the 95% confidence interval of the mean reanalysis AR width for a random 21-member sample drawn from the pool of reanalysis ARs based on 10,000 iterations. The error bar centered on the blue dashed line indicates the 95% confidence interval of the difference between the blue and red dashed lines based on a two-tailed, paired t-test. (right) As in the left but for total integrated water vapor transport (108 kg s⁻¹) across AR widths.