# Surface Pressure¶

In this section, MERRA-2 surface pressure is compared to observations reported to the National Center for Environmental Information (NCEI – formerly National Climatic Data Center). Selected surface sites from the NCEI Integrated Surface Database (ISD) files are used for the hourly MERRA-2 comparisons. Global "Summary of the Day" (GSOD) files are used for the comparison to the daily mean MERRA-2 surface pressure.

## Hourly¶

Scatter plot of the MERRA-2 hourly adjusted surface pressure (see info box below on the hypsometric equation) vs. the hourly station observations values from the selected NCEI ISD files for the years 2001 - 2019. Also compare the Daily surface pressure sections below of unadjusted vs. adjusted for the full justification of using the adjusted surface pressure for assessment purposes.

The hourly validation for the adjusted surface pressure shows good agreement with clustering around the 1:1 correlation line, especially for values above 700 hPa.

## Daily Average¶

The daily mean surface pressure is taken directly from the re-gridded MERRA-2 and represents estimates of the atmospheric pressure at 2 m above the local surface averaged from the original source spatial resolution.

Scatter plot of the re-gridded MERRA-2 daily un-adjusted pressure vs. the daily station observations values from the NCEI GSOD files for every 3rd year from 1981 – 2014.

\begin{align} Where: \\ z_1: & \text{ elevation of the MERRA-2 grid. } \\ z_2: & \text{ elevation of the reporting surface site. } \\ p_1: & \text{ MERRA-2 surface pressure. } \\ p_2: & \text{ Surface site pressure. } \\ R: & \text{ Gas constant for dry air. } \\ g: & \text{ Gravitational constant. } \\ T: & \text{ Temperature at mid-point between $z_1$ and $z_2$. } \\ \end{align}