Lunar gravitational atmospheric tide, surface to 50 km in a global, gridded data set

The lunar gravitational semidiurnal atmospheric tide L 2 is detected in the geopotential field in the ERA‐Interim reanalysis by regressing the data for each of the four daily analysis times onto periodic lunar half‐synodic cycle time series and differencing the resulting regression maps. A robust pattern dominated by zonal wave number 2 emerges, which agrees with tidal theory and with previous observational studies based on station data in terms of its amplitude, phase, and seasonality at the Earth's surface. Above the 100 hPa level, the tidal signal amplifies with height and it is downward propagating; below that level it is almost barotropic. The L 2 signal is resolvable even in data for individual months of individual years. Monthly time series of tidal amplitude and phase exhibit a well‐defined pattern of seasonality year after year. Since the ERA forecast model has no L 2 gravitational forcing, the source of its L 2 signal must be in the observations.