The diurnal cycle from observations and ERA5 in surface pressure, temperature, humidity, and winds.

The diurnal cycle is a prominent feature of Earth's weather and climate. Partly due to lack of sub-daily data, our understanding of the diurnal cycle is incomplete. This study analyzes 3-hourly surface observations and ERA5 hourly data to evaluate ERA5's performance in simulating the diurnal cycle and to quantify the diurnal variations in common meteorological and climate fields over the globe. The goal is to provide a standard reference for the diurnal cycle in various surface and atmospheric fields. Here I focus on surface pressure (Ps), air temperature (Tas), relative (RH) and specific humidity (q), and winds. Results show that ERA5 captures well the observed diurnal variations in Ps, Tas and RH, showing the dominant westward-propagating wavenumber-2 pattern in Ps and wavenumber-1 pattern in Tas and RH. Over many land areas, Ps 24 h cycle (S1) can reach an amplitude of 1.0–1.5 hPa, comparable to its semidiurnal cycle (S2), and the S1 enhances the 1st peak around 1000 local solar time (LST) but suppresses the 2nd peak around 2200 LST in Ps. Diurnal variations in Tas and RH are out-of-phase and dominated by S1, with a mean amplitude of 3–4 °C for Tas and 10–15% for RH over land (but with considerable variations) and a maximum for Tas (a minimum for RH) around 1500 LST. Tas and RH diurnal variations are much weaker over ocean, with an amplitude of ~ 0.7–0.8 °C for Tas and 1–2% for RH. Surface q is slightly higher during the day than at night with a dip in middle afternoon in the tropics and warm-season extratropics. Surface wind speed shows a diurnal amplitude of 0.5–1.0 m/s over land and about half of that over ocean and a peak in the early-middle afternoon. ERA5 data suggests that the surface Tas and RH diurnal cycle extends to the middle-upper troposphere with similar phases but reduced amplitudes. Sea surface temperatures in ERA5 lack any diurnal variations, which dampens diurnal variations in its marine Tas and RH and causes incorrect diurnal cycle in marine q. [ABSTRACT FROM AUTHOR]