An analysis of 10-year ASOS data from east of Rockies: The detection of the diurnal continental tide and eastward-propagating waves

Harmonic analysis has been applied to 10-year nearly 1000 ASOS over the United States to separate the diurnal component of surface pressure, temperature, precipitation, etc. The calculated phase can be converted to the time when surface observation reaches its maximum. In warm season (JJA), for stations over the Great Plains and Midwest, the phases of the surface pressure component present a diurnal atmospheric tide pattern; with consistent phases around 105 degree. The longitudinal variations of diurnal surface pressure amplitude are tightly related to the temperature amplitude. So we assume that there exists a continental enhanced diurnal atmospheric tide that is mainly contributed by the sensible heating from the ground and is proportional to the diurnal surface temperature variations. The increasing amplitudes of both pressure and temperature toward west are because of the elevation and vegetation. Nelder Mead Simplex optimization method has been used to separate the Continental enhanced tide from other signals.

The residue parts of pressure show a clear east-propagating signal, with phases increasing eastward and the estimated phase speed is about 25m/s. The phases for diurnal precipitation component also show increasing phases with longitude with a phase speed of nearly 25m/s. Our derived phase speeds for pressure signal and precipitation signal are similar to the propagating speed of the summer storms in Carbone et al (2002). In our results, the precipitation phase is about zero degree at 105W, and in Carbone et al (2002), the storms start at midnight east of the Grand Junction.

The data from different months shows that the eastward propagating pressure signal exists almost the whole year, but the correspondence with the precipitation only happens in warm season (JJA). These results raise the question whether the eastward propagating pressures signal is the cause or the consequence of the eastward propagating precipitation bands. If the precipitation is caused by the pressure, then what is the source of this east propagating pressure signal? In order to answer these questions, linear model simulation is deployed and the preliminary results show that with the existence of the vertical mean wind shear, the potential vorticity pulse generated by the elevated diurnal heating (diurnal convection over the Rockies) can trigger convection in the lower troposphere downwind side even far away from the original source. The simulation results show that these PV-pulses generate an east-increasing phases that is similar to what we observed over east of the Rockies.