The diurnal cycle of warm season rainfall frequency over continents

It has become increasingly evident that a sizeable fraction of warm season continental rainfall results from long-lived “episodes” as reported in JAS by Carbone et al. 2002. More recently, we have expanded this study to portions of several continents where there is evidence for a high frequency of organized convection. This paper synthesizes the diurnal cycle findings for these diverse geographical regions under environmental conditions best described as “weakly-forced” in a balanced dynamical sense.

The following signals have emerged from our comparative studies over four continents:

• A disproportionately large fraction of events have their origin in the lee of major cordillera, a finding consistent with that reported by Laing and Fritsch 2000 [QJRMS]. Obvious instances are the Tibetan Plateau, the Rockies, the Ethiopian Highlands, and the eastern cordillera of Australia. While clearly insufficient in itself, a major factor in the genesis of events is thermal forcing associated with regional scale elevated heat sources.

• Many events are of long duration, sometimes spanning several diurnal cycles. In the presence of steering winds and shear, the events steadily propagate across continents, while exhibiting periods of dissipation followed by periods of phase-coherent convective regeneration.

• The combined effects of thermal excitation over mountains and steady propagation leads to sharply defined patterns in the diurnal cycle. This pattern may be characterized as “streaks” of alternately high and low frequency of occurrence in a Hovmoller diagram. We refer to this pattern as “globally phase-locked”, since a local diurnal cycle is controlled or influenced by remote forcings with a delayed phase of arrival.

• One consequence of the above phenomenon is the superposition of a local maximum of “ordinary” convection with the delayed-phase arrival of remotely-forced events, both of which were forced diurnally. Nevertheless, this gives rise to semi-diurnal signals in traditional harmonic analyses. Such signals have been mistakenly interpreted as evidence of the role of atmospheric tides in semi-diurnal convective triggering.

This presentation will illustrate the diurnal cycle structure over portions of several continents and the importance of systematic propagation and coherent regeneration in the lee of major cordillera.

Figure 1. The increasing seasonal influence of thermal forcing over the Rockies (~106W) on the diurnal frequency of precipitation as estimated from the WSR-88D weather radar network. The diurnal cycle is repeated for clarity. Some events span 2-3 diurnal cycles. Scale on r.h.s is % time in each UTC hour that radar echo from precipitation is present between 30-48N. Adjacent regions can vary by more than 30%, owing to systematic propagation. (after Carbone et al. 2002, JAS)