M8 Diurnal Cycle

Topic

The diurnal cycle is the periodic twenty-four-hour oscillation of atmospheric variables, driven by the variation in solar heating between day and night. It is the most regular and predictable forcing mechanism in the entire atmospheric system; this very regularity causes the diurnal cycle to exhibit a much greater amplitude over landmasses than over oceans, as land heats and cools far more rapidly than water. The effects of this cycle extend to convection, the atmospheric boundary layer, atmospheric chemistry, and the hydrological cycle, though it is convection and the atmospheric boundary layer that most clearly reveal the cycle's hour-by-hour dynamics.

Over tropical landmasses, convective activity peaks in the afternoon—between 3:00 and 6:00 PM local time—coinciding with the daily maximum of available convective energy. This alignment between peak convective energy and the warmest part of the afternoon causes convection over land to occur almost exclusively during that same time window.

Over tropical oceans, by contrast, the peak in convection occurs not in the afternoon but in the early morning—between 3:00 and 6:00 AM local time—driven by a mechanism distinct from the one dominating over land: during the night, cloud tops lose heat through radiative cooling into space, and this cooling increases the cloud's internal instability, triggering convection just before dawn. Mesoscale convective systems forming over these regions do not always dissipate at nightfall; many continue to propagate overnight, driven by the combination of their own cold pool—the dense air left near the ground by downdrafts—and the nocturnal low-level jet, a wind current that intensifies after sunset. It is this combination of the cold pool and the nocturnal low-level jet that allows these mesoscale convective systems to keep advancing and producing rain long after the sun has set.

Within the realm of mesoscale meteorology, the diurnal cycle of the atmospheric boundary layer is the most extensively studied process: at sunrise, surface heating generates turbulence that erodes the nocturnal temperature inversion and causes the convective boundary layer to grow, whereas at sunset, surface cooling leads to the formation of a stable nocturnal boundary layer. Within this stable nocturnal boundary layer, the aforementioned nocturnal low-level jet reappears; reaching speeds of fifteen to twenty-five meters per second, it plays a critical role in both transporting moisture and sustaining the nocturnal convection that relies on it.

Thus, from the peak convection occurring over continents in the afternoon and over oceans in the early morning, to the nocturnal low-level jet generated by the atmospheric boundary layer at sunset—which subsequently fuels mesoscale convective systems throughout the night—the diurnal cycle ultimately organizes, hour by hour, nearly everything that occurs in the atmosphere near the surface. It is this very twenty-four-hour regularity that makes the diurnal cycle the easiest atmospheric forcing to predict, while simultaneously making it a primary shaper of the weather experienced each day.

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