The timing and intensity of rainfall events over the United States is changing in response to a warming atmosphere. The changing behavior of extreme precipitation events such as drought and floods may have costly economic and human safety consequences if not forecasted and planned for correctly. On the forecasting side, the diurnal mode of the hydrologic cycle needs to be well represented especially in areas where precipitation is strongly linked to diurnal processes, as is the case over the U.S. Corn Belt. The major theme of this dissertation project was to better understand changing precipitation patterns over this region as a dynamic response to the intensification of the diurnal mode of the hydrologic cycle. A very thorough understanding of the dynamic system can help us improve model representation of the diurnal cycle mode to improve climate forecasts.

The structure of this dissertation began with first understanding how the diurnal mode of the hydrologic cycle was linked to the Great Plains low-level jet (GPLLJ) to maintain warm season rainfall over the central U.S. The majority of this rainfall is produced by nocturnal Mesoscale Convective Systems (MCSs) which may be initiated or enhanced by the GPLLJ. The diurnal variation of water vapor supply to maintain this central U.S. rainfall is an integrated part of the continental and global scale circulation. GPLLJ events were collected over June, July, and August (JJA) during the years 1979-2010. Diurnal components of precipitation and water vapor transport were isolated and compared between GPLLJ events and the climatological composite state. Three distinct synoptic environments in which GPLLJ events occurred were identified and assessed by their impact on water vapor transport to the central U.S. The climatological warm season phase of diurnal rainfall was shown to exhibit a clockwise rotation around the continental U.S.

Next, the interannual and interdecadal variation of rainfall over the U.S. Corn Belt was investigated as a response to diurnal cycle mode intensification. Extreme precipitation events were also investigated. The phase and amplitude of diurnal hydrologic fields were evaluated and compared between months JJA over years 1979-2010. 20 flood events and 6 drought periods were examined to determine diurnal deviations that support extreme events. Over the U.S. Corn Belt, the component of diurnal water vapor transport in phase with diurnal precipitation is the streamfunction, whereas over the southeast U.S. it is the potential function. The amplitude of climatological JJA diurnal rainfall was similar, though the phase of August diurnal rainfall was 3 hours later in the nighttime. The major difference between diurnal water vapor transport between the 1993 flood and 1988 drought was timing, not amplitude. Comparing all extreme events, amplitude rather than phase of diurnal rainfall tends to deviate during drought events while both amplitude and phase deviate from the climatological mean during flood events.

Intense GPLLJ activity was shown to be increasing on a decadal scale consistent with an increasing decadal trend in rainfall over the Great Plains. Comparing the 1979-1988 and 2001-2010 decades suggests that maximum diurnal precipitation over the U.S. Corn Belt is falling earlier in the warm season as a response to changing GPLLJ activity. The Empirical Orthogonal Function (EOF) analysis was used to illustrate the large percentage of total variance explained by the diurnal modes of precipitation and water vapor transport. Despite being closely linked dynamically, the variance of the top three EOF modes of precipitation showed a decreasing decadal trend that was not matched by the trend in water vapor transport.