Scale analysis of the diurnal cycle of precipitation over Continental United States

Abstract

Rainfall initiation is related to diurnal and semidiurnal radiation forcing (e.g. Wallace 1975, Carbone et al. 2002, Surcel et al. 2010). Much of the observed warm season rainfall results from a thermodynamic response to strong diurnal cycle of land surface temperature. Therefore, over some continental regions deep convection tends to peak around local afternoon and early evening hours. However, there is regional uniqueness in the precipitation pattern that implies a connection between regional characteristics and the behaviour of the precipitation field (Wallace 1975, Carbone et al. 2002, Lee et al. 2007). Over western US the diurnal precipitation pattern becomes well organized with a late afternoon maximum along the eastern slopes of the Rocky Mountains (Carbone et al. 2002, Ahijevych et al. 2004, among others). This mountain-initiated convection tends to propagate away, leading to the local evening maximum over the adjacent plains (Lee et al. 2007). The daily occurrence of propagating systems has a high impact on the continental diurnal cycle of precipitation. Parker and Ahijevych (2007) found that approximately 90% of the episodes identified in the east-central US were due to propagating systems from the west. A consequence of these systems result on the transport of the diurnal cycle from west to east (Surcel et al. 2010). The objective of this work is to study the scale dependence of the diurnal cycle and the variability of the rainfall field with the location and time of the day, with special focus on the role of the different spatial scales in such variability.