Low‐level jets and the summer precipitation regimes over North America

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Abstract
The summer precipitation regimes over the United States and their relation to two low level jets are examined using a daily gauge‐based precipitation data set, the NCEP‐NCAR reanalysis and the 10‐yr summer (June–September) simulations based on the 50‐km NCEP regional spectral model (RSM) with the initial and boundary conditions provided by the global reanalyses. The quality at regional scales of the RSM simulations is assessed through the comparison with the NCEP's operational Eta Data Assimilation System (EDAS) analyses. The RSM, as EDAS, captures the seasonal evolution of the North American monsoon rainfall and the related vertically integrated moisture fluxes, but details differ. The fluxes associated with the Great Plains low level jet (GPLLJ) and the Gulf of California low level jet (GCLLJ) depicted by the two data sets are similar, but the 50‐km RSM has difficulty in capturing the vertical structure of the meridional moisture flux associated with the GCLLJ. The recurrent summer precipitation regime is a three cell pattern that consists of an inverse phase relationship between the Great Plains and the core monsoon region with an additional weak center over the southeastern United States. Over the southwestern United States, Arizona and New Mexico belong to two different rainfall regimes and have different moisture sources. The out of phase precipitation relationship is consistently related to an out of phase relationship between the two LLJs, whose variations are in turn associated with the upper level jet streams. The strong GCLLJ cases imply a weaker GPLLJ and less rainfall over the central United States and the Mississippi Valley. The strong GPLLJ cases only imply weaker meridional moisture fluxes from northern Mexico or Gulf of California to the southwestern United States and less monsoon rainfall there, but they do not have an impact on moisture fluxes along the Gulf of California.