Diurnal precipitation variations in south-central New Mexico

Orographic forcing of diurnal-precipitation variations in south-central New Mexico is examined. Harmonic analysis reveals a strong diurnal cycle in precipitation frequency at all stations studied. In addition, relatively high amplitudes in the second, third, and fourth harmonics were present at several stations in the region. Cumulant methods confirm the importance of the higher harmonics and can also divide the stations into precipitation regimes. At each of the stations one of the maxima in the precipitation frequencies appears to be due to surface convergence caused by a mountain-valley circulation system. Surface wind data support this explanation. All stations have a maximum near midnight local time, which seems to have its source in larger-scale forcing. A possible cause is diurnal variations in the plateau circulation system of the western United States. Upper-air wind data indicate that such variations could result in the formation of a low-level jet that would destabilize the atmosphere near midnight local time.