The vertical extent of drought in the western United States, 1973--2002

In this research, I address the vertical extent of drought in the western United States. Spatial examinations of drought have historically ignored a vertical moisture component. To answer the question, what is the vertical extent of drought, I calculate moisture profiles for drought conditions in the western U.S. I also determine to what extent the vertical moisture profiles of drought are distinguishable from vertical moisture profiles of wet surface conditions.; Using a Geographic Information System (GIS), I created surface "PDSI regions" based on climate divisions and Palmer Drought Severity Index (PDSI) data. These surface PDSI regions identify severe to extreme drought as "dry" and severe to extreme wet conditions as "wet." I calculated vertical moisture profiles in the GIS using standardized NCEP/NCAR Reanalysis (NNR) specific humidity data for each PDSI region at four levels in the atmosphere (850, 700, 600, and 500 hPa).; The results show that the vertical extent of drought in the western U.S. for the period, 1973--2002, extended through the lowest levels of the profile. Discriminant analysis showed that low-level moisture was the most important discriminator between the dry and wet PDSI regions. Temporal and spatial sensitivity analyses revealed finer differences by decade, season, and sub-region of the dataset. Moisture profiles became moister through the decades of the analysis, though only the latest (1993-2002) decade yielded a significant discriminant model. Low-level moisture was also the most important discriminator during the summer and fall seasons. These profile characteristics coincide with seasonal changes in dominant precipitation mechanisms over the study area (convection in warm seasons, and cyclonic activity in winter). Similar regional differences emerged in the sensitivity analyses. In the southern and eastern portions of the study area, dry conditions extended only through low-level moisture was most important in discriminating between the wet and dry regions, while in northern and western portions of the study area, moisture extended through the full profile, and all levels were important in discriminating between the wet and dry PDSI regions.