HR: 1330h
AN: A13A-08    [Abstracts]
TI: The Sensitivity of the Northeast Colorado Moist Convective Environment to Upstream Soil Moisture Conditions
AU: * Grimaldi, R T
EM: grimalr@oneonta.edu
AF: SUNY Oneonta, Ravine Parkway, Oneonta, NY 13820 United States
AB: Statistical evidence supports a hydro-dynamic link between severe thunderstorm activity in Northeast Colorado and antecedent snow condition in the upstream higher elevations. Two subsets of seven runoff seasons were created, based on the criteria of anomalously high and low cumulative streamflow discharge from the Colorado Rockies. Observational evidence suggests that the morning time lower atmosphere, during the months of May and June, over Denver, is cooled and moistened following an anomalously large runoff season when compared to seasons of meager runoff. Furthermore, comparison of Northeast Colorado severe thunderstorm reports reveals that severe weather occurrences of hail greater than 1 inch in diameter, tornados, and damaging thunderstorm downdrafts, occurred on average 51 minutes earlier following years of anomalously high runoff compared to the low runoff years. The character of severe weather also appears to be altered so that high runoff years yield a significantly reduced percentage of tornadic reports over the Northeast Colorado plains. The proposed mechanism put forth to explain the presumed alteration of the lee plains' convective environment and the nature of severe thunderstorm activity, links alpine surface moisture conditions to lagged thermal and moisture attributes of the downstream elevated mixed layer which caps the convective boundary layer. Moist surfaces attributed to snowpack, ponding of melt water, and saturated soils are known to increase evapotranspiration so that the coupled boundary layer is cooler and moister than would be observed under drier conditions. The nocturnal decoupling of the boundary layer from the surface forms a residual layer which is surmised to retain the attributes imparted to it according to the degree of soil moisture present during the previous day. Mean late spring/early summer prevailing wind velocity supports the likely presence of an elevated mixed layer, with similar attributes of the aforementioned residual layer, to overly the downstream lee plains during the course of the following day. Consequently, the apparent atmospheric response over Northeast Colorado to (increased /decreased) surface moisture at upstream high elevations is to (weaken/strengthen) the capping layer. It follows that the warming of the capping layer, which would follow dry conditions in the upstream higher terrain, should lead to a reduction in boundary layer depth, a delay in the diurnal timing of thunderstorm formation, and an increase in the spacing between penetrating deep moist convective cells, all of which are known to increase the likelihood and/or magnitude of severe weather in this region, while at the same time, reducing areal coverage of convective rainfall. Observational evidence supports this conclusion.
DE: 0315 Biosphere/atmosphere interactions
DE: 1818 Evapotranspiration
DE: 1833 Hydroclimatology
DE: 1878 Water/energy interactions
SC: Atmospheric Sciences [A]
MN: 2005 Joint Assembly