Observational analyses of heavy spring rainfall during the IFloodS campaign

Abstract:

We examine the regional water cycle in Iowa during the IFloodS campaign period, April-June 2013. The period had exceptionally large rainfall accumulations and rainfall was distributed over an unusually large number of storm days. We utilize high-resolution radar rainfall fields; precipitable water from a network of GPS measurements; vertically-integrated water vapor flux derived from GPS precipitable water, WSR-88D VAD wind profiles, and radiosonde humidity profiles. We show that heavy rainfall is relatively weakly correlated to precipitable water and precipitable water change, with a somewhat stronger correlation to differential water vapor flux in the north-south direction. Thermodynamic properties tied to the vertical distribution of water vapor play an important role in determining heavy rainfall distribution, especially for periods of strong southerly water vapor flux. We examine interrelationships among southerly water vapor flux, low-level jet frequency and rainfall accumulations. It is found that the magnitude of southerly water vapor flux is considerably higher in wet years and lower in dry years. The low-level jet frequency, however, does not exhibit strong interannual variability; the frequency and diurnal cycle are similar for wet and dry years. The finding from the IFloodS period that the relative humidity in the lower atmosphere plays an important role in rainfall amount under strong southerly water vapor flux conditions is consistent with that from a multi-year analysis.