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Trent W. Ford, Anita D. Rapp, and Steven M. Quiring


Soil moisture is an integral part of the climate system and can drive land–atmosphere interactions through the partitioning of latent and sensible heat. Soil moisture feedback to precipitation has been documented in several regions of the world, most notably in the southern Great Plains. However, the impact of soil moisture on precipitation, particularly at short (subdaily) time scales, has not been resolved. Here, in situ soil moisture observations and satellite-based precipitation estimates are used to examine if afternoon precipitation falls preferentially over wet or dry soils in Oklahoma. Afternoon precipitation events during the warm season (May–September) in Oklahoma from 2003 and 2012 are categorized by how favorable atmospheric conditions are for convection, as well as the presence or absence of the Great Plains low-level jet. The results show afternoon precipitation falls preferentially over wet soils when the Great Plains low-level jet is absent. In contrast, precipitation falls preferentially over dry soils when the low-level jet is present. Humidity (temperature) is increased (decreased) as soil moisture increases for all conditions, and convective available potential energy prior to convection is strongest when atmospheric humidity is above normal. The results do not demonstrate a causal link between soil moisture and precipitation, but they do suggest that soil moisture feedback to precipitation could potentially manifest itself over wetter- and drier-than-normal soils, depending on the overall synoptic and dynamic conditions.

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Anita D. Rapp, Alexander G. Peterson, Oliver W. Frauenfeld, Steven M. Quiring, and E. Brendan Roark


Tropical Rainfall Measuring Mission Precipitation Radar precipitation features are analyzed to understand the role of storm characteristics on the seasonal and diurnal cycles of precipitation in four distinct regions in Costa Rica. The distribution of annual rainfall is highly dependent on the stratiform precipitation, driven largely by seasonal increases in stratiform area. The monthly distribution of stratiform rain is bimodal in most regions, but the timing varies regionally and is related to several important large-scale features: the Caribbean low-level jet, the ITCZ, and the Chorro del Occidente Colombiano (CHOCO) jet. The relative importance of convective precipitation increases on the Caribbean side during wintertime cold air surges. Except for the coastal Caribbean domain, most regions show a strong diurnal cycle with an afternoon peak in convection followed by an evening increase in stratiform rain. Along the Caribbean coast, the diurnal cycle is weaker, with evidence of convection associated with the sea breeze, as well as a nocturnal increase in storms. The behavior of extreme precipitation features with rain volume in the 99th percentile is also analyzed. They are most frequent from May to November, with notable differences between features at the beginning/end of the rainy season versus those in the middle, as well as between wet and dry seasons. Convective rain exceeds stratiform in winter and midsummer extreme features, while stratiform rain is larger at the beginning and end of the wet season. Given projected changes in precipitation and extreme events in Costa Rica for future climate change scenarios, the results indicate the importance of understanding both changes in total precipitation and in the storm characteristics.

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