UCLA

Predecessor rain events (PREs) are coherent mesoscale rainstorms that occur well in advance of recurving tropical cyclones (TCs) and have a high potential to cause flooding and adverse societal impacts. In 2007, a PRE associated with TC Erin produced record-breaking rainfall (> 350 mm) across southern Minnesota and Wisconsin, contributing to seven fatalities and over $170 million in property damage. A series of idealized numerical simulations is conducted using an aquaplanet version of the Weather Research and Forecasting model Advanced Research core (WRF-ARW) v.3.6 to examine the influence of cloud-radiative processes on the development of PREs.

This study finds that cloud-radiative feedback (CRF), the interaction of hydrometeor cloud species with longwave (LW) and shortwave (SW) radiation, produces a more robust PRE structure with stronger convective activity and, ultimately, more precipitation. It is demonstrated that LW cooling associated with clouds outside of the PRE region induces a stronger horizontal pressure gradient that enhances low level con ergence and drives more vigorous ascent. Therefore, the primary radiation driver of PRE formation occurs outside of the PRE itself and is based on how the model responds radiatively to low clouds. In addition, the warming component of CRF enhances parcel buoyancy and reduces vertical stabilities within the PRE structure. In particular, the distribution of cloud ice produces a greater depth of in-cloud, primarily LW warming conducive to stronger convective processes. In contrast, SW CRF effects weaken PRE development through a combination of in-cloud cooling and low level cloud top warming, countering the LW CRF effects responsible for greater rainfall production. Moreover, PRE formation is found to be sensitive to the diurnal cycle, resulting in faster development at night and slower development during the day. A seasonal sensitivity also brings about weaker PRE intensification during the summer season; shorter nights and longer days minimize favorable LW CRF effects and maximize unfavorable SW CRF effects.

PRE development depends on the balance between subtle, competing processes involving radiation and microphysics schemes. The high sensitivity of PRE initiation and intensification to variations in CRF cannot be ignored and necessitates further research and observations to improve operational forecasts. Because the recipe for PRE development requires many individual ingredients to come together correctly, accurately predicting heavy rainfall events associated with tropical cyclones remains a daunting forecast challenge.