The effects of organized upstream convection on downstream precipitation: Physical processes and model representation

As communicated by National Weather Service forecasters in the Southeast US and ongoing case-study research, past events have demonstrated a weakness in the ability of numerical weather models to accurately simulate the effects of upstream convection on quantitative precipitation forecasts downstream of the convection, especially for cases with rapidly moving mesoscale convective systems (MCSs).

Preliminary case studies demonstrated that the presence and character of organized convection upstream of the study area did exert an influence on the downstream QPF. Further inspection of these cases indicates that there are at least two primary differing scenarios for upstream convection in the Southeast US: The first of these scenarios features upstream convection that moves quickly eastward with respect to the speed of the parent system. In these cases, the presence of intense upstream convection southeast of the parent cyclone often reduces precipitation in the downstream region to the east of the cyclone by (i) disrupting moisture transport as the convectively altered momentum field becomes more westerly in the wake of the MCS, (ii) convective moisture removal in the upstream air mass, (iii) convective stabilization of the upstream air mass, or (iv) alteration of the downstream synoptic forcing for ascent. Analysis of several cases of this type reveals that model QPF often exhibits a strong positive bias in these instances, and a brief climatology shows that of the two primary scenarios, this one occurs with the most frequency. Alternatively, the other scenario features convection moving slowly with respect to the main system that may enhance moisture transport via a diabatically-enhanced low-level jet (LLJ), and has been observed to thereby increase downstream precipitation.

The purposes of this research are to identify the physical mechanism(s) associated with the convection that most strongly affect downstream QPF, and also to pinpoint the mechanism(s) that are misrepresented in operational numerical forecasts in order to eventually improve downstream QPF in upstream convection events.

In the case of the first scenario (featuring a quickly-propagating upstream convective feature) a detailed case study shows that NWP model inability to sufficiently resolve the near-storm convective environment in fast-moving convective lines may inhibit accurate downstream QPFs. Initial results suggest that model misrepresentation of convective propagation is tied to the QPF bias. As most operational cumulus parameterization (CP) schemes do not sufficiently account for the process of convective momentum transport, it is hypothesized the omission of this process may inhibit numerical models from accurately representing squall line movement, particularly for fast-moving cases. This idea is explored through high-resolution forecasts using the WRF modeling system, as well as model experiments that compare model forecasts produced both with and without the use of a CP scheme.

Implications for operational model interpretation, as well as forecast practices in the presence of upstream convection are discussed, along with model configuration considerations for optimal representation of organized convective systems.