The Effect of Upstream Convection on Downstream Precipitation

Abstract

Numerical weather prediction (NWP) models have demonstrated a weakness in the ability to accurately forecast precipitation amounts downstream of strong, organized convection in the Southeast US. This weakness has also been communicated by operational forecasters, as past events have exhibited reduced downstream precipitation amounts relative to the model quantitative precipitation forecast (QPF), particularly when the upstream convection (UC) feature moves rapidly eastward. Conventional forecaster wisdom evolving from such events thus advocates reducing downstream QPF in the presence of UC.

The purposes of this study are (i) to identify the physical processes by which downstream precipitation may be reduced or enhanced in the presence of UC; (ii) distinguish UC cases in which downstream QPF should be reduced from those in which it should be enhanced; (iii) understand why operational models are challenged to produce an accurate downstream forecast in these situations; (iv) identify synoptic settings associated with different types of UC events; (v) find ways in which human forecasters may anticipate and improve upon erroneous model forecasts; and (vi) investigate optimal model configurations for the representation of UC and downstream QPF.

A brief climatology of National Centers for Environmental Prediction (NCEP) Eta model QPF error revealed that the UC problem is observed to occur in three relatively distinct synoptic settings, in which different QPF biases tended to occur. The three scenarios are based on the orientation and movement of the UC. Scenario 1 (S1) is characterized by UC that is oriented parallel to mean flow, and the organized convective system propagates quickly eastward relative to the primary synoptic system in a direction perpendicular to flow. These cases were generally characterized by a positive model QPF bias. Scenario 2 (S2) features UC that is oriented parallel to mean flow as in S1, but that propagates slowly (or not at all) with respect to the primary synoptic system. It was hypothesized that in these cases a diabatically-enhanced low-level jet (LLJ) would act to enhance moisture transport ahead of the synoptic system, and these cases were therefore hypothesized to yield increased downstream precipitation amounts, albeit with smaller QPF errors. Scenario 3 (S3) is characterized by UC oriented perpendicular to flow, usually aligning with a coastal and/or warm frontal features. S3 cases were found to often transition into an S1 or S2 event. They were consequently treated as precursors to S1 or S2 cases and are not investigated in great detail here.

Several potential physical mechanisms of downstream precipitation alteration were examined for each scenario. These physical mechanisms include (i) moisture consumption; (ii) stabilization of the downstream environment; (iii) alteration of lower-tropospheric moisture transport through interruption (S1) or enhancement (S2) of the LLJ; and (iv) alteration of synoptic dynamics.

Two main case studies were undertaken to investigate S1 and S2. For the S1 case it was found that a high-resolution (4-km grid-spacing) convection-resolving forecast using the Weather Research and Forecast (WRF) model vastly improved the downstream QPF relative to the large overprediction of precipitation produced by the operational Eta forecast. Comparisons were made between the successful WRF run, analyses, and the erroneous operational Eta forecast. Errors in the low-level wind and horizontal moisture flux fields from the operational Eta forecast revealed that the delayed speed of the model-forecasted UC did not permit an accurate representation of low-level moisture transport (physical mechanism (iii)), causing the model to allow too much moisture to infiltrate the downstream area, and likely contributing to the model overprediction of precipitation. The S1 case showed that despite excellent operational Eta forecasts of the placement and intensity of the synoptic system, errors in model-forecasted convective motion resulted in large operational QPF errors downstream.

The S2 case study revealed that UC does not always lead to a decrease in downstream precipitation. Potential vorticity (PV) diagnostics were used to demonstrate that the diabatic influence of the UC acted to enhance the low-level jet (LLJ) and increase moisture transport (physical mechanism (iii)). A quasi-geostrophic PV inversion showed that the diabatic cyclonic QGPV anomaly associated with the UC feature contributed significantly to the southerly LLJ preceding the convective line. S2 proved to be an important counter-example to the notion that UC generally reduces downstream precipitation, as this case study demonstrates a physical process associated with the UC that instead acts to enhance downstream precipitation.

The findings of this study may be helpful to forecasters in anticipating future UC events, as well as to numerical modelers facing decisions regarding what type of model run will yield the best results for future UC events. The results are further analyzed in the context of the ability of cumulus parameterization (CP) schemes to accurately represent convective movement, and the implications of such a shortcoming for UC forecasts. Future work in this area is suggested.