The 4 June 1999 Derecho Event: A Particularly Difficult Challenge for Numerical Weather Prediction

Abstract

Warm season convective system rainfall forecasts remain a particularly difficult forecast challenge. For these events, it is possible that ensemble forecasts would provide helpful information unavailable in a single deterministic forecast. In this study, an intense derecho event accompanied by a well-organized band of heavy rainfall is used to show that for some situations, the predictability of rainfall even within a 12-24-h period is so low that a wide range of simulations using different models, different physical parameterizations, and different initial conditions all fail to provide even a small signal that the event will occur. The failure of a wide range of models and parameterizations to depict the event might suggest inadequate representation of the initial conditions. However, a range of different initial conditions also failed to lead to a well-simulated event, suggesting that some events are unlikely to be predictable with the current observational network, and ensemble guidance for such cases may provide limited additional information useful to a forecaster. KeywordsAgronomy, atmospheric thermodynamics, climate change, climatology, heat convection, mathermatical models, rain, numerical weather prediction, rainfall forecasts, warm season convective system, weather forecasting, ensemble forecasting, parameterization ABSTRACT Warm season convective system rainfall forecasts remain a particularly difficult forecast challenge. For these events, it is possible that ensemble forecasts would provide helpful information unavailable in a single deterministic forecast. In this study, an intense derecho event accompanied by a well-organized band of heavy rainfall is used to show that for some situations, the predictability of rainfall even within a 12-24-h period is so low that a wide range of simulations using different models, different physical parameterizations, and different initial conditions all fail to provide even a small signal that the event will occur. The failure of a wide range of models and parameterizations to depict the event might suggest inadequate representation of the initial conditions. However, a range of different initial conditions also failed to lead to a well-simulated event, suggesting that some events are unlikely to be predictable with the current observational network, and ensemble guidance for such cases may provide limited additional information useful to a forecaster.

On the Impact of WRF Model Vertical Grid Resolution on Midwest Summer Rainfall Forecasts

“…3b). Both the 31L and 62L runs had a low bias for lighter rainfall thresholds and a high bias for heavier thresholds, similar to what was found in Jankov et al (2007) and Shaw (2004) for runs with explicit rainfall, a feature uncommon for model runs using convective parameterizations (e.g., Gallus et al 2005).…”

“…Although the runs using the KF CP, namely, 31L_KF and 62L_KF, performed better than both the 31L and 62L runs using the BMJ CP for all rainfall thresholds, the use of the KF CP did not result in a different degree of sensitivity to VGR (Table 2). Bias scores were lower for all thresholds in the KF CP runs compared to the BMJ CP runs, a result also found in Gallus and Segal (2001).…”

“…3a), a result common for warm-season events (e.g., Gallus and Segal 2001;Jankov and Gallus 2004). The ETS values were noticeably worse in 62L than in 31L for the lowest rainfall threshold for five microphysical schemes, possibly because of the more frequent grid cell saturation that was found (as evaluated later in this section), which lead to higher biases (more grid points with rainfall) and higher bias errors (Fig.…”

“…First, because some studies (e.g., Gilmore et al 2004;Jankov et al 2005) have shown that the choice of microphysical scheme can have a large impact on the simulated rainfall, the VGR impact associated with the changes in model microphysics was examined for the 23-24 June 2003 case, chosen because of its relatively high QPF skill. The following microphysical schemes were used for this evaluation: WRF single-moment three-class (WSM3; Hong et al 2004), WRF singlemoment five-class (WSM5; Hong et al 2004), WRF single-moment six-class (WSM6; Hong and Lim 2006), Thompson (Thompson et al 2006), Lin (Lin et al 1983), and Ferrier (Rogers et al 2001).…”

On the episodic nature of derecho-producing convective systems in the United States

“…Further, derechos continue to be forecasted poorly by operational numerical guidance utilized by forecasters, and often, they cannot be simulated, even after the event, using contemporary mesoscale models and superior data assimilation methods (e.g. Gallus et al, 2004). Hence, it is important to continue to increase our understanding of these inadequately forecasted, often devastating, windstorms through applied research such as this climatology.…”