Submitted to Weather and Forecasting December 1994, revised September 1995

PREDICTING THE MOVEMENT OF MESOSCALE CONVECTIVE COMPLEXES

S.F. Corfidi

Storm Prediction Center

Norman, OK 73069

J.H. Merritt and J.M. Fritsch

Department of Meteorology

The Pennsylvania State University

University Park, PA 16802


ABSTRACT

A procedure for operationally predicting the movement of the meso-beta scale convective elements responsible for the heavy rain in mesoscale convective complexes is presented. The procedure is based on the well known concepts that the motion of convective systems can be considered as the sum of an advective component, given by the mean motion of the cells comprising the system, and a propagation component, defined by the rate and location of new cell formation relative to existing cells. These concepts and the forecast procedure are examined using 103 mesoscale convective systems, 99 of which are mesoscale convective complexes.

It is found that the advective component of the convective systems is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional (but opposite in sign) and well correlated to the speed and direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the meso-beta scale convective elements are 0.80 and 0.78, respectively. Mean absolute errors of the speed and direction are 2.0 ms-1 and 17 degrees, respectively. These errors are sufficiently small so that the forecast path of the centroid of the meso-beta scale elements would be well within the heavy rain swath of the typical mesoscale convective complex.


1. INTRODUCTION

Mesoscale convective complexes (MCCs; Maddox, 1980) are responsible for most of the warm-season rainfall over the Great Plains (Fritsch et al. 1986). They also produce damaging winds and hail, with nearly one quarter of them resulting in injury or death (Maddox et al. 1986). Moreover, MCCs substantially alter high-altitude wind fields and can be a significant factor in flight safety and efficiency (Fritsch and Maddox 1981, Maddox et al. 1981). Nevertheless, in spite of their substantial contribution to the production of significant weather, these systems are not forecast very well. This is evident from the historically low levels of skill in quantitative precipitation forecasting (QPF) during the warm season (Ramage 1982, Heideman and Fritsch 1988, Funk 1991, Junker et al. 1992, Olson et al. 1995).

The purpose of the present study is to try to increase skill in warm season QPF by developing a procedure for forecasting MCC movement. The foundation of the procedure rests on the well known concept that the motion of convective systems can be considered as the sum of an advective component, given by the mean motion of the cells comprising the system, and a propagation component, defined by the rate and location of new cell formation relative to existing cells (Newton and Katz 1958, Newton and Newton 1959, Chappell 1986, Jiang and Scofield 1987). It is hypothesized that 1) the advective component of MCC movement is proportional to the mean flow in the cloud layer (VCL) and 2) that the propagation component (VPROP) is directly related (but opposite in sign) to the speed and direction of the low-level inflow feeding new cell development. In order to explore these hypotheses, we will select parameters indicative of the advective and propagative components, determine empirically if these parameters relate to the observed motion of MCCs, and then use the resulting relationships to forecast the movement of the convective systems.

It is important to note here that since the emphasis of the study is on QPF, the measure of movement of MCCs will not be the cold cloud shield centroid used by Maddox (1980). Rather, we will focus on the radar-observed movement of the meso-beta scale (Orlanski 1975) convective elements (MBEs) that McAnelly and Cotton (1986) have shown are responsible for the heaviest rainfall. Thus, we have constructed a very simple conceptual model (Fig.1) which shows the MBE movement as the vector sum of the mean flow in the cloud layer and the propagation component. In this model, it is assumed that the low-level jet (VLLJ) is a good indicator of the low-level inflow to the convective systems and that VPROP is equal and opposite to VLLJ . The model therefore implies that: 1) the greater the mean flow in the cloud layer, the greater the advective component, 2) the stronger the low-level jet, the stronger the propagation component toward the source of the inflowing air, and 3) the movement of the MBEs (VMBE) is given by the difference between the mean flow in the cloud layer and the low-level jet, i.e.,

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