On the importance of environmental factors in influencing the evolution of morning Great Plains MCS activity during the warm season
Carl E. Hane, NOAA/NSSL, Norman, OK; and D. L. Andra, J. A. Haynes, T. E. Thompson, and F. H. Carr
Mesoscale convective system (MCS) activity maximizes during nighttime hours over the central United States. Forecasters are aware that this activity generally decreases in intensity or dissipates during the four hours or so before local noon. The reason or reasons for this evolutionary behavior in the late morning are not well understood. Whether these systems maintain themselves or decrease in intensity through the late morning therefore represents a significant forecast problem.
A climatological study has been carried out including 145 of these morning systems that affected the county warning areas of Norman, Oklahoma and Dodge City, Kansas over a period of five summers (1996-2000). From that work it was learned that a majority of the systems that affected this limited area were initiated during the previous afternoon or evening near terrain features in Colorado and New Mexico. It was also learned that about 60% of these systems either decreased in intensity or dissipated in the 13-17 UTC (7-11 AM) period, and another 12% dissipated between 09 and 13 UTC. Recently, systems that occurred during 4 additional summers (2001-2004) were examined and results added to this climatological study.
Examination of the influence of various environmental factors on the evolution of these systems has been undertaken in the hope of finding keys to aid in MCS forecasting. Because high temporal and spatial resolution data are necessary to assess these influences, Rapid Update Cycle (RUC-2) model data were used to characterize the environment of 48 systems that occurred during 1999 and 2000 by deriving model soundings at hourly intervals 50 km in advance of each system as it moved through the area. Work in the near future will add environmental influence analysis during another 5 summers (2001-2005) to increase the sample size.
Recent work as part of this project has included examination of combinations of environmental parameters in relation to the observed evolution of these 48 systems during the 13-17 UTC period. Two categories of evolution are defined: a ‘decreasing' category includes 32 cases that either decreased or dissipated, while a ‘non-decreasing' category includes the remaining 16 cases that either remained steady or increased in intensity during the period. Among the many environmental parameters that are being examined are stability measures (e.g., CAPE and lifted index), low-level and deep-level vertical wind shear in the plane of system motion, low-level horizontal flux of water vapor toward the system, and shear offset (a measure of the vertical shear vector deviation to the left of the system motion vector in an elevated layer).
Discriminant analysis has been used in an attempt to identify combinations of environmental factors that have strong influence on determination of system evolution between the two above-described categories. To date a total of 61 combinations have been evaluated using this approach. The table below lists results for some of the more promising combinations. Efforts are underway to incorporate these results into the forecasting procedures at the Norman WFO for testing purposes.
Table 1. The first column includes 3-variable combinations including convective available potential energy (ca), direction of system motion (dir), shear offset (sof), system speed (spd), 0-2 km water vapor flux toward system (qf2), 350 hPa north-south wind component (v35), lifted index (li), 0-2 km shear in the plane of system motion (s02), and 0-4 km water vapor flux (qf4). The second column includes the number and percentage of correct classifications within the “non-decreasing” evolution regime. The third column lists the same quantities within the “decreasing” evolution regime, and the last column lists the total number of correct classifications (out of 48 possible).
COMBO G1 (steady) G2 (decr) CC
ca/dir/sof 12/16 75.0% 28/32 87.5% 40
ca/spd/sof 12/16 75.0% 28/32 87.5% 40
ca/qf2/v35 13/19 68.4% 26/29 89.7% 39
li/s02/spd 10/13 76.9% 29/35 82.9% 39
li/qf4/dir 11/15 73.3% 28/33 84.8% 39
li/sof/dir 12/15 80.0% 29/33 87.9% 41
Extended Abstract (472K)
Poster Session 3M, Mesoscale Processes, Dynamics, and Predictability
Tuesday, 25 October 2005, 6:30 PM-8:30 PM, Alvarado F and Atria
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