SECTION 1: PROJECT OBJECTIVES AND ACCOMPLISHMENTS
During 15-17 November 1987 exceptionally heavy (and unforecast) rains fell over the lower Mississippi River Valley (rainfall totals in Louisiana exceeded 50 cm) in association with the passage of two very large mesoscale convective complexes (MCC). Each MCC was associated with a migratory upper-tropospheric potential vorticity (PV) anomaly and associated jet streak that could be readily tracked by means of dynamic tropopause (DT) analyses. Noteworthy aspects of this case, in addition to the heavy rains, included the existence of multiple long-lived wake-low troughs (WLTs) and large-amplitude inertia-gravity waves (IGWs). Surface pressure perturbations associated with these WLTs and IGWs reached ~10 hPa. Widespread rain-cooled air contributed, in part, to the formation of an extensive stable layer in the lower troposphere that served as a wave duct for WLT and IGW amplification and propagation. The details of earlier findings for the 15-17 November 1987 case are contained in Bracken (1995) and Corfidi et al. (1990).
The goal of the new work was to see if we could make an improved precipitation forecast with a finer resolution numerical simulation, given that neither the KF or BM versions of the 29 km MEM were able to predict explicitly the outflow boundary over Louisiana, the prominent wake troughs behind the main convective lines, and the observed inertia-gravity waves. The expectation was that a 10-km version of the MEM run with both the KF and BM schemes would do a better job resolving the important mesoscale boundaries that were crucial to focusing the precipitation than the 29-km MEM. Our scientific hypothesis rested on the physical idea that the failure of the 29-km version of the MEM to simulate the important mesoscale boundaries could be attributed to the inability to resolve the low-level pool of rain-cooled air. The importance of the low-level cool pool is that it serves to focus convergence and low-level warm-air advection. Likewise, if the rain-cooled air is deep enough then subsidence behind the active convective elements in an MCC/MCS or a squall line (e.g., subsidence associated with the rear-inflow jet) can act to produce a prominent surface pressure perturbation that can be associated with the wake-low trough and/or cyclone.
The project goal was not achieved because of severe operational and resource constraints at NCEP as described in section 4. Likewise, at the Albany end, Ed Bracken is no longer available to assist, having graduated and moved on to full-time employment. Even though the project (and project extension) has terminated, the two PIs (Bosart and Rogers) will attempt to complete the work on a time-available basis and subject to a relaxation of the resource constraints at NCEP.
SECTION 3: PRESENTATIONS AND PUBLICATIONS
No additional publications were completed beyond what were prepared in the first project. A paper summarizing the synoptic and mesoscale aspects of the 15-17 November 1987 rainstorm and wake low/trough event is expected to be prepared and submitted to the Monthly Weather Review in 2000.
SECTION 4: SUMMARY OF BENEFITS AND PROBLEMS ENCOUNTERED
4.1 (University side):
The main benefit to the university was that advanced graduate students (e.g., Bracken) were exposed to the day-to-day scientific and operational challenges associated with running an international modeling center such as NCEP. The primary problem is the no one at Albany anticipated the extraordinary challenges and obstacles in the way of getting cooperative research done with participating scientists of a resource-starved organization such as NCEP. The lasting lesson from this experience is that successful cooperative research with NCEP scientists must await the adequate base funding of the National Test Facility.
4.2 (NCEP side):
The main benefit to NCEP from this work was the opportunity to assess and diagnose the performance of the operational BM and the experimental KF scheme in the MEM for an extreme rain event. The performance of the operational MEM with the BM scheme made it clear to NCEP that the current operational MEM is incapable to resolving the fine scale structures observed, due to insufficient horizontal resolution, inadequate initialization of relevant surface / boundary layer features, and deficiencies in the model physics. The marked improvement in the precipitation forecast obtained from the KF scheme shows much promise. However, the results from the 29-km MEM BM and KF runs of this case show that NCEP needs to put considerable work into 1) successfully forming and maintaining mesoscale structures (e.g., cool-pools) and 2) improving the operational 3-d variational analysis (3DVAR) to analyses observed mesoscale features at and near the surface. NWS partners have been unable to perform any work on this project during the last six months due to 1) computing time limitations on the NCEP Cray supercomputers; 2) work on conversion of Eta and NGM codes from the NCEP Cray to the IBM-SP computer, and 3) the fire which destroyed the NCEP C-90 computer of 27 September which eliminated any use of the remaining Crays for non-operational use and rendered all codes used to run the KF forecasts for the Nov 87 case inaccessible.