University of Wisconsin (Suzanne Wetzel, COMET graduate student fellow): "Development of diagnostic tools for forecasting heavy precipitation in the occluded sector of winter cyclones"

Final Report

1 Overview

The final work presented in this report differs somewhat from the original project titled "A proposal for the development of diagnostic tools for forecasting heavy precipitation events in winter cyclones." It was proposed that there would be an emphasis on "investigating the dynamical signals responsible for the veracity of [empirical rules of thumb], thereby increasing their value as forecast tools." This analysis was performed for one traditional snowfall accumulation prediction technique (the Garcia Method); however, it was determined that a more significant improvement to forecasting winter precipitation could be made by introducing an entirely new technique that builds on the idea of an ingredients-based methodology. Furthermore, the instability ingredient was identified to be an important yet often overlooked element of winter season precipitation forecasting; thus, a detailed investigation of instability in mid-latitude cyclones was also performed. This instability analysis is in preparation for submission to Weather and Forecasting by June 2000.

The highlights of the operational accomplishments of this project include the development and implementation of an ingredients-based diagnostic tool for forecasting winter season precipitation events. Through a series of seminars and a web page, this tool was introduced to NWSFOs throughout the central region and has been employed operationally (see Appendix A). Currently, the ingredients approach material from the web page and thesis is being adapted for NWS training by Scott Bachmeier of UWMadison's SSEC and Richard McNulty of the National Weather Service Training Center.

2 Synopsis of Research Project

The first objective of this project was to explore an ingredients-based alternative to traditional rule-of-thumb techniques for the analysis and prediction of mid-latitude winter season precipitation. The ingredients-based methodology (IBM) provides a systematic approach to forecasting winter season precipitation by establishing a framework for interpreting numerical forecast model output and observations. The IBM presented in this work diagnoses five key ingredients in a winter precipitation event: quasi-geostrophic forcing for ascent, moisture, instability (i. e., gravitational, inertial, or slantwise instability), precipitation efficiency (specifically cloud microphysical properties), and temperature.

The forcing ingredient was combined with the instability ingredient to form a new parameter, QPV, that serves as an indicator of heavy precipitation potential by identifying regions where these two ingredients coexist. The ingredient diagnostics and QPV were incorporated into ingredients maps to facilitate a systematic approach to forecasting the duration, intensity, and type of precipitation. Examples of the application of the IBM to forecasting winter precipitation events were investigated, including a case study of a heavy snow event that occurred on March 13-14, 1997 in the upper Midwest.

The second objective of this project was to perform a detailed investigation of the instability ingredient in mid-latitude cyclones. Instability is arguably the least understood and most often overlooked ingredient in winter storm events. Though neither a necessary nor sufficient condition for precipitation to occur, instability modulates the response of an air column to forcing for vertical motion. A given forcing will produce considerably more vertical motion if it coincides with a region of instability. As a result, forecasts tend to under-predict snow amounts when convection, the release of instability, enhances the precipitation rate. This investigation focused on evaluating the instantaneous distribution and temporal evolution of conditional and potential instability in winter cyclones by considering the local rate of change of saturated geostrophic equivalent potential vorticity (PV,,) and geostrophic equivalence potential vorticity (PV,,), respectively.

3 Discussion of Results

3.1 Ingredients Analysis Tool

The ingredients-based methodology was applied to forecasting winter season precipitation through the ingredients analysis tool. This tool combined diagnostics for assessing each ingredient into a systematic forecast technique. Q-vector convergence was employed to qualitatively assess quasi-geostrophic forcing for ascent. Saturated geostrophic equivalent potential vorticity, PV,,, was used to identify regions of conditional or conditional symmetric instability (CI or CSI). Relative humidity and mixing ratio quantified moisture availability, and atmospheric temperature was used for both a determination of precipitation type and as a means to assess the efficiency ingredient.

Isobaric ingredients maps (see Figure 1) were introduced to assist in the visualization of the ingredient parameters, and an ingredients table (see Table 1.) was used to organize the ingredients information from numerical model data for all forecast hours and in three isobaric layers (800-850 hPa, 700-750 hPa, 600-650 hPa). In some situations, the isobaric ingredients maps and ingredients table analysis at the three standard pressure layers do not fully capture the relevant distribution of the ingredients. In these cases, cross-sectional ingredients maps (not shown) provide a more complete picture of the vertical distribution of the ingredients.

The IBM can be used by itself to forecast precipitation duration, intensity, and type. It does not, however, independently provide an estimate of snowfall accumulation. This project also introduced an approach that combines the physical basis and flexibility of the IBM with the quantitative nature of a traditional forecast technique to make a preliminary prediction for snowfall accumulation. In this approach, the Garcia Method (Garcia, 1994) forecast for snowfall accumulation is used as the "normal conditions" forecast. Normal conditions for the Garcia Method correspond to moderate forcing for vertical motion, no instability, a 10:1 snow to liquid water ratio, and snow as the sole precipitation type. Thus, if the IBM indicates "abnormal conditions" (i. e., the potential for strong forcing, instability, or very cold surface temperatures), the Garcia Method-predicted snowfall accumulation should be increased. Garcia (2000) recommends a doubling of the inches of snowfall predicted by his original technique for heavy or convective snow events.

The utility of the IBM was illustrated for a number of Wisconsin snow events, including a case study of a poorly forecasted strong snow storm that affected Wisconsin on March 13-14, 1997. Together, the early onset of precipitation and the locally enhanced precipitation rates (as a result of convective snow) led to storm total accumulations of up to 30 inches. When applied to this case, the IBM provided an accurate description of precipitation duration, intensity, and type. The IBM identified a mid-level instability coincident with ample moisture, QG forcing, and an air temperature that could support maximum depositional growth of ice crystals. These processes were likely involved in an unanticipated snow swath that fell prior to the predicted onset of precipitation. Later in the storm, significant clues to the potential for additional convective precipitation were identified using the IBM. Using the 4:1 ratio of snowfall to mixing ratio suggested by Garcia (1999) for convective snowfall events, an accurate estimate of the snowfall accumulations was obtained.

3.2 Focus on the Instability Ingredient

A comprehensive investigation of instability, the most often overlooked ingredient in winter season mid-latitude cyclones, was performed. Saturated equivalent potential vorticity (PV,,) and equivalent potential vorticity (PV,) were employed as diagnostics for identifying regions of conditional instabilities (CI or CSI) and potential instabilities (PI or PSI), respectively. Through the analysis of numerical model data from Midwestern snow events, the mechanisms that influence the evolution of PV,, and PV, with time were explored. This analysis revealed that the evolution of PV,, and PV, in a mid-latitude cyclone is dominated by horizontal advection A comparison of the horizontal advection of PV, with the Eulerian change in PV, (APV,) revealed substantial agreement in structure and location of these features. A similar situation existed for the agreement between horizontal advection of PV,, and APV,,.

Although horizontal advection is the dominant mechanism by which PV, is changed, adiabatic generation, controlled by thermal wind advection of 0, also contributes to the reduction of PV,. The most negative AG occurred primarily upshear of the occluded thermal ridge and along the cold front of mature or decaying cyclones, where isopleths of layer-averaged 0, were rotated counter-clockwise with respect to contours of the isobaric thickness. No analogous adiabatic generation term exists for PV,,; however, the evolution of PV,, was shown to be influenced by what is termed the saturation deficit term. The saturation deficit term is generally of secondary importance to the advection of PVe,, though it may contribute significantly in the dry regions behind a strong cold front and upshear of a thermal ridge in mature and decaying cyclones where the relative humidity is low and horizontal temperature gradients are large.

These preliminary findings about the time tendencies of PV, and PVe, were not incorporated into the ingredients-based forecast tool; however, further investigation of the evolution of PV, and PVe, may have utility for the IBM. An understanding of the mechanisms that transport and generate regions of instability in mid-latitude weather systems would enable forecasters to anticipate the evolution of regions of instability given initial conditions including the PV, or PV,, field. For example, when numerical forecast model simulations are not verifying with observations, current analyses of winds, temperature and moisture parameters, and PV, or PV,, could be used to infer the short term evolution of regions of instability. Additionally, one could predict the location or intensity of areas of instability at times between the standard 6-hour numerical forecast model time interval by anticipating the behavior of PV, or PVe,.

4 Operational Elements of the Project

This project involved significant interactions with National Weather Service Forecast Offices. During the first year, through weekly visits to the Dousman, Wisconsin, forecast office, Suzanne became familiar with the demands placed on operational meteorologists and with their daily routines. An understanding of their approach to forecasting, and for their needs with respect to forecasting tools was acquired. This experience helped compile the following guidelines to aid in the preparation of an operational tool. A tool should be clear and simple and broken down into distinct steps. It should be well backed up with case studies and continually reinforced with real-time examples. It should be presented in a familiar context and available through a familiar interface. Most importantly, the tool should be concept-based and not a black box. 'Magic' formulae are too often trusted blindly and used beyond their capabilities. The forecaster must remain well informed of the limitations of the tool and must be able to recognize situations where further analysis is required. Suzanne also became familiar with software in operation at the NWSFO, including:

• NTRANS: Adapted the 4-Ingredient Graphic to be generated twice daily from ETA and UW-NMS data and available through NTRANS.
• AWIPS: Attended the AWIPS On-Site User Training and compiled notes from these sessions for distribution among the office forecasters. Adapted IBM to AWIPS.
• WSR-881): Interpreted radar imagery for case study analysis.

A web page was also part of this project:

http://www.speedy.meteor.wisc.edu/,swetzel/winter/winter.html

This page has been submitted to the COMET SOO Training Resource Center catalogue and is currently being adapted for NWS training by Scott Bachmeier (UW-Madison, SSEC) and Richard McNulty (NWS Training Center). The web page includes an introduction to the ingredients approach and a presentation of the theory behind each ingredient, a discussion for the ingredients analysis tool and access to 12z and Oz realtime runs of the ingredients maps (isobaric and cross-section) access to GEMPAK and NTRANS code to run the ingredients maps, case studies from 1995-1999, and a summary of traditional techniques for forecasting snow.

By the second winter season, the IBM forecast analysis tool was completed and running operationally at Dousman, WI on AWIPS and NTRANS, as well as at UWMadison, providing web access to the ETA model runs. A seminar titled "An Ingredients-Based Approach to Forecasting Winter Season Mid-Latitude Precipitation Systems" was presented by Suzanne at the NWS Central Region Winter Weather Workshop hosted by the Dousman NWSFO on November 3, 1999. Following this presentation, she was invited to present the seminar at other forecast offices:

• NWS Central Region Winter Weather Workshop, November 3, 1999
• NWSFO Green Bay, WI, on November 19,1999
• NWSFO Marquette, MI, on January 11, 2000
• NWSFO Dousman, WI, on January 25, 2000

Following are comments from Ed Fenelon, SOO, Marquette, MI NWS (through e-mail):

"I thought your talk at the winter weather workshop at WFO Milwaukee/ Sullivan was excellent. Your methodology for an ingredients based forecast brings together a number of excellent parameters and I thought summarized a number of important concepts forecasters must be aware of.''

"I think the concepts you cover provide an excellent perspective on uses of Q vectors and various forms of instability and PV, and I am very interested in having our forecasters apply the ingredients approach to winter storms in Upper Michigan.''