NASA/Global Hydrology and Climate Center, Huntsville, Alabama
1Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts
2Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts
3National Weather Service Office, Melbourne, Florida
4NASA/Kennedy Space Center, Florida

*Corresponding author address: Ravi Raghavan
Global Hydrology and Climate Center
320 Sparkman Drive
Huntsville, AL 35805
email: ravi.raghavan@msfc.nasa.gov

1. INTRODUCTION

A study was initiated in the Summer of 1996 to determine how future (geo-synchronous orbit) lightning measurements might provide added knowledge to the Weather Forecast Officer in the determination and identification of severe thunderstorms in real-time. The Melbourne Weather Office was selected as a preliminary site to conduct this study because it serves as a experimental forecast office for the National Weather Service (NWS) and as an Applied Meteorology Office for NASA. The demonstration system is based on the Integrated Terminal Weather System (ITWS) and currently displays a suite of products in real-time that includes data from the Melbourne WSR-88D radar, Lightning Detection and Ranging (LDAR) network at NASA/Kennedy Space Center, and the National Lightning Detection Network (NLDN).

In the future, total lightning measurements from space will be acquired from a lightning mapper in geostationary orbit. This instrument referred to as the NASA Lightning Mapper Sensor (LMS) will provide continuous day/night monitoring of individual storms. A prototype of the LMS is currently flying on the MicroLab-1 satellite in low earth orbit. This instrument, named the Optical Transient Detector (OTD), observes an individual storm for about 3 minutes from an altitude of 750 km. An LMS prototype instrument, named the Lightning Imaging Sensor (LIS) will also be flown on the Tropical Rainfall Measuring Mission (TRMM) satellite beginning in August 1997. This instrument will observe an individual storm for about 80 seconds from an altitude of 350 km. Retrospective display of NASA's orbiting OTD provides space-based lightning measurements approximately twice per day over Central Florida. OTD data are available the next day to the forecasters for analysis.

The real-time data sets already being acquired by MIT/Lincoln Laboratory and NASA are being integrated to produce new algorithms and products. These algorithms and products will be based on prior studies suggesting relationships between lightning and storm growth, decay, convective rain flux, vertical distribution of storm mass and echo volume in the mixed phase regions, and storm energetics (Goodman et al., 1988; Williams et al., 1989). This study extends these concepts to real-time by developing an interactive display environment that promotes the use of total lightning measurements as an indicator that aids in identifying severe storms and storm morphology.

Total lightning measurements from the LDAR network are used to demonstrate the value of continuously available storm diagnostics (growth, decay, and intensity). Incremental forecast skill accrued through utilization of the lightning data will be assessed quantitatively for the first time through scoring of products generated with and without total lightning data and through interviews with Melbourne Weather Forecast Office personnel. This will demonstrate the incremental value of using total lightning observations to identify and track severe storms. The expected results from this experiment will be extrapolated to derive a quantitative estimate of the benefits that would be possible on a national basis from the use of data from an operational LMS. Lastly, the integrated database acquired through this experiment can be used to validate and calibrate total lightning measurements acquired from space based sensors like LIS and LMS.

2. THE LISDAD SYSTEM

LISDAD is an acronym for the Lightning Imaging Sensor Data Applications Demonstration. It is an interactive data acquisition and display system that allows the user to interact in real-time with the multi-sensor data being displayed. The user interface to the LISDAD system is based upon the interface developed by Lincoln Laboratory for use by the Integrated Terminal Weather System (ITWS) currently under development for the Federal Aviation Administration (FAA).

The LISDAD display consists of multiple images. The background image displayed is the WSR-88D composite radar reflectivity product. The areal coverage of this product extends to 100 nm from the Melbourne doppler radar. Multisensor lightning observations (flashes) are overlaid on this background image. The lightning data are obtained from the NLDN and LDAR networks. Hooks are in place within the LISDAD system to create overlay images using the OTD data. Since the current turnaround time on the processed OTD data is on the order of a day, separate retrospective displays are generated using the OTD, composite reflectivity, NLDN and LDAR data. Considerable emphasis was placed on the design of the interactive features of the LISDAD system. The interactive features provide the user the ability to perform quick diagnostics on storm cells of interest. When the user selects a storm cell, a "pop-up box" appears displaying the time history of various storm-cell parameters (e.g. Maximum Radar Reflectivity, Height of the Maximum Reflectivity, Echo-Top height, Probability of Severe Hail (POSH), NLDN and LDAR lightning-flash rates).

A brief description of the multisensor data flow and processing follows. The ITWS test-bed operating at Orlando, Florida provides most of the information utilized by the LISDAD system via a dedicated 56k phone line. The radar-derived storm-cell parameters are generated by the Severe Storm Algorithm (SSA) developed by NSSL, and runs as a member of the ITWS algorithm suite. The raw LDAR lightning source data (X, Y, Z, t) are transmitted from the NASA/Kennedy Space Center (KSC) via a dedicated T-1 line. These lightning sources are then grouped into flashes by associating all sources that occur within a certain time/ space distance of each other into a single flash. Specifically for sources that occur within 50 km of KSC, a flash is constructed from sources that are located within 5 km of each other and occur no more than 300 ms apart (i.e.a flash ends when 300 ms have elapsed with no new sources been added to the flash). For sources located beyond 50 km from KSC, the maximum separation of a single source to be included in a single flash is increased in a quadratic manner to a maximum value of 20 km for locations 200 km from KSC. The maximum range of the LDAR data displayed by the LISDAD system is approximately 200 km, and so covers an appreciable fraction of the state of Florida.

3. STORM CHARACTERIZATION USING LIGHTNING OBSERVATIONS

An important goal of this study is to determine the added value of providing total lightning data in real-time to the weather forecast officers given their access to GOES imagery, WSR-88D radar data and NLDN data. To accomplish this objective, the LISDAD display was integrated into the workstation environment of the forecast officer. The forecast officers were also provided with a questionnaire which sought their response to specific questions related to their use of the lightning data in determining and identifying thunderstorms in real-time. It should be noted that at the time this paper was written only a limited number of questionnaires had been received and evaluated.

The responses to some of these questions were compiled and analyzed. The results of the analyses are presented in Figures 1 and 2. The response to how useful the lightning measurements were to the forecast officers in characterizing each aspect of thunderstorms is shown in Fig. 1. The responses were categorized on a rank scale of 1 - 5. In Figs. 1a,b, the vertical axis represents the number of responses received at each rank (rank 1= least useful, and rank 5 = most useful). The thunderstorms were also classified as severe and non-severe. Fig. 1a represents responses for the non severe thunderstorms. It indicates that while in general the lightning data were very useful in identifying most storm aspects, it was used least in tracking storm movement. The latter response was primarily due to the transient nature of lightning data when compared to the persistence of radar echoes.

Fig. 1a also indicates that the lightning data were very useful in characterizing storm development. In this context it was determined that the true value of the LDAR data was the ability to assess the frequency of the electrical activity with extremely high temporal resolution. The radar updates require 5 minutes, so the continuously sampled LDAR data fills the 5 minute radar data gap with vertical (growth/frequency) information thus aiding the assessment of the strength of the updraft. The high temporal resolution of the LDAR data also explains the utility of LDAR data in identifying the existence of storms. The high utility of the LDAR data in the identification of storm dissipation is also evident in Fig. 1a. The high temporal resolution and vertical information presented by the LDAR data enabled the forecast officers to determine storm dissipation. If the LDAR points on the display associated with a storm were decreasing, it indicated to the forecast officers that the storm was dissipating or weakening. This information may assist decision making in the resumption of activity at airports, on golf courses or in public swimming areas, following lightning activity.

Fig. 1b , shows a similar chart as in Fig. 1a, for the severe cases. These results also emphasize the extreme usefulness of the LDAR data in identifying the various aspects of the storm. The inferences made about Fig.1a can also be applied here. Fig. 2 presents a chart showing the frequency with which the LDAR data were used by the forecast officers during active weather in characterizing the storms. These results indicate that 55% of the time during active weather, the LDAR data were used as an aid on a time scale of 5 minutes or less. The LDAR data were used every 10 minutes, or every 30 minutes, 22% of the time during active weather. This clearly highlights the utility of total lightning measurements as an aid to characterizing thunderstorms.

The real-time diagnostic value of the total lightning measurements are illustrated in the pop-up boxes shown in Figs. 3a,b,c. These figures display the maximum reflectivity, height of the maximum reflectivity, height of the echo tops, the NLDN cloud-to-ground lightning, and the LDAR flashes for any selected storm cell in the radar coverage area. The distance of the storm cell from the radar and the observation time at which the selection was made are also shown on the figure. The negative numbers on the time axis refer to minutes prior to the current observation time. Fig. 3a presents the evolution of one storm cell for which the ratio of intercloud (IC) to cloud-to-ground (CG) flashes increases substantially at a time of rapid growth of the maximum cloud top height. This cell produces the first CG flash 20 minutes after the first LDAR activity was recorded. This example also illustrates a case where the cell is still producing intercloud lightning after the last recorded CG flash. Fig. 3b also displays a scenario with in which the ratio IC to CG flashes increases in association with the vertical growth of the storm. Fig. 3c presents an example where there is only intercloud lightning and no cloud-to-ground lightning. Clouds in this category go undetected as thunderstorms by NLDN but may still present significant hazard to aviation. Another point to be noted in these examples is that the maximum reflectivity of these clouds is the same even though they exhibit different lightning characteristics. A detailed analyses of these cases is underway. The LISDAD pop-up displays will be further enhanced to include information on the vertical profiles of radar reflectivity, the vertical centroid of LDAR sources, and differential radial velocity.

4. SUMMARY

The results presented in this study demonstrate in several ways the incremental value of lightning measurements in diagnosing convective storm characteristics. The high temporal resolution of the LDAR data combined with the vertical information (growth/decay) enables continuous characterization of storm cells. It was shown in this study based on responses obtained from forecast officers, that total lightning measurements from LDAR were extremely useful for identifying storm existence, intensity, growth, and dissipation. It is noteworthy that the total lightning measurements were very useful in identifying storm dissipation from the view of inclusion/exclusion of thunder in aviation terminal forecasts (and eventually Transcribed Weather Broadcasts). This is especially critical in the modernized weather service as the temporal accuracy of forecasts is being improved. There are cases where the cloud is electrically active for a while after the CG discharge appears to have diminished. Total lightning information adds considerable confidence to the safety margin forecasters include in their aviation products.

Total lightning measurements with high temporal resolution can also be used to address lightning hazards. It has been found that the LDAR flashes precede the CG discharges by about 4 minutes. Thus, it may be possible to use this lead time in issuing Lightning Watches/ Warnings. In a related study conducted by the National Weather Service office at Melbourne, LDAR information was successfully used to issue Lightning Watches/Warnings for the Olympic Soccer Venue in Orlando. It is noted here that the LMS data will be available at the forecaster's workstation within 30 seconds. This high temporal resolution may assist forecasters to evaluate the threats of a lightning hazard more accurately.

References:

Goodman, S.J., D.E. Buechler, and P.J. Meyer, 1988: Convective tendency images derived from a combination of lightning and satellite data, Weather and Forecasting, 3, 173-188.

Williams, E.W., M.E. Weber, and R.E. Orville, 1989: The relationship between lightning type and convective state of thunderclouds, J. Geophys. Res., 94, 13213-13220.

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