THE USE OF HIGH- FREQUENCY ACARS SOUNDINGS IN FORECASTING CONVECTIVE STORMS

INTRODUCTION

Many commercial aircraft automatically report wind and temperature data that can be extremely useful in weather research and forecasting. This data is often referred to as ACARS, which is simply an acronym for the communications system that transmits much of the data in the United States. ACARS has been used for many years in research and modeling (it is currently used in the Rapid Update Cycle), but has only recently been made available to National Weather Service meteorologists in forecast offices.

Because ACARS data is often available at places far from radiosonde sites, and at asynoptic times, it can be very useful in forecasting convective weather and other phenomena. Aircraft near airline "hub" airports (such as Chicago O'Hare) sometimes transmit soundings of wind and temperature every 15-30 minutes during the busiest times of the day. The availability of frequent soundings allows the forecaster to monitor the degree of instability and wind shear throughout the day, and issue improved forecasts of convective initiation, severity, and dissipation.

Meteorologists at the NWS office in Chicago used ACARS data May 5,1997 to monitor the destabilization of the atmosphere, and to forecast the time of convective initiation and type of severe weather. ACARS and surface dewpoint were used early in the morning to forecast the convective temperature, which helped forecasters determine when surface-based convection was likely to develop. ACARS soundings were then used throughout the day to show the lifting and erosion of the low level inversion, the destabilization of the atmosphere, and the directional and speed shear of the wind. The soundings were also transmitted to the Storm Prediction Center in Norman, Oklahoma, who found it useful in the decision to issue a Severe Thunderstorm Watch.

ACARS DATA ACCESS AND QUALITY

Meteorological data from certain commercial jet aircraft is provided by several airlines and airfreight companies (United, Northwest, Delta, and UPS), and is transmitted by ARINC (Aeronautical Radio Inc.) to the National Centers for Environmental Prediction (NCEP), the Forecast Systems Laboratory (FSL), and a few other users via ACARS (ARINC Communications, Addressing and Reporting System). FSL makes this information available to the NWS and certain other users via an Internet web page that allows display of wind plots, soundings, and alphanumeric data. This figure shows one way that data may be displayed on the FSL ACARS Internet page. (Click on any figure in this paper to see a larger-sized version.)

Several studies have compared ACARS with other data sources. Among these, Lord, et. al. (1984) compared ACARS flight-level winds with radiosonde, cloud-motion and VAS thermally-derived winds. When ACARS was compared to radiosondes, root mean square (RMS) deviations were 7.4 degrees in direction and 5.3 m/s in speed. More recently, Schwartz and Benjamin (1995) compared ACARS ascent/descent winds and temperatures with radiosondes and found temperature differences were less than 2C on 94 percent of all occasions, and less than 1C better than 68 percent of the time. Wind speed RMS deviations were 4.1m/s while direction RMS differences were 35 degrees (mostly due to light and variable wind situations).

The FSL ACARS site has many quality control checks. Data failing any of the several tests is color-coded on the various graphic presentations, while more specific information is included with the alphanumeric data. While much of the quality control function is automated, some manual intervention is needed (Moninger 1997, personal communication).

MAY 5th 1997 CONVECTIVE EVENT

A 500mb shortwave trough was moving rapidly across Minnesota and western Iowa at 12UTC. While this trough was expected to provide favorable dynamics for convective development across Illinois, the atmosphere below 700mb was quite dry (dewpoint depressions greater than 10C across the entire state). Surface temperatures across northern Illinois at 12UTC were near 60F (15C), with dewpoints in the middle to upper 40sF (7-8C). A surface trough stretched from western lake Superior to central Iowa (Fig. 2), with even drier air (dewpoints mostly in the 30s [-1 to 4C] ) upstream. Strong warm air advection was producing scattered elevated showers and thunderstorms over Michigan and Indiana (these had moved across Illinois during the overnight hours). The forecast question of the day was whether there would be enough surface heating and mid-level cooling to initiate surface-based convection across northern and central Illinois. In an effort to better monitor the atmosphere, and make improvements to the forecast, ACARS data from aircraft near O'Hare airport were requested frequently during the late morning and early afternoon hours (about every thirty minutes).

An ACARS sounding from 1515UTC (below, left) showed the atmosphere to be stable from the surface to 900mb. This sounding coupled with a surface dewpoint of 50F (10C) resulted in a convective temperature of 72F (22C). With strong warm air advection occurring, forecasters felt this temperature was attainable in the mid-afternoon hours (around 1900UTC). At 1623UTC another sounding was obtained (below, right).

This showed that the inversion had lifted, and that surface heating had produced a nearly dry-adiabatic layer between the surface and 875mb. Even so, there was a sufficient cap to prevent any convective initiation.

Surface temperatures were warming rapidly, however, and it was still felt that the convective temperature would be reached between 1800 and 1900UTC. A nowcast was issued at 1645UTC stating thunderstorms would develop by mid-afternoon, some with gusty winds and hail (the cold temperatures aloft and 50-60 knot [25-30m/s} winds below 800mb from the ACARS soundings were useful in determining the severe weather potential).

This sounding from 1738UTC (right), coupled with the surface temperature and dewpoint of 71F (21.5C) and 53F (11.5C), respectively, showed that the atmosphere had become unstable enough to initiate surface-based convection.

A Severe Thunderstorm Watch was issued by the Storm Prediction Center at 1811UTC. The first radar echoes associated with this event were noted about twenty minutes later. The storms produced several reports of severe weather (winds of 60 to 70 mph [30 to 35 m/s] and golfball-sized [1.75in./4.0cm] hail).

The ACARS data was a very useful supplement to the usual tools found in an NWS office (ADAP, SHARP, WSR-88D, GOES, etc.), for there is no other system in place to get very frequent soundings of both wind and temperature.

ACARS data can be extremely useful in many weather situations. The NWS Chicago forecast staff has used it over the past several years to forecast low-level wind shear, fog, the precipitation type of winter storms, and convective storms such as this. Another account of how ACARS can be used to monitor the atmosphere in severe weather situations is detailed in Mamrosh and Labas (1996).

Although ACARS data is currently available only from jet aircraft near large and medium cities, there is an aggressive effort by the NWS to get more ACARS data. It is expected that ACARS data will be available from commuter aircraft that fly to small cities sometime in the future. In addition, there are a few UPS aircraft with a water vapor sensor. Soundings with dewpoints are available from these aircraft. The NWS has an agreement with UPS to install this sensor on sixty more aircraft in 1998 (Fleming 1997, personal communication).

ACKNOWLEDGMENTS

The author wishes to thank Dr. Bill Moninger at FSL for his support in getting ACARS data to NWS offices in a reliable, user-friendly format. Mr. Carl Knable of United Airlines graciously provided ACARS data to our office via facsimile for several years until we were able to utilize the FSL ACARS page. Information concerning the future of ACARS data (especially in regards to water vapor measurements) was provided by Dr. Rex Fleming of the National Center for Atmospheric Research.

REFERENCES

Mamrosh, R. D., and K. M. Labas, 1996: Real-time Monitoring and Reconstruction of a Severe Thunderstorm Environment Using Unique Data Sets, Central Region Technical Attachment, National Weather Service, No. 96-03, May.

Corresponding author address:

Richard Mamrosh
National Weather Service,
333 W. University Drive,
Romeoville, IL 60446
(815) 834-0600 ext 286
richard.mamrosh@noaa.gov