# Presentation: 025270

##    Stories from the Sonoran Desert		

-  Severe Weather Technology Workshop
- Silver Spring MD
- 12 July 2005
- Doug Green
- Science Operations Officer
- WFO Phoenix Arizona

##    Microburst Detection Challenge

- Microbursts are a key severe weather producer over the Phoenix CWA, especially during the summer convective season.  Wind damage or wind speeds greater than 57 mph verify the vast majority of our severe thunderstorm warnings...and severe microbursts...with winds approaching or exceeding 100 mph...occur nearly every summer over the Phoenix CWA.
- Weak-moderate vertical wind shear and CAPE, combined with large DCAPE, lead to high-based storms with relatively short lifetimes (< 35 minutes).  Microbursts, should they occur, develop quickly (10 minutes lead time is about the best we can provide for a particular storm).  Forecasters don’t have much time to interrogate a storm and determine whether or not to warn.  “Subtletly reigns supreme” in all aspects of the microburst environment.
- Bottom line:  WFO Phoenix warning forecasters must be able to quickly discriminate among storms likely to produce damaging microburst winds vs storms that produce more typical non severe outflow winds.

## Slide 3

## WDSS...gone, but not forgotten...

- 1994-2004:  WFO Phoenix had access to NSSL’s Warning Decision Support System (WDSS).
- WDSS provided cell-based time-height displays of radial convergence/divergence, as well as base reflectivity.   The Damaging Downburst Prediction and Detection Algorithm (DDPDA) was also installed on our system...it was designed to indicate when a moderate or severe microburst was predicted or detected via an entry on the Cell Algorithm Output table.
- WFO Phoenix forecasters much preferred WDSS-based outputs to SCAN-based outputs.  I say preferred, because...
- Recent ORPG loads have rendered the WDSS output unreliable, so the WDSS has been removed from the ops area.

## WDSS displays...including storm-based time-          height displays of maximum reflectivity and convergence/divergence (from NSSL)

## Recommendations...

- Modify SCAN’s cell algorithm output table to include velocity-based information.
- Have SCAN provide time height displays of maximum reflectivity and radial velocity convergence/divergence.
- Install the WDSS-II at WFO Phoenix (hey, nothing ventured, nothing gained!!).  We already receive information from the Sky Harbor TDWR...via a stand-alone monitor.  Soon, we should have TDWR data available via AWIPS...but it will still be stand-alone information.  Data from the 2 nearby Doppler systems needs to be integrated, processed, and displayed via one easy-to-use warning system.

## Evolution of microburst-producing storm at Phoenix Sky Harbor Int’l                  Airport  – 14 July 2002

- Sequence of seven 4-panel displays, over a period of 35 minutes, will be shown (0.5 and 4.3 degree reflectivity and velocity images)
- Assuming standard refraction, height of the beam centerline over Sky Harbor is approximately 1500 feet AGL for the 0.5 degree slice, and 10,000 feet AGL for the 4.3 degree slice.
- Phoenix Sky Harbor ASOS is situated approximately 1.5 miles ESE of where the microburst appears to have been most intense.

## Intense but small tstm 5 miles NNE of Sky Harbor

- Redevelopment continues on SW flank
- Weak divergence near surface... very weak convergence just above cloud base
- 6 c-g lightning strikes in 5 minutes (5 -, 1+)
- Leading edge of strong gust front/ dust storm was approaching from the southeast

- 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:08 pm 14 July 2002

## Rapid and intense development on south flank just north of Sky Harbor.

- Lightning activity increased to 11 strikes (9-,2+).
- Very strong convergence just above cloud base with new updraft development.
- Gust front/dust storm colliding with outflow from mature cell...increased sfc-based convergence just southeast of developing cell.

- 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:18 pm 14 July 2002

## 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:23 pm 14 July 2002

- Leading edge of dust
- storm reaches Sky
- Harbor...ASOS wind at
- 823 pm: 143/26kt, peak
- gust 156/34kt.
- Outflow collides with convergence zone.
- Convergence still exists
- just above cloud base coincident with updraft just north of Sky Harbor.
- Not much lightning: 4 strikes.

## 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:28 pm 14 July 2002

- Rapid development of very strong updraft
- directly over Sky
- Harbor.  Peak Z > 65 dBz at 4.5 deg.  When this collapses, the microburst(s) will occur.
- Leading edge of out-flow/dust storm has moved west of Sky Harbor...ASOS indicated peak mean wind of 31 kt (36 mph) from the south-southeast at 827pm and 828 pm (40 kt gust at 826 pm).
- AWW verified.

## 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:33 pm 14 July 2002

- High-reflectivity core (dBz > 65) and strong anticyclonic divergent
- signature over Sky
- Harbor on 0.5
- deg slice (microburst in progress over west end of airport).
- Lightning increased
- dramatically during past
- 5 minutes...28 strikes
- (26 -, 2 +).
- Phoenix ASOS wind at
- 833 pm: 154/28; peak
- gust 172/33.  ASOS situated on SE corner of airport, a couple miles away from microburst.

## 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:38 pm 14 July 2002

- Outflow from microburst
- begins to trigger new
- updraft development
- south of Sky Harbor.
- ASOS wind at 838 pm:
- 323/28kt; peak wind 333/38kt.  Wind shifted from 158/26, with peak gust 151/33 at 834 pm,  to 304/21 with peak gust 320/37kt at 837 pm (3- minute vector wind peak gust diff: 305/70kt!!)
- Anticyclonic divergent signature expands...still very strong.  Microburst still in progress? New microburst?
- Frequent lightning: 27 strikes (22-,5+).

## 0.5 degree base reflectivity   4.5 degree base reflectivity

- 0.5 deg base velocity          4.3 deg storm-relative velocity

- 8:43 pm 14 July 2002

- Reflectivity echoes south of Sky Harbor begin to move/develop toward the east and south as leading edge of outflow from first microburst spreads southeast.
- Wind at Sky Harbor
- ASOS switches from NW to SE between 839 pm and 841 pm.  At 843 pm, wind is 165/22kt, peak gust at 844pm: 158/36kt.  SE wind continues past 9 pm.
- C-G lightning strikes
- Increase noticeably:
- 41 strikes (36-, 5+).

##   Sky Harbor Microburst tidbits...

- Microburst resulted in $45M damage at Sky Harbor Airport.   Couple of damage examples:  500 pound mail carriers (no, not big human beings)  were blown several hundred feet and smashed into parked cars.   A Cessna aircraft was “harpooned”, from top to bottom through the pilot’s seat, by a large steel I-beam from a nearby hangar.
- Peak wind gusts estimated near 100 mph...may have been higher.
- Dust storm, with low visibility and wind of 40-50 mph, had caused a temporary cessation of takeoffs and landings at Sky Harbor...the fact the microburst occurred during the height of the dust storm may have resulted in the avoidance of an aircraft accident.

## FAA Sky Harbor TDWR radar was operational, but...

- Unofficial, second-hand report:  TDWR detected a 95 knot wind...which led to the evacuation of the Sky Harbor tower.
- Unfortunately, we will never know what the TDWR actually detected...because TDWR data were not archived, and TDWR data were not available to WFO Phoenix at that time.
- TDWR data now streams into WFO Phoenix via a T1 link, reflectivity/velocity images are displayed via a stand-alone system, and data collected during the past 30 days can be archived.

##    KIWA Data Quality woes

- For many years, data quality problems have been observed with the KIWA (Williams Gateway) WSR-88D.  Periodic episodes of unreliably strong radial velocities and excessively high reflectivity values were commonplace.   Occasionally, very high radial velocities would be embedded in otherwise moderate outbound flow, which made it difficult for forecasters to estimate the true magnitude of the wind.
- Poor clutter suppression was suspected...however, a series of local tests/experiments conducted by WFO Phoenix ET Mike Schumacher confirmed that KIWA’s clutter suppression was working extraordinarily well.  Still, data quality problems persisted.
- The ROC was called in to lend a hand...tests performed jointly by Mike and Dwight Lee, contractor for the ROC, indicated that an outside source was likely responsible.  But, what was that source, and what could be done about it?

## Could a nearby radar be the culprit? 

- Given the azimuth at which the signal was strongest, the source was north-northeast of KIWA.  After a little sleuthing, ET Mike Schumacher discovered that a powerful FAA radar was situated only a couple of miles north-northeast of KIWA.  This radar had been installed at Williams Gateway back in 1994, a year after KIWA became operational.
- The FAA radar transmits at a similar frequency to KIWA, and at a high power level (1 MW).  Some of its transmitted signal appears to be getting picked up and processed by KIWA.

## Spectral display of KIWA while it was radiating.  KIWA transmission is centered at 2.77 GHz.  The FAA radar transmission, centered at 2.825 GHz, is easily detected.   KIWA azimuth = 24 deg; elevation = 0.5 deg.

**Spectral display of KIWA while it was radiating.  KIWA transmission is centered at 2.77 GHz.  The FAA radar transmission, centered at 2.825 GHz, is easily detected.   KIWA azimuth = 24 deg; elevation = 0.5 deg.**

## Spectral display from KIWA site...same azimuth and elevation angle...but KIWA was not radiating.

##                Now what???

- If, after further testing, the FAA radar is determined to be the cause of the KIWA data quality problems, what could be done?   Possibilities include programming the FAA radar not to transmit thru a few degrees of azimuth centered on KIWA, or changing the transmitting frequency of the FAA radar.  If the FAA indicates nothing can be done to change its radar’s operations, it is possible that KIWA could be moved.

##              Summary

- WFO Phoenix forecasters feel the WDSS was an extremely important tool, and are not happy it no longer is available for their use.  Forecasters feel they need better tools to help them make timely determinations regarding the potential for damaging microburst winds from pulse storms or poorly-organized multicell storms  (e.g., no velocity-based output available via SCAN cell algorithm table or trend windows).
- TDWR and WSR-88D data need to be integrated and displayed via WDSS-II, or a similar software package.
- An FAA radar was installed within several miles of a pre-existing WSR-88D, the KIWA radar.  The FAA radar is quite powerful, and appears to be interfering with the KIWA radar.

##           It never rains in Arizona...it pours! Quasi-stationary MCS east of Phoenix on 8-9 Sept 2003