# Presentation: 433631

## Tornado Detection & Limitations*

**David Craft**
**Weather Forecaster**

***This briefing covers the major limitations common to all Doppler weather radars**

**April 2004 Media Workshop**

**Notes:**
This briefing describes limitations of detecting tornadoes common to all Doppler weather radars, be they owned and operated by news media, research institutions, or the National Weather Service (NWS).

## Overview

**Background**
**Doppler Radar Limitations**
**Examples**
**Other Tornado Detection Capabilities **
**Prepare & React Appropriately**
**Bottom Line**
**References**

##  National Average Lead Times & Probability of Detection

**WSR-88D network installed early 1990s**
**Doppler technology**
**Greater sensitivity**
**Improved volume scanning     & computer processing**
**Training advancements**
**Improved local training on Doppler radars & storm structure/evolution**
**Four-week course in residence**

**Significant improvement in the last 10 years**

**Notes:**
 WSR-88D Advancements 
- Doppler technology
- Greater sensitivity
- Improved volume scanning
- Computer processing of primary data into useful derived data and products

 Training Advancements (radar use and storm structure/evolution)
- Intensive 4-week course in residence
- Local training programs

  NWS Weather Event Simulator
- Recently deployed to field offices
- Replays data from an archived event
- Simulates technological, scientific, and human facotr (fatigue, stress) challenges
- Allows forecasters to gain experience and expertise w/o actually working an event and w/o impacting operations

## Weather Radar Basics

- (NWS, 2003)

**Doppler radars obtain data by:**
**Transmitting electromagnetic energy in brief pulses at specific angles**
**Energy returns from precipitation, cloud droplets, mountains, etc.**
**Doppler measures strength of return and the component of object motion toward and away from the radar**

**Angles used by the WSR-88D**

**Notes:**
 To obtain data
- Transmit electromagnetic energy in brief pulses at specific angles
- Energy returns from precip, cloud droplets, mountains, buildings, etc.
- Doppler measures strength of return and object motion toward/away from radar

 Let’s look at data from a supercell near Clovis

## Weather Radar Basics

**Technically, radar shows larger shear zones, not the actual tornado**
**Base reflectivity shows hook echo due to precipitation thrown out of/around rotating updraft core**
**Storm relative velocity**
**Speed of wind toward (cool colors) and away (warm colors) from radar**
**The stronger, tighter, & deeper (more than one elevation angle) the rotation, the greater the likelihood a tornado is present**
**Time continuity is also important (more than one volume scan)**

**Supercell Thunderstorm**

**Notes:**
 Reflectivity image may show hook echo due to precip thrown out of/around rotating updraft core

 Velocity image shows speed at which wind carrying particles moves toward/away from radar
- The stronger, tighter, and deeper the rotation, the greater the likelihood a tornado is present
- Time continuity is also important
- Technically, radar shows larger shear zones, not the actual tornado 

## Supercell Tornadoes

**Occur during a supercell thunderstorm’s mature stage**
**Preceded by strong mid-level [15-25,000 feet (ft)] rotation (where radars have a better view)**
**In NM, occur mostly along eastern border with TX during May, June, and July**
**WSR-88Ds earned excellent reputation with these**
**Supercell thunderstorms produce the majority of NM’s confirmed tornadoes each year, but not all of them**

**Notes:**
 AKA: land spouts, gustnadoes, or spin-ups

 Rarely exceed F2 intensity

- May cause damage, injuries, or even deaths.

- Lets take a look at two common formation mechanisms in New Mexico

## Non-Supercell TornadoesA.K.A. landspout, gustnado, or spin-up

**Probably more frequent in Western U.S.**
**Often unseen or unreported**
**Form early in thunderstorm lifecycle, sometimes before lightning strikes**
**May form rapidly near surface then extend upward**
**May form simultaneously at low and mid levels**
**Shallow and/or narrow (rarely exceed F-2 intensity)**
**Larger atmospheric circulations usually cause these to dissipate in only a few minutes**

**Notes:**
 AKA: land spouts, gustnadoes, or spin-ups

 Rarely exceed F2 intensity

- May cause damage, injuries, or even deaths.

- Lets take a look at two common formation mechanisms in New Mexico

## Spin-Up Tornado  Formation

**Cause 1: small and shallow circulations along surface convergence zone, stretched upward by strong updraft**

** **

** **

- (Wakimoto et. al., 1988)

- (NOAA, 1999)

**Cause 2: horizontal vorticity roll downwind of mountains, tilted to vertical when crossed by a strong updraft**

**Notes:**
 Shearing instabilities
- Rapidly growing cumulus (Cu) or cumulonimbus (Cb) moves over small near-surface circulation along a boundary
- Tornado forms rapidly near the surface then extends upward as the updraft below the cloud stretches and tightens the circulation by pulling it upward
- Similar stretching can occur when a cumulus or cumulonimbus moves over a dust devil.

Vertically tilted horizontal vorticity
- Cu or Cb moves over horizontal roll (east of mountains) and tilts into vertical
- Tornado may form at low levels or simultaneously at low and mid levels

 Now that you understand some basic information about how Doppler weather radars sample the atmosphere and about the types of tornadoes we see out west, let’s examine radar tornado detection limitations more closely

## Doppler Radar Limitations

**Too far from radar**
**Beam broadening**
**Overshooting**
**Too close to radar**
**Blocked from radar view**
**Dissipate too quickly**

**All Doppler radars have difficulty detecting circulations...**

**Notes:**
- These limitations are common to all weather radars

## Limitation 1:Too Far From the Radar

**Cause 1: Beam Broadening**

- (WSR-88D Operational Support Facility, 1998)

**Beam grows too large compared to size of the circulation**
**Large # of slow wind returns outside tornado outweigh small # of fast wind returns within**
**Averaged away/toward velocities too small to represent a threat**
**Effective detection range depends on circulation size; effective detection range **_**<**_** ****63 statute miles (sm) for the WSR-88D ****(WSR-88D Operational Support Facility, 1997)**
**For very small tornadoes, or radars with larger beam diameters, the range shrinks even shorter**

**Notes:**
 Beam grows too large compared to the size of the circulation
- Large number of slow wind returns from outside the tornado outweigh the relatively small number of fast wind returns from inside the tornado
- Averaged away/toward velocities too small to represent a threat

 Thus, w/ any radar, beam broadening limits the range of confident tornado detection.  Effective range depends on the size of the circulation.

 Example
- WSR-88D’s 0.96 degree angular beam width helps it resolve smaller features
- Practical range of tornado detection still limited to around 55 nm (63.3 sm)
- For very small tornadoes, the range shrinks even shorter

## Limitation 1:Too Far From the Radar

- (NWS, 2003)

**Cause 2: Overshooting**

**Angles used by the WSR-88D**

**Beams rise above low-level circulations because beams travel away from the radar at an angle**
**Many NM spin-ups only extend up to about 3,000 ft**
**Overshot by center of lowest beam at 40 sm from radar**
**Overshot by center of second lowest beam at 29 sm, so spin-ups may only be detectable out to 29 sm (WSR-88D Operational Support Facility, 1997)**
**New scan strategy coming**
**Adds 3 new angles at lowest levels**
**Better vertical display of low-level storm structure **

**Notes:**
 To obtain data
- Transmit electromagnetic energy in brief pulses at specific angles
- Energy returns from precip, cloud droplets, mountains, buildings, etc.
- Doppler measures strength of return and object motion toward/away from radar

 Let’s look at data from a supercell near Clovis

## Limitation 2:Too Close to the Radar

**Cone of Silence**

**Doppler radars don’t sample the atmosphere directly above them**
**Radars cannot detect circulations that move into the cone of silence**
**WSR-88D’s cone prevents detection of mid-level circulations within about 11.5 sm of the radar**
**Neighboring radar beams overshoot the low levels and the lower portion of the mid levels over both the Albuquerque and Cannon AFB radars ***** ***

**Angles used by the WSR-88D**

- (NWS, 2003)

**Notes:**
Weather Radars don’t sample the atmosphere directly above them

 Circulations that move into the cone of silence cannot be detected

 Example
- WSR-88D’s cone inhibits searches for mid level circulations w/ in 10 nm (11.5 sm) of the radar
- Unfortunately, neighboring radar beams overshoot the low levels and the lower portion of the mid levels of the atmosphere above KABX and KFDX 

## Limitation 3:Blocked From View

**Radar Coverage at 10,000 Ft**

**Mountains can shield low-level circulations from view**
**If not for mountains, we’d see perfect circles of coverage like those in TX**
**Since lower elevation angles intercept more terrain, coverage decreases below 10,000 ft **
**Since higher elevation angles intercept less terrain, coverage improves above 10,000 ft**
**When implemented, the new scan strategy will slightly improve detection capabilities by increasing sampling of the atmosphere just beyond and above mountain tops**

**Notes:**
Mountains can shield low level circulations from view

 For example
- If not for mountains, we’d see perfect circles of coverage around each radar
- Since lower elevation angles intercept more terrain, there is less coverage below 10,000 ft
-- Better coverage exists above 10,000 ft, except for the area within the cone of silence

- NWS’s new scanning strategy will improve the detection capabilities of radars by increasing the sampling of the atmosphere just beyond and above mountain tops.

## Limitation 4:Dissipate Too Quickly

**Many spin-ups may last only a few minutes**
**Complete atmosphere scan may take 5 or 6 minutes**
**Forecaster analysis of images and dissemination of a tornado warning takes extra time**
**Even when detected, spin-ups frequently dissipate before a warning can reach people**
**New scan strategies will shrink scans to 4.1 minutes**

**Notes:**
Forecaster analysis of images and dissemination of a tornado warning takes extra time

 Although these four weather-radar limitations are separate phenomena, they can work together to prevent forecasters from detecting a tornado.  Let’s look at a couple examples, then we’ll discuss the NWS’s other tornado detection capabilities and the actions people should take to mitigate New Mexico’s tornado threat.

## Example 1:Spin-Up Below Cumulus Cloud

**Photo 1**

**Photo 2**

**Photo 3**

**Photo 4**

**March 2003, Torrance County, east of Manzano mtns.**
**Not detected by forecasters for 3 reasons**
**Too far: 63 sm from Albuquerque radar and 128 sm from Cannon AFB radar**
**Manzanos blocked the lowest 1 1/2 beams from Albuquerque radar; radar sampled no lower than 6,000 ft above spin-up**
**No rain or thunderstorm activity on this day.  Forecasters use the prominent returns from rain, hail and thunderstorm outflow boundaries to identify possible tornado locations.**

**Notes:**
March 2003, Torrance County, east of Manzano Mountains

 Not detected by forecasters for 3 reasons
- Too far away for its size: 63 miles from KABX, 128 miles from KFDX
- Manzanos blocked lowest 1 1⁄2 beams from KABX; radar sampled no lower than 6K ft above spin-up
- No rain, hail or thunderstorm activity on this day.  Forecasters use the prominent returns of rain shafts, hail shafts and outflow boundaries to help identify updrafts and other favored areas for tornado formation.

## Example 2:Spin-Up Below Thunderstorm

**May 2003, near Willard in Torrance County, east of Manzanos**
**Storms nearly stationary this day**
**Forecasters issued a severe thunderstorm warning about 40 minutes before this photo; this spin-up may have developed along the earlier storm’s outflow boundary**
**59 sm from Albuquerque radar; this tornado may  have been big enough to detect**
**Manzanos blocked lowest 1 1/2 beams from Albuquerque radar; radar sampled no lower than about 5,000 feet above this spin-up**

**Notes:**
May 2003, near Willard in West Central Torrance County, east of Manzanos

 Not detected, but forecasters did issue a severe thunderstorm warning nearby 40 minutes before photo.  Storms were nearly stationary on this day.
- No strong and sizeable mid level rotation
- At 59 sm from KABX, may have been large enough compared to beam diameter for detection
- Manzanos blocked lowest 1 1⁄2 beams from KABX; radar sampled no lower than around 5K or 6K feet above spin-up

 Fully aware of the limitations of weather radars, NWSFO ABQ uses a variety of other means to detect and warn for tornadoes.  I’ll close our discussion of tornado detection limitations with a brief explanation of these other means, and give you some ideas how to prepare and respond appropriately to the tornado threat.

## Other Tornado Detection Capabilities

**Storm Prediction Center guidance**
**Weather models**
**Satellites, profilers, soundings, upper-air data, surface obs**
**Storm spotters**
**News media, law enforcement and public reports**
**Research collaboration**
**Testing new scan strategy**
**Will help confirm sonar’s ability to detect tornadoes**

**Notes:**
 Research collaboration
- Currently operationally testing new scanning strategies on KABX
- Will soon help confirm sonar’s ability to detect tornadoes  

## Prepare & React Appropriately

**FEMA’s tornado safety tips brochure: www.fema.gov/hazards/tornadoes/tornadof.shtm **
**Watch for small tornadoes early with all thunderstorms; remember our first spin-up example**
**Useful NOAA forecasts**
**U.S. hazards assessment**
**Severe weather outlook, mesoscale discussions, watches**
**Hazardous weather outlook & warnings**
**Use NOAA weather radios**
**Report tornadoes to NWS, if safely possible**

**Notes:**
Remember our first spin-up example: brief but dangerous tornado-like circulations can occur without any other signs of a thunderstorm

 We will always need spotter, news media and public reports, because they may provide ground truth information that wouldn’t otherwise be available

## Bottom Line

**WSR-88D’s do a good job at what they’re designed to do: detect strong mid-level circulations**
**All weather radars have limitations**
**Small tornadoes frequently exploit these limitations in New Mexico**
**NWS forecasters use all available resources to overcome tornado detection limitations**
**Weatherwise media & public can prepare for this dangerous threat and react appropriately when tornadoes strike**

**Notes:**
Future technology
- Polarimetric radars (dual-polarization radars)
Most weather radars, such as the WSR-88D, transmit radio wave pulses that have a horizontal orientation. Polarimetric radars, transmit radio wave pulses that have both horizontal and vertical orientations. The horizontal pulses essentially give a measure of the horizontal dimension of cloud (cloud water and cloud ice) and precipitation (snow, ice pellets, hail, and rain) particles while the vertical pulses essentially give a measure of the vertical dimension. Since the power returned to the radar is a complicated function of each particles size, shape, and ice density, this additional information results in improved estimates of rain and snow rates, better detection of large hail location in summer storms, and improved identification of rain/snow transition regions in winter storms. Engineers at the National Severe Storms Laboratory are currently working to develop a polarimetric NEXRAD Doppler radar. Polarimetric technology should be available for installation into the national radar network in 5-10 years. 

- Phased array radars
-- Uses multiple beams and frequencies to reduce scans to 1 minute
-- more detail of lowest elevations
-- allows observation of rapidly changing low-level wind flow

## References

- NOAA, 1999: Thunderstorms...tornadoes...lightning... nature’s most violent storms.  A preparedness guide.  NOAA/PA #99050, ARC 1122, 16 pp. Cited 2004.  [Available online at http://www.nws.noaa.gov/om/brochures/ttl.pdf].
- NWS, cited 2003: NWS radar FAQs page.  [Available online at http://weather.noaa.gov/radar/radarinfo/radarinfo.html].
- Wakimoto, R.M., and Wilson, J.W., 1988: Non-supercell tornadoes.  Mon. Wea. Rev., **117**, 1113-1140.
- WSR-88D Operational Support Facility, May 1997: WSR-88D operations course student guide.  Version 9704.
- WSR-88D Operational Support Facility and Titan Systems Group, July 1998: *WSR-88D Principal User Processor Operator Handbook*.  Volume II, Applications Terminal, Software Version 10.0, 267 pp.

## Tornado Detection & Limitations*

**Questions for David Craft?**
**David.Craft@noaa.gov**

**April 2004 Media Workshop**

***Limitations common to all Doppler weather radars**