# Presentation: 815505

## Tornado Detection & Limitations

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

**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**
**References**
**Bottom Line**

##  National Average Lead Times & Probability of Detection

**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)

**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

**Base Reflectivity**

**Storm Relative Velocity**

**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 

TYPES OF NEW MEXICO TORNADOES
 Supercell Tornadoes 
- Can be large and damaging
- Occur during a thunderstorm’s mature stage
- Preceded by mid-level (15-25 k ft) mesocyclones, where radars have better view
- Most common in the east, near the Texas border in May, June, and July
- WSR-88Ds have earned excellent reputation w/ these storms
- Supercells produce the majority of confirmed tornadoes each year, but not all of them

## Non-Supercell Tornadoes

**Probably more frequent in Western U.S.**
**Often unseen or unreported**
**Form early in thunderstorm lifecycle**
**May form rapidly near surface then extend upward**
**May form simultaneously at low and mid levels**
**Shallow and/or narrow, brief **

**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: shear instabilities stretched upward by updraft**

**Cause 2: horizontal vorticity tilted to vertical by updraft**

** **

** **

- (Wakimoto et. al., 1988)

- (NOAA, 1999)

**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 and Titan Systems Group, July 1998)

**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

**Angles Used**

**Radar Coverage**

**Cause 2: Overshooting**

**Notes:**
Beams rise above low-level circulations when they travel away from the radar at an angle

 Example
- Many NM spin-ups develop from ground upward to around 3,000 ft w/o mid level rotation
- Overshot by center of WSR-88D’s lowest beam at 40 miles
- Meteorologists prefer 2 beams bisect tornado-related circulations because vertical continuity of the signature increases warning confidence
- But, most of two beams only sample the lowest levels w/ in 29 miles of the radar 
- Thus, many spin-ups may only be detectable out to a range of 29 miles.

 In the near future, the NWS will employ a new scanning strategy with 3 new elevation angles between 0.5 degrees and 2.4 degrees, that will provide better vertical definition of low-level storm structure.

 Radar Coverage
- NWS distributed 122 WSR-88D radars w/ equation that considered population density and severe weather frequency
- Radar coverage along the NM/TX border, where supercells occur the most, is superior to the rest of the state

## Limitation 2:Too Close to the Radar

**Cone of Silence**

**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**

**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**
**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**

**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

**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**

**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 with all thunderstorms**
**U.S. hazards assessment**
**Severe wx outlook, mesoscale discussions, watches**
**Hazardous wx 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 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**