# Presentation: 726138

## National Weather Service

- Steve Davis
- Lead Forecaster

- Internet Web Page : _[www.crh.noaa.gov/mkx](http://www.crh.noaa.gov/mkx/welcome.htm)__[ ](http://www.crh.noaa.gov/mkx/welcome.htm)_
- Email : steve.c.davis@noaa.gov

## Outline

-  - Radar Principles
-  - Doppler Velocity Interpretation
-  - SRV vs Base Velocity
-  - Pre-storm Environment Analysis
- Part II: Radar/Storm Interpretation
-  - Thunderstorm Spectrum
-  - Severe Storm Generalities
-  - Les Lemon Criteria
-  - Pulse Storms
-  - Multicell Clusters/Lines
-  - Supercells
- Part III
-  - Build 10 - New TVS Algorithm

*Outline*

## Radar Basics

## Beam Power Structure

- 1⁄2 Power Point

- 1⁄2 Power Point

- Side Lobe Energy

- Side Lobes cause most of the clutter in close proximity to the radar

- The Radar Beam is defined by the half power points

## .96 Degree Beam Resolution

**If R =       60 NM        120 NM         180 NM       240 NM**

**    ****D =       1  NM          2  NM             3 NM           4  NM**

**D**

- D = Beam Width

- Radar resolution with respect to beam width / range

## Azimuth Resolution Considerations

- Weak inbound, weak outbound

- Rotation too small to be resolved

- Stronger inbound than outbound

- Strong inbound, strong outbound

- Azimuth 3

- Azimuth 2

- Azimuth 1

- Enlarged image along a radial. Individual “blocks”
- represent one sample volume.  This graphically shows the radar  resolution.

- Rotational couplet identification can be affected by azimuth resolution.
- As the diagram shows, the closer a rotation is to the radar the more likely it will be identified correctly.  If the rotation is smaller than the 10 beam width (possible at long ranges) then the rotation will be diluted or averaged by all the velocities in that sample volume.  This may cause the couplet to go unidentified until it gets closer to the radar.

## Pulse Repetition Frequency- PRF

-  PRF is the number of pulses per second transmitted by a radar

- PRF controls the Max Radar Range and Max Unambiguous Velocities

## The Doppler Dilemma

- Rmax  and  Vmax depend on PRF

- Rmax = The range to which a transmitted pulse can travel and return to the radar before the next pulse is transmitted.
- Vmax = The maximum mean radial velocity that the radar can unambiguously measure (before dealiasing).

- * Rmax is inversely related to PRF
- * Vmax is directly related to PRF

- As PRF increases, Rmax decreases and Vmax Increases!

_**The Doppler Dilemma**_: There is no single PRF that maximizes both Rmax and Vmax

## Defeating the Doppler Dilemma

- The WSR-88D employs a dual PRF scanning strategy to help defeat the “Doppler Dilemma”

- CS = Contiguous Surveillance 	B = Batch
- CD = Contiguous Doppler   	CDX = Contiguous Doppler X

- The 88D performs redundant sampling on the lowest 2 elevation slices and interlaced sampling on the “middle” slices to maximize range/velocity data, and minimize ground clutter.

- In this example of Volume Coverage Pattern (VCP) 21, the lowest two elevation slices are sampled twice.  Once using a low PRF (CS) to maximize range data and then again using a high PRF (CD) to maximize velocity data.  The middle slices (blue) are sampled once but use an alternating, or interlaced, high and low PRF (B) on each radial.  The upper elevation slices use only a high PRF (CDX) to maximize velocity data.  Range issues are not a problem in the higher elevations, precluding the use of a low PRF.

## Volume Coverage Patterns of the 88-D

- Precipitation Mode:
-  VCP 11   14 Slices*/5 minutes
-  VCP 21    9 Slices*/6 minutes
- Clear Air Mode:
-  VCP 31    5 Slices*/10 minutes
-         	        (Long Pulse)
-  VCP 32    5 Slices*/10 minutes
-         	        (Short Pulse)

- * Add 2 more slices to every VCP because the bottom two slices are sampled twice.  See previous slide.

## Atmospheric Refraction

- The radar assumes the beam is undergoing standard refraction.  The beam height will be misrepresented under super/sub-refractive conditions.

- Max cores may be displayed
- at wrong heights

**Superrefraction**: The beam refracts more than standard.  The beam height is lower than
-                               the radar indicates.

**Subrefraction**:    The beam refracts less than standard.  The beam height is higher than the radar 	              indicates.  Beam can overshoot developing storms.

## Super/Sub Refraction

**Super Refraction**
-        This occurs when the beam propagates through a layer where :
- 	- Temperature increases with height
-  Moisture decreases sharply with height
- 		* Radiation or subsidence inversion
- 		* Warm, dry air advecting over cooler water surface
- 		* Thunderstorm outflow
-  Will likely produce ground clutter

**Sub Refraction**
-        This occurs when the beam propagates through a layer where :
- 	- Temperature lapse rate is ~ dry-adiabatic
-  Moisture content increases with height
- 		* Inverted V sounding (mid-afternoon, well
-   		   mixed environment)
-  Will help eliminate ground clutter

## Beam Height vs. Range

- Standard Refraction Assumed

- Range (nm)

- Height AGL
- in Kft

**4.3****0**

## Odd Phenomena Seen on Radar

-  Chaff- Look for it coming from the Military        	Operations Areas
-  Migrating birds rising from nesting areas 	around sunrise and sunset
-  Smoke from fires
-  Sunrise/Sunset spike
-  The unexplainable...

## Doppler Velocity Interpretation

## The Zero Isodop “Problem”

- When the wind velocity is parallel to the radial, the full component of the wind is measured

- When the radial is perpendicular to the the wind, the radar displays zero velocity - This “zero zone” is called the “Zero Isodop”.

**What percentage **
**of actual wind **
**will the radar detect?**
**0****0**** = 100% - Parallel**
**15****0 ****= 97%**
**30****0**** = 87%**
**45****0 ****= 71%**
**60****0**** = 50%**
**75****0**** = 26%**
**90****0**** = 0% - Perpendicular**

## Large Scale Winds

- “S” Shape
- “S” shape of the zero isodop indicates veering winds with height.  Veering may imply warm air advection.

- The combination shape of the zero isodop indicates both veering and backing winds with height.
- Combination

- Backward “S” Shape
- Backward “S” shape of the zero isodop indicates backing winds with height.  Backing may imply cold air advection.

- Use the Zero Isodop to assess
- the vertical wind profile.

## Large Scale Winds

- Uniform Flow
- Straight Zero Isodop  indicates uniform direction at all levels.

- Uniform Flow with Jet Core
- Straight Zero Isodop indicates uniform direction at all levels.  The inbound/outbound max’s show a jetcore aloft with weaker winds above and below.

## Example from KMKX 88D

- Low level
- jet max

- January 5, 1994
- Steady snowfall

## The VAD Wind Profile

## Small Scale Winds- Divergence/Convergence -

- Divergent Signature
- Often seen at storm top level or near the ground at close range to a pulse type storm

- Convergence would show colors reversed

- In all of the following slides, note the position of the radar relative to the velocity signatures.  This is critical for proper interpretation of the small scale velocity data.

## Small Scale Winds- Cyclonic Convergence/Divergence -

- Anticyclonic convergence/ divergence would show colors reversed in each panel.

- Cyclonic Convergence

- Cyclonic Divergence

## Small Scale Winds- Pure Cyclonic Rotation -

- Pure Cyclonic Rotation

- Anticyclonic
- rotation would show colors reversed

- Example

## Small Scale Velocity Example

## Small Scale Velocity Example 

- Rotation seen with the Big Flats Tornado.  August 27, 1994 ~ 9 PM.

## Storm Relative Velocity - SRVvs.Base Velocity

- In General:
- 	When diagnosing rotational characteristics, use SRV
- 	- SRV subtracts out the motion of a storm to display pure  	 	   rotational characteristics of that storm.  Often, the motion of 	   the storm will mask or “dilute” the rotational information.  	  This is especially true when rotations are subtle.
- 	When diagnosing Straight Line Winds (bow echo, derecho,           	microbursts), use Base Velocity
- 	- The strength of an advancing line of storms producing straight 	  line winds is a sum of the winds produced by the storms, plus 	  the movement of the storms.  Using SRV would take one 	  	  component away.

- Examples

## SRV vs. Base Velocity- Strong Rotation - 

- Base Velocity

- Storm Relative Velocity

- Persistent rotation from Big Flats Storm

## SRV vs Base Velocity- Subtle Rotation -

- Base Velocity

- Storm Relative Velocity

- Janesville F2 tornado.  June 25th, 1998 ~ 700 PM

- Interesting note: These scans are at 3.40 elevation.  The 0.50 elevation showed little rotational information.

## SRV vs Base Velocity- Subtle Rotation -

- 0.50

- 3.40

- Base Velocity

- Storm Relative

- Little/no rotation seen at lowest elevation

## SRV vs Base Velocity- Oakfield -

- Base Velocity

- Storm Relative Velocity

- Oakfield F5 tornado.  July 18, 1996.  Although the rotation was intense, the low precip (LP) nature of the storm at this time, limited the amount of energy returned back to the 88D by precipitation targets.  In this case, though the rotation was strong, the SRV clearly was the better tool for diagnosing the strength of the rotation.

## SRV vs Base Velocity- Straight Line Winds -

- Base velocity shows max inbound winds of 55 to 60 kts.

- SRV shows max inbound winds of 30 to 40 kts.

## Pre-Storm Environment

- Dewpoints/Precipitable Water

- CAPE

- Cap Strength/CIN

- Wet Bulb Zero

- Helicity

- Energy Helicity Index -EHI

- BRN

- Boundaries

- Jet Position (coupling?)

- LI’s

- The three main elements to assess are:
- Moisture, Stability and Lift

## LI’s and Moisture

- LI = -3 to -6 	Moderately Unstable
- LI = -6 to -9 	Very Unstable
- LI = < -9      	Extremely Unstable*
-   	* LI’s even lower are increasingly likely to
-     	   exist under a capped environment
- Best to use the most unstable parcel in a layer up to about 850 mb.  A surfaced based LI may be unrepresentative if boundry layer is under a shallow inversion.

- Surface - 600 F dewpoint or higher
- 850 mb - 120 C dewpoint or higher
- 1000-500mb Precipitable water - 1.5" or higher

- _LI’s_

- _Moisture_

## CAPE \ CIN and Cap

- 700 mb +100 C used as edge of cap

- The edge of a cap is often a good place to watch for “Back-Building”, nearly stationary, flood producing storms.  This is especially true if there is a focusing, trigger mechanism available.

## Upper (low) Level Jet Influence

## Shear and Thermal Instablility

- The most severe, organized storms occur in environments where the shear and thermal instability are both moderate or strong and well balanced.

- Supercells seem to be the favored mode of convection when the low-level, storm relative winds are greater than 19 knots and veer by roughly 900 in the lowest 4 km.

## Bulk Richardson’s Number

- The BRN usually is a good overall indicator of convective storm type within given environments.  It incorporates buoyant energy (CAPE) and the vertical shear of the horizontal wind, both of which are critical factors in determining storm development, evolution, and organization.

**BRN < 10**       Strong vertical wind shear and weak CAPE.  The shear may be too strong given the weak buoyancy to develop sustained convective updrafts.  However, given strong enough forcing, rotating supercells could evolve.
- (I1, M1)
**BRN = 10 to 45**   “Sweet Spot”   Associated with supercell development.
- (M3,P3,H3)
**BRN > 50**   Relatively weak vertical wind shear and high CAPE which suggests pulse/multicellular storm development is most likely.
- (N3,K3,B2)

## S-R Helicity and EHI

- Storm-relative helicity is an estimate of a thunderstorm’s potential to acquire a rotating updraft given an environmental vertical wind shear profile.  It integrates the effects of S-R winds and the horizontal vorticity (generated by vertical shear of the horizontal wind) within the inflow layer of a storm.

- Hs-r = 150              The approximate threshold for supercell development
- Hs-r = 150 to 299    Weak tornadoes (F0 and F1) possible
- Hs-r = 300 to 449    Strong tornadoes (F2 and F3) possible
- Hs-r > 450               Violent tornadoes (F4 and F5) possible

- An intense rotating updraft can form with relatively weak CAPE if the vertical wind shear and storm-relative inflow are strong.  Relatively low S-R helicity usually can be compensated by high instability to produce a rotating updraft.  The EHI attempts to combine CAPE and S-R helicity into one index to assess the potential for supercell and mesocyclone development.  High EHI values represent an environment possessing high CAPE and/or high S-R helicity.

- EHI < 1.0               Supercells and tornadoes unlikely in most cases
- EHI = 1 to 2           Supercells and tornadoes are possible but generally tornadoes are
-                                not of violent or long lived nature
- EHI = 2 to 2.4        Supercells more likely and mesocyclone-induced tornadoes possible.
- EHI = 2.5 to 2.9     Mesocyclone-induced supercellular tornadoes more likely.
- EHI = 3.0 to 3.9     Strong mesocyclone-induced tornadoes (F2/F3) possible.
- EHI > 4.0               Violent mesocyclone-induced tornadoes (F4/F5) possible.

- H+12 ETA model produced an EHI of 5.5 over Oakfield area on July 18, 1996.

## Scatter diagram- S-R Helicity vs CAPE -

- Hs-r = 150 to 299        Weak tornadoes
- Hs-r = 300 to 449        Strong tornadoes
- Hs-r > 450                   Violent tornadoes

- CAPE < 0                    Stable
- CAPE = 0 to 1000       Marginally 			     unstable
- CAPE = 1000 to 2500  Moderately 		          	      unstable
- CAPE = 2500 to 3500  Very unstable
- CAPE > 3500 to 4000  Extremely 			      unstable (capped?)

- “Sweet Spot” :
- - Hs-r of 250 - 400
- - CAPEs 1500 - 3000

## Wet Bulb Zero

- The wet bulb temperature represents the lowest temperature a volume of air at constant pressure can be cooled to by evaporating water into it.   The height of the wet bulb zero is that level on the sounding where the wet bulb drops to 00 C.

- In general, WBZ heights from 5Kft to 12Kft are associated with hail at the ground.
- The potential for large hail is highest for WBZ heights of 7Kft to 10Kft, with rapidly diminishing hail size below 6Kft and above 11Kft.
-            * Above 11Kft, hail is less common since it has a smaller depth in which to form and may          	melt before reaching the ground due to a deep warm layer below.
-              * WBZ values too low indicate shallow warm cloud depth with less warm cloud  	collision- coalescence occurring to provide necessary liquid drops to increase hail size.

- The WSR-88D uses the height of the 00C and -200C isotherm in the Hail Algorithm.
- We adjust this continually using either actual soundings or grid point soundings from the models.  The RUC is very useful here.  Slight adjustments to these numbers has a dramatic influence on Hail Size output from the 88D.

## End Part 1Part 2 - Tracking and Identifying Storms