# Presentation: 641689

## Hail Forecasting and Formation Processes

**DLOC Workshop 2008**
**Paul Schlatter**
**Warning Decision Training Branch**

- Photo by Gene Moore

## Outline

-  Hail formation/growth

-  Hail melting

-  Severe Hail Forecasting

- Quiz game where you are the SPC Outlook forecaster!

## Learning Objectives

- Understand the important hail growth factors
- Explain the 2 reasons why small hail melts proportionately faster than large hail
- Identify important environmental characteristics for severe hail forecasting

## Hail Formation Concepts

- Formation/growth due to differences in terminal fall speeds of hydrometeors
- Vt related to hydrometeor diameter

**Freezing Level**

**Cloud Parcel in  Weak Updraft**

- Vt small

- Vt large

**Notes:**
None of this is to scale....
Large hydrometeors fall faster compared to smaller ones, and if well above freezing level, freeze on contact
Fall velocity increases as it grows
Continues to collect and freeze more droplets while inside supercooled cloud
Updraft/LWC sufficient, it becomes a hail stone, typically defined as 5 mm in diameter

## Terminal Fall Velocity

## Slightly Painful Impact

- >100 mph when it reached the surface

**Notes:**
US record largest hailstone size ever

## Ingredients for Hail Growth

- In a basic sense, 3 Main “ingredients” are required:
- 	1. **Adequate updraft** to keep hailstone aloft for an appropriate amount of time, -10 to -30oC
- 2.  **Sufficient supercooled water** near the hailstone to enable growth during transit through an updraft
- A piece of ice or snow (**embryo**) for it to grow upon

**Notes:**
Updraft strength:  strong but not too strong.  LWC:  high in the right region, fresh source when depleted  Embryo:  can’t have too many, otherwise just small hailstones in a given region.  Embryos of any type can produce large hail: graupel, liquid drop, either one can go onto produce large hail.

## How/Where does Hail Grow?  Examine Growth Trajectories

- A wide variety of trajectories are capable of producing hail in a storm.
- Stronger the updraft/shear, more diverse the trajectories.
- Number and size (width) of updrafts also important.
- Straight up/down through or around the main updraft most common

## Hail Trajectories

- _[Link to Hail Trajectories Animation](http://www.wdtb.noaa.gov/courses/awoc/dev/hail_trajetories.swf)_

- Trajectories are important because of residence time

## Growth Concepts Summary

-  Hail growth occurs between -10 and -30 C

- Sufficient updraft and LWC
-  Maximize residence time for embryo

## Melting Hail

- _[Link to Melting Hail Phases](http://www.wdtb.noaa.gov/courses/awoc/dev/hail_melting_cycles.swf)_

## Why small hail will melt proportionately faster than large hail

- Larger hail falls faster
- Surface area vs. volume
- 			Surface Area	Volume
- 1” Hail	3.1 in2		0.52 in3
- 3” Hail	28.3 in2		14.1 in3
- Ratio	6:1 1” hail; 2:1 for 3” hail

## Melting Hail Dependencies on Environmental and Microphyscial Conditions

- Rasmussen and Heymsfield (1987) Varied size, density, RH, temp profile, LWC
- Low RH=less melting
- Cooler T profile=less melting (duh)
- Higher LWC=slightly greater melting

-  Size/density by far most important, RH and temp very important for smaller hail sizes

**Notes:**
Regardless of what type of environment the hail is falling through, smaller hail (say less than golfball) will melt much more proportionately than larger hail.  Golfball hail just doesn’t melt very much even in very moist, warm, high freezing level environments.  However, at smaller hail sizes, environmental conditions make all the difference.  Warm, moist, high freezing level environments are most likely to completely melt small hail.

## Melting Hail Model

- _[Link to Melting Hail Model Graphic](http://www.wdtb.noaa.gov/courses/awoc/dev/hail_melting_model.swf)_

## Hail Formation/Melting Summary

-  The Essence of Hail
-  Embryo input lower levels of updraft, max growth occurs by accreting supercooled water between -10oC and -30oC, is either ejected out the side by mid level winds, out the top because too small, or falls through the updraft because Vt exceeds w.
-  Melting only significant for smaller hail, and then density, RH, temperature, freezing level height in/out of cloud, all come into play

**Notes:**
Essence of hail sounds like a cheap cologne that I’d buy

## Forecasting Hail (0-12 hr prior)

## Forecasting ANY Hail

- _Deep, Moist Convection has 3 Ingredients:_
- Sufficiently deep low level moisture
- Steep lapse rates (related to instability)
- Sufficient lifting from LCL to LFC

-  Once DMC identified as possibility, look for hail signal

**Notes:**
Long range hail forecasting has changed very little since the early 1990s.  Johns & Doswell (1992) detail process as looking at deep moist convection, then trying to identify ingredients for an enhanced hail threat.

## Hail Ingredients Provided DMC Possible

**Hail Ingredients Provided DMC Possible**

- Strong updrafts necessary but not sufficient
-  High CAPE in hail growth zones

**Notes:**
Edwards and Thompson study found that golfball hail could be formed with any value of CAPE from a RUC proximity sounding.  Just about all golfball hail reports were associated with at least 1000 J/kg, while all baseball hail was associated with larger than 1300 J/kg.

## Hail Ingredients Provided DMC Possible

- 2. Storm scale winds:  Speed/Direct-ional shear important

**Notes:**
Very large hail events (> golfball) tend to be associated with strong veering with height wind profiles that are favorable for supercells

## What’s Right/Wrong with this Sounding for Hail Threat?

**What’s Right/Wrong with this Sounding for Hail Threat?**

**Notes:**
Edwards and Thompson study found that golfball hail could be formed with any value of CAPE from a RUC proximity sounding.  Just about all golfball hail reports were associated with at least 1000 J/kg, while all baseball hail was associated with larger than 1300 J/kg.

## Hail Ingredients Provided DMC Possible

- Studies of sounding or model derived parameters:
- Height of Freezing level (or T at any height)
- CAPE between –10 to –30oC
- SR winds and wind shear at various levels
- Wet Bulb 0oC height
- PW
- Dewpoint Depression at various heights/depths
- Varying degrees of skill, but none are in fact particularly skillful

**Notes:**
Reasons why not skillful:  Problem with not having representative soundings and model inaccuracy on the storm scale
--physically, all these predictors have physical merit, but the resolution with which they are measured is horribly inadequate, and even if high resolution in time/space, we simply don’t understand enough about microphysics of real hailstorms to know for sure if these predictors are highly skillful, or even if they could EVER be highly skillful.
SPC uses:  Steep mid level lapse rates. Strong deep layer shear through the cloud layer. Moderate CAPE. Low wet bulb zero levels

## 0-12 hr Hail Forecasting

- DMC potential
- CAPE and Lapse rates in hail growth zones (-10 to -30oC)
- Shear (for giant hail production potential)
- “Traditional” parameters should be fine in low shear environments and marginal CAPE
- Examine numerical model output and Hail-Size estimation algorithms (SPC)

**Notes:**
Supercells are responsible for hail larger than about golfball in size.  Thus, forecasting for very large hail must examine wind shear, both directional and speed shear.
The hail size estimator in AWIPS on the sounding parameters is worthless.  A few recent studies have attempted to predict hail size off a sounding with mixed results.  SPC currently uses this model to aid in their hail probability graphics.

## 0-12 Hr Hail Forecasting Summary

-  Know your environment!
-  Keep in mind when to ditch certain parameters
-  Utilize SPC data/outlooks for awareness of threat and consistency checks
-  CAPE, SHEAR, Thermodynamic profile all important but rarely reliable
-  Melting only important for smaller hail sizes

## Time for a Game

- Time for a game:  _[Hail soundings](https://www.wdtb.noaa.gov/courses/awoc/ICsvr1/HailSoundings/)_