# Presentation: 354534

## Thundersnow Proximity Soundings

**Patrick S. Market**
*Atmospheric Science Program*
*University of Missouri-Columbia*

## Introduction

- Horizontal composite fields useful, but...
- Reveal otherwise typical snowstorms
- Many of the standard approaches to snow forecasting would apply
- Need to distinguish what sets these convective snow storms apart from
- Aspects of the sounding

## Data and Approach

- 12 proximity soundings were found
- All associated with an extratropical cyclone
- Assume
- some feature (instability) makes these events unique, and
- convection resulting from instability is the source of charge separation for lightning production

## Data and Approach

- Four events occurred at exact time and location of a sounding
- Four events occurred at exact time of a sounding but were ***not*** at the exact location the sounding occurred
- the location of thundersnow was within 90 nautical miles (167 km) of the sounding station
- Four events occurred within 90 nmi of sounding station and the TS report occurred 0-3 h after balloon release

## What we found – Part I

- Thundersnow northeast of a cyclone center
- NEC
- N=4
- Potentially neutral, but
- profiles rich in
- moisture
- forcing for ascent

## What we found – Part II

- Thundersnow northwest of a cyclone center
- NWC
- N=8
- Elevated layer of potential instability
- Not deep, but exists in a region of
- deep moisture
- forcing for ascent

## Previous work

- Curran and Pearson (1971)
- Found only 76 obs of TS  (period - 2/68 TO 4/71)
- Obs eliminated if they did not occur
- Three hours before or after
- 0000 or 1200 UTC, and
- within 90 nmi of a rawinsonde station
- Only 13 observations remained
- All of these occurred east of the Rocky Mountains

## Previous work (CP71 reanalysis)

## Our results – Mean profiles

- Mean profiles used raw data from 12 balloon flights
- not fitted to a grid first
- Proximity soundings show
- mean temperature
- mean dew point
- mean wind speeds, and
*median* wind direction

## Our results – Mean profiles

- NWC mean proximity sounding
- N=8
***NO*** CAPE present  &   ***NO*** PI present
- However, moist-neutral LR from 700 mb to 550 mb
- Also, Winds
- veer from surface to 600 mb (WAA)
- back from 600 mb to 400 mb (CAA)
- veer from 400 mb to 200 mb (WAA)

## Our results – Mean profiles

## Our results – Mean profiles

- NEC mean proximity sounding
- N=4
***NO*** CAPE present  &   ***NO*** PI present
- Moist-neutral LR from 500 to 400 mb
- Shallower than NWC
- Higher that NWC
- Deep WAA profile

## Our results – Mean profiles

## Additional Analyses

## Our results – Derived profiles

- Mean derived (e.g., F  ) profiles generated
- Raob data objectively analyzed to a grid
- horizontal grid spacing of 150 km
- 50 mb vertical spacing.
- Only TSSN at 0000, 0300, 1200, or 1500.
- Location of the TSSN w.r.t. location of the sounding ***not*** a limiting factor as before
- Samples now larger
- NEC   N=9
- NWC  N=13

## Our results – Derived profiles

- NWC cases
- Potentially stable
- Negative EPV not a recurrent theme
- Weak values do manifest at ~600 mb

## Our results – Derived profiles

## Our results – Derived profiles

## Our results – Derived profiles

***log***

## Our results – Derived profiles

- NEC cases
- Potentially stable
- Negative EPV not a recurrent theme
- Weak values do manifest
- Deeper layer than NWC cases
- 700-500 mb

## Our results – Derived profiles

## Our results – Derived profiles

## Our results – Derived profiles

***log***

## Our results – Typical profiles

- Also based upon objectively analyzed data fitted to a grid, but...
- Blind averages were *not* used here, as before
- Cases examined individually to assess the existence of PI, and if present, where
- While not a quantitative assessment of profiles, typical profiles reflect patterns seen most often.

## Our results – Typical profiles

- NWC cases
- 10 of 13 profiles revealed a PI layer somewhere
- Two most common locations for PI
- near the surface, usually 950 mb to 900 mb
- usually late-fall or early spring
- in the range of 700 mb to 600 mb
- occurs throughout the year

## Our results – Typical profiles

- NWC cases
- Profiles speaks of a source of heat and moisture aloft
- In a well-developed cyclone, warm moist air northwest of the surface low is most likely the result of the warm conveyer belt, especially the ***trowal***

## Our results – Typical profile (NWC)

## Our results – Typical profiles

- NEC cases
- 5 of 9 profiles revealed at least one PI layer
- near the surface, from 1000 mb to 950 mb
- Second location of nearly neutral **e change was found higher in the atmosphere
- ~650 mb to 600 mb
- profile here is moist neutral

## Our results – Typical profile (NEC)

## Conclusions

- Not statistically significant, yet
- Profiles would seem to support the hypothesis
***potential instability*** related convection dominating in ***NWC*** thundersnow cases
***potential symmetric instability*** related convection dominating in ***NEC*** cases