Mesoscale Circulations during VTMX

John Horel

Lacey Holland, Mike Splitt,

Alex Reinecke

jhorel@met.utah.edu

Overview

• Temporal and spatial context for VTMX IOPs

• Synoptic and mesoscale conditions during IOPs

October 1999500 mb geopotential height

October 1999500 mb geopotential height anomaly

October 2000500 mb geopotential height

October 2000500 mb geopotential height anomaly

Cooler and wetter thannormal

PM-10 concentrationshalf those inOctober 1999

MesoWest

• Data available from January 1997 to present

• www.met.utah.edu/mesowest

• Paper describing MesoWest submitted to BAMS

9 Oct. 0900 UTC

Diurnal Temperature Range: Fall

Alex Reinecke

MesoWestObservations

1997-2000

Diurnal Temperature Range- FallLatitude vs. Elevation

SLC

Diurnal Temperature Range: October 2000

RushValley

TooeleValley

Salt LakeValley

October 2000: Salt Lake Valley

SLC

U42

Great Salt Lake: 21C- 10C

Surface Wind Convergence in Salt Lake Valley

IOP

1

IOP

2IO

P 3

IOP

4

IOP

5IO

P 6

IOP

7

IOP

8IO

P 9

IOP

10

Mike Splitt- Linear regression fit

Stability and Wind

• Surface-based inversions (greater than 5C in the lowest 100 mb) observed during 15 of the 31 morning (1200 UTC) soundings at the Salt Lake City International Airport

• Weak surface inversions with stable layers aloft below the crest of the Wasatch Mountains on 5 other mornings

• Well-mixed conditions present during the other 11 mornings

• Winds at 700 mb (near the crest of the Wasatch Mountains) were less than 10 m/s in 19 of the 31 morning soundings.

IOPs with Well-Developed Drainage Circulations

• 5 (14-15 October)• 6 (15-16 October)• 8 (19-20 October) • Clear skies, weak winds aloft at crest level, strong

nocturnal radiational inversions• Limited moisture in the boundary layer• Pronounced drainage flow into the Salt Lake Valley from

the west, south, and east• Surface based inversions and drainage circulations

developed after sunset  and persisted without significant interruption until sunrise

IOP-8: 9 UTC 20 October

SLC

500 mb

700 mb

IOPs Modulated by Synoptic and Mesoscale Systems

• IOP 1 ( 2-3 October) • Test operational procedures • During evening:

– clear skies with drainage flows developing as the evening progressed• Synoptic-scale northerly pressure gradient developed overnight

– Northerly winds penetrated into northern end of the Salt Lake Valley before midnight

– Eventually  reversed the down-valley (southerly) flow through the center of the valley

– Drainage circulations down into the valley from the Oquirrh and  Wasatch Mountains were largely unaffected

  IOPs modulated by synoptic and mesoscale weather systems

• IOP 4 (8-9 October), IOP 7 (17-18 October) • Similar boundary-layer structure to that in IOPs 5,

6, 8 until early morning• Prior to that time, clear skies, weak winds aloft,

and strong surface-based radiational inversions • As a result of approaching upper-level troughs

from the west, the nocturnal inversions eroded both by surface heating and by mixing due to the downward penetration of southerly winds

IOP-4 9 October

SLC

500 mb

700 mb

Wheeler

IOPs Modulated by Synoptic and Mesoscale Systems

• IOP 2 (6-7 October) and 3 (7-8 October)• Split flow aloft with weak upper-level short waves to the southwest and

northeast of Utah• Strong outbreak of cold air to the east of the continental divide

progressed westward after 0 UTC• After 1000 UTC, the depth of the cold air  to the east of the Wasatch

Mountains built to sufficient height to spill over the lower terrain from Mill Creek Canyon to near the University of Utah in the northeast corner of the Salt Lake Valley

• IOP-3 began at 2200 UTC 7 October and was terminated before midnight

• Strong downslope conditions persisted into the evening in the northeastern corner of the Salt Lake Valley

• Winds in the western part of the valley were turbulent

IOPs Modulated by Synoptic and Mesoscale Systems

• IOP 9 (20-21 October ) and IOP 10 (25-26 October) • Affected significantly by approaching upper-level

troughs. • weak short-wave ridge aloft initially• Skies were broken to overcast • Weak nocturnal surface inversion and drainage

circulations • Cold-front at 1200 UTC 21 October • Southerly surface winds were enhanced during IOP

10

IOP 9- 3UTC 21 October

500 mb

700 mb

Summary

• Mountain/valley circulations and radiational inversions occurred on over half of the days

• Local circulations dominated several IOPs (5, 6, 8)- but each had unique characteristics

• Synoptic and mesoscale influenced IOPs:– IOP 1: interruption of drainage circulations in the north

end of the Salt Lake Valley– IOPs 2, 3: downslope wind event– IOPs 4, 7: erosion of invesion from aloft– IOPs 9, 10: approaching weather systems

IOP-1 300 UTC 3 October

6UTC

IOP 2: 1200 UTC 7 October

IOP 3- 300 UTC 8 October

0Z

IOP-4 9 October

SLC

500 mb

700 mb

Wheeler

IOP 5 600 UTC 15 October

IOP 6 12 UTC 16 October

IOP 7 600 UTC 18 October

IOP-8: 9 UTC 20 October

SLC

500 mb

700 mb

IOP 9- 3UTC 21 October

500 mb

700 mb

0Z

IOP-10 600 UTC 26 October