Moisture Variability and Nocturnal Convection

David B. Parsons, Crystal Pettet, and Tammy Weckwerth

NCAR/ATD

Acknowledgements to Ed Browell et al., Cyrille Flamant et al., and Steve Koch andthe bore folks

Sounding-based Schematic of Nocturnal Convection Initiation

From Trier and Parsons 1993

Moisture Upstream of MCS Inflow

LLJ

2nd LLJDeeper Mixed Layer from near the dryline

Nocturnal MCS June 6th

~Frontal Position

~DC-8 Flight track1

2

3

4

MCS Inflow

Conv

Water Vapor: 20 June

Sounding Susceptible to Elevated ConvectionSounding Susceptible to PBL-Based Convection

DVN (0000 UTC 9/12/2000) GRB (0000 UTC 9/12/2000)

From Trier et al. 2002, abstract 130, Reading QPF meeting

A key factor in nocturnal convection is the depth of the moist inflow, which can be aloft

Research Goals

• How common are bores and wave train events?

• What are the characteristics of these events?

• How are they generated and in what environments?

• How strong is the lifting and does it modify the inflow thermodynamics?

Research Technique

• Identify events in S-Pol and NEXRAD composites

• Independently identify events in MAPR and ISS surface data

• Identify event characteristics

• Link to environmental characteristics

Time of Occurrence

Bore/Wave Passage at MAPR

0

1

2

3

CST (h)

# of

Eve

nts

End Time of Bore/waves Event

0

1

2

3

4

5

6

CST (h)

# of

Eve

nts

Time of Generation (S-Pol)

0

1

2

3

4

5

Local Time (h)

# of

Wav

e/B

ore

Even

ts

1 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43

S1

0

2

4

6

8

10

12

14

16

IHOP Date

Max Nocturnal Wind Speed (m/s)

Max Surface Wind vs. Bore/Wave Events

S-Pol and MAPRbore/wave events

~18 bore and 8 S-Pol and MAPR wave events were observed. Bore events are observed in the later stages of LLJ periods when precipitation occurs.

BOREExample From MAPR4 June

Pre-bore height Post height

Bore Height Displacements

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25 30 35 40 45 50 60 65 75

Time (mins)

LayerHeight(km)

Approximate Spatial Dimension of S-Pol Bore/Wave Events

0123456789

10

50 150 250 350 450 550Along line length (km)

# of

Eve

nts

Implications and Speculations•Undular bore-like disturbances are ubiquitous over

this region when convection and low-level jets are present. Bore lifting varies.

•These disturbances can promote intense lifting with displacements of up to ~2 km. Theyinitiate convection and help maintain existing systems by lifting their inflow (removing CIN) and by creating a deeper moist inflow.

•These disturbances surely contribute to the nocturnalprecipitation maximum over this region allowingsystems to cross over this unfavorable environment into more moist air.