An Overview of VTMX Activities at NCAR

David Parsons and William BrownResearch Technology Facility/ATDNational Center for Atmospheric Research

Initial Questions of Scientific Interest

1. How do terrain-induced circulations “pattern” (i.e., where, when and why) mixing and vertical

transport in a stable, urban basins?---to be presented by Dave Parsons

2. How best to measure mixing and vertical transport in stable, urban basins?---was to be presented by Bill Brown

3. How well do numerical simulations represent these mesoscale circulations and the patterning of mixing and vertical transport? How can simulations be improved?---James Pinto, unable to be here

Outline

1. Background (Instruments, location, status of data)

2. Generalization of observed terrain induced circulations at our site (Zeroth order findings)

3. Variations in structure of the terrain induced circulations and cold pool generation (1st order findings)

4. Thoughts on where and when mixing and vertical transport occur (Hypotheses to be tested)

5. How best to measure stable cold pool environments, mixing and transport (Pure speculation)

Location: Just north of the Jordan Narrows

Great Salt Lake

Utah lake

Wasatch Range

RTF

Special thanks to John Horel as this site was selected partly in response to his request about the need to know how much flow enters the Salt Lake Valley from the south

NCAR Instruments:

• MAPR – wind profiler• Metek Doppler SODAR• Two surface stations• TAOS -tethersonde (5 levels)• Rawinsondes (~2-3 h)• SABL (high resolution

backscatter lidar)

Status: • Archival and data quality work for the variety sensors is

well underway or completed for the six sensor systems. Profiler is the last.

• Interactive data perusal for data from these sensors can be found at http://www.atd.ucar.edu/sssf/projects/vtmx/

• Analysis has begun on suspected mixing and transport events and on placing these events within the context of the circulations within the basin

•Based on the need for improved vertical resolution in stable regions, we are modifying our wind profiler

•Modeling efforts (MM5) have commenced

1) Well defined nocturnal flow through the gap is a common occurrence

during the VTMX IOPs.

2) Often the lake breeze arrives in the late

afternoon.

3) Therefore the southern end of the valley “typically” resides in a very

different air mass than the northern portions.

Zeroth Order Findings

SODAR Winds: 0000 UTC on 16 Oct.- 0000 18 Oct. Lake Breeze and Nocturnal Drainage Flows

(From H.-J. Kirtzel)

0th Order (cont.)

4. Lake breeze,gap flow, canyonwinds, and thedeep dry desertABL means multiple air masses enterthe basin. Often not just the same air mass subject to changesin the pressure gradient.

2200 UTC on 8 October

Lake breeze layer

Dry western ABL

Residual of deep…western ABL

1100 UTC9 October

..of lake breezeGap flow

1. The structure and intensity of the flow through the gap varies (case-to-case and during a case)

2200 UTC 6 Oct. –2200 UTC 7 October

1st Order Findings

SODAR derived winds during the previously shown

intermittent orographic flow

event.

Tethersonde measurements

through the second pulse.

Richardson numbers next

Observed Radiosonde Cooling Rate vs. Radiative Cooling(Used Streamer Code (Key, 1996))

During 1st Drainage Pulse: Warm, shallow (overshooting) drainage with complex structure aloft

Launch just before 2nd pulse:Closer match to radiationwith evidence of mixing or displacements aloft

1st Order Findings2. Large long wave cooling

rates.3. Cooling rates can be reduced

by gap flow.4. Sounding “budgets” might

prove useful.

Where and when does mixing occur?

1. Near the top of the gapflow.

SABL Data:0800 –0830 UTC3 October

Scanning Aerosol Backscatter Lidar

Where and When Does Mixing and Vertical Transport Occur?

2. At the leading edge of transientflows (i.e., northerlysynoptic surge).

SODAR measurementsof the impulsive arrival of aNortherly surge

Where and When do Mixing and Vertical Transport Occur?

3. Evening transition is often a time for wave activity (lowstability withshear). Note that the transition atthis site is whenthe lake breeze transitions to gapflow.

SABL

SABL

Wave layer at 2 km, period 8m

Clear layer at 1 km

How Best to Measure Terrain Induced Circulations and Vertical

Transport and Mixing?

• Environmental characteristics– Dry air masses

– Light Winds

– Turbulence is intermittent

– Night-time conditions

– Inversions shallow

– Flows can be shallow

– Fine-scale structures

– Clear, cloudy or foggy

Profiler and Radars

• There is typically a diurnal cycle in the signal to noise ratio with low ratios at night when turbulence decreases.

• Signal to noise ratio is relatively poor when conditions are dry.

• High vertical and temporal resolution is relatively difficult whenever the signal to noise ratio is poor.

• Seasonal bird migration are a problem at night. We at NCAR learned that the Jordan Narrows efficiently channel the winds, but it also channels birds.

• Calm winds difficult for spaced antenna techniques.

MAPR

• 915 MHz Wind Profiler• Uses spaced antenna techniques• Rapid wind measurements (1 – 5 minute winds)• Turbulence

Photo: Charlie Martin

Multiple Antenna Profiler Radar

MAPR

• Observations heavily contaminated with birds

• Some layers visible before birds

MAPR• Bird algorithm under development and special processing

• Adapting special processing for our spaced antenna system (Spectral filtering (Merritt SAM), Wavelet filtering (Jordan), and NIMA)

• Layers clearer

• More “real” winds

Profilers and Radars• Therefore, VTMX is a very challenging environment for

profilers and radars. Some evidence– NIMA applications at Shay’s lounge and our struggle with

NCAR data– “turbulent events visible with radar systems were spare” –

UMASS presentation– Profiler data “I have problems with it.” – Rich Coulter

• For next time– At NCAR we have purchased a new transmitter, we are going

to change our pulse coding to work better with anti-bird alogirthms and we are going to multiple frequency efforts (FDI) to increase vertical resolution

SODAR• Diurnal variation in the signal-to-noise ratio with best

signal at night.• Light winds and low turbulence are good for acoustic

systems.• Some evidence of mixing events might be visible in sodar

power return, variance and vertical motions.• Lessons for NCAR

– The performance of the METEK sodar exceeded expectations so we just bought one.

– Try getting winds by tracking acoustic shell (RASS), speed of sound is far from clutter sources (birds, ground clutter, and wire beating etc.)

Lidars and soundings

• GPS soundings do not resolve problems of accurate winds near the surface – Lose lock at launch Vaisala is aware of the

problem and are trying to fix it– LORAN systems gave you winds but just a

smooth interpolation

• Lidars are fantastic (dry and slightly dirty air is good) until the clouds and fog arrive later in winter

Conclusions

• We collected data in stable layers with several instruments with some pleasant surprises:

Truly complex flow in complex terrain, suspect it is hard to accurately predict concentrations in these western urban basins

Frequently observed waves and breaking waves under specific conditionsLarge-scale (100s of meters) displacements and mixing commonPulsing drainage flowMultiple layering of aersolTransition periods with wavesStrong winds through the gap with favorable large-scale conditionsRadiative cooling very large and the “budgets” instructiveWith a 10-20 C/day cooling rate, large flows needed for shear driven mixingTAOS was successful in its first field testSODARs and lidar work well (calm conditions)

What’s Next• Expand detection of mixing and transport events• Richardson numbers from tethersondes and sondes to

understand the wave and mixing events• Understanding the observed variations in the gap flow• Understand the differences in predicted versus observed

temperature changes (gap flow adds to the mass of the cold pool, but not directly to its intensity?)

• Become serious about modeling efforts• Field work: Next for us at NCAR may be HVAMS due to

NSF request for instrumentation, but with instrument upgrades

SABL

Rapid change in aerosol height

SABL

More evening transition

SABL

Morning transition from stable to convective

SABL: 6 Oct.

MAPR

• Observes a strong layer about 750 m above ground level.

• Lowest layers could be ground clutter