An Overview of VTMX Activities at NCAR David Parsons and William Brown Research Technology Facility/ATD National 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 (Zero th order findings) 3.Variations in structure of the terrain induced circulations and cold pool generation (1 st 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 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. Zero th Order Findings

SODAR Winds: 0000 UTC on 16 Oct Oct. Lake Breeze and Nocturnal Drainage Flows (From H.-J. Kirtzel)

0 th Order (cont.) 4. Lake breeze, gap flow, canyon winds, and the deep dry desert ABL means multiple air masses enter the basin. Often not just the same air mass subject to changes in the pressure gradient UTC on 8 October Lake breeze layer Dry western ABL

Residual of deep …western ABL 1100 UTC 9 October..of lake breeze Gap 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 1 st 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 1 st Drainage Pulse: Warm, shallow (overshooting) drainage with complex structure aloft Launch just before 2 nd pulse: Closer match to radiation with evidence of mixing or displacements aloft 1 st Order Findings 2. 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 gap flow.

SABL Data: 0800 –0830 UTC 3 October Scanning Aerosol Backscatter Lidar

Where and When Does Mixing and Vertical Transport Occur? 2. At the leading edge of transient flows (i.e., northerly synoptic surge).

SODAR measurements of the impulsive arrival of a Northerly surge

Where and When do Mixing and Vertical Transport Occur? 3. Evening transition is often a time for wave activity (low stability with shear). Note that the transition at this site is when the lake breeze transitions to gap flow.

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 conditions Large-scale (100s of meters) displacements and mixing common Pulsing drainage flow Multiple layering of aersol Transition periods with waves Strong winds through the gap with favorable large-scale conditions Radiative cooling very large and the “budgets” instructive With a C/day cooling rate, large flows needed for shear driven mixing TAOS was successful in its first field test SODARs 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