Ground-based Lidar Operations

TEA CO2 Doppler lidar Scan Strategy

  • When we regularly repeat the same series of scans over time, we can document the evolution of winds with time over a given region. The following series of scans will be executed on the hour and half-hour during every shift. They should take about 12 min to complete.
    • PPIs: 0, 2, 4, 6, 10, 25 degrees above the horizon, 360 degrees in azimuth
    • RHIs: 0, 30, 60, 90, 120, 150 degrees azimuth, horizon-to-horizon
  • Special scans to fill out the half-hour:
    • PPIs, RHIs, Raster scans are to be chosen according to what was seen in the survey scans.

Morning

Objective: transition from offshore to onshore flow

  • Shallow (0-30 degrees in elevation) constant-azimuth scans to capture the transition from offshore flow to onshore flow. Choose angle(s) according to what was seen in the survey PPIs, or scan at 30, 60, 90, and 120 degrees azimuth.

  • High-resolution horizontal raster scans to measure horizontal variability of bay breeze and convergence zone between offshore and onshore flow. Choose an azimuth sector approximately 45° wide, according to what was seen in survey PPI s. Scan from 0-5° elevation, incrementing in elevation by 0.25°.
  • Horizontal and/or vertical raster scans from approximately 315-45 degrees azimuth (over the ship channel and the inlet to the ship channel). Try to capture windshifts over the ship channel or inlet. Scan over a smaller area if interesting features warrant.
  • If the bay breeze front is obvious, do repeated vertical slices through the front until it passes out of range. Alert OPAL crew to point vertically until after the front passes.
  • If synoptic conditions are such that a bay breeze is not likely to form, cycle through the survey scans. If interesting flows are seen, create scans to document them and intersperse these between sets of survey scans.

Afternoon

Objectives: continued evolution of bay breeze (if present), appearance of gulf breeze

  • Continue with survey scans and special scans as in the AM
  • Look for approaching gulf breeze front by executing shallow vertical scans along southwest to southeast azimuths

Other things to look for and document:

  • Merging of bay breeze and gulf breeze
  • If a return flow becomes established, do OPAL ozone measurements increase above the onshore flow layer? It is hypothesized that pollutants are transported back toward Galveston Bay in the return flow layer, sink in the subsidence region, and then get carried back onshore the next day.
  • Stagnation region over Galveston Bay
  • Look for thunderstorm outflows, and document with vertical and horizontal raster scans.

Evening

Objective: transition from onshore to offshore

  • Continue with survey scans.
  • After sunset use scans similar to those listed in the AM section, but look for the flow reversal from onshore to offshore (land breeze). The reversal will not be as sharp as the shift from offshore to onshore. OPAL ozone measurements may show an increase in O3 after the wind shift.
  • Continue looking for thunderstorm outflows, and document with vertical and horizontal raster scans.