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LCL, Cloud Base, and Wind Direction Analysis

LCL, Cloud Base, and Wind Direction Analysis. Andrew Dzambo – 6/25/2012. Topography. Things to look for…. How does the height of the PBL vary with synoptic conditions? Is wind direction a big factor in determining the LCL? Does local topography influence the height of the PBL?. Caveats.

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LCL, Cloud Base, and Wind Direction Analysis

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  1. LCL, Cloud Base, and Wind Direction Analysis Andrew Dzambo – 6/25/2012

  2. Topography

  3. Things to look for… • How does the height of the PBL vary with synoptic conditions? • Is wind direction a big factor in determining the LCL? • Does local topography influence the height of the PBL?

  4. Caveats • Ceilometer data contains days ONLY when clouds were observed. • Tower data contains ALL data (clear, cloudy, etc.). • Some graphs will show an average lower cloud base than LCL – this will be corrected in the next segment of my study.

  5. Graphical Notes • LCL / cloud base height vs. time: • Black line corresponds to LCL height. • Green points correspond to the cloud base. • LCL / cloud base height vs. wind direction: • Black line with segments corresponds to LCL height. • Green line corresponds to cloud base height. • Temperature / dew point vs. wind direction: • Red line corresponds to air temperature. • Green line corresponds to dew point.

  6. March 2011

  7. April 2011

  8. May 2011

  9. July 2011

  10. August 2011

  11. November 2011

  12. Findings • LCL is consistently highest from 0o to ~30o (wind direction) and from ~300o to 359o (WNW to ENE). • Likewise, LCL is lowest when the wind comes from the SW. • Average air pressure is lowest around the same wind direction ranges as listed above. • Highest dew points are measured between 50o and 250o, with a peak around +/- 50o of 200o.

  13. Why this makes sense… • Cold fronts and low pressure systems generally approach from the south and west directions. • These systems bring moist gulf moisture with them, as well as warmer air. • As a result, the LCL tends to remain at a steady height when the wind direction comes from one of those directions.

  14. Why this makes sense… • Air pressure during the winter and spring months observes a minimum within +/- 50o of 200o. • Cold fronts and low pressure centers from Nor’easter-type storms are generally observed from this direction. • Air pressure during the summer months, however, observes a minimum closer to 300o. • A ridge of high pressure is often found over the SE United States (e.g. the Carolinas, Georgia). • Geostrophic winds, therefore, come from the W and WNW. Low pressure systems, therefore, are more likely to come from this direction. • The thermal gradient across the Northern Hemisphere is weaker at this time as well; the observed change in air pressure from fronts is not as significant as a result.

  15. Why this makes sense… • When these low pressure systems move away… • Winter/Spring months: • High pressure quickly builds in behind these systems; geostrophic winds from high pressure systems observed from WNW to ENE. • Cold, dry air builds in. • Dew points are especially low; this is a key factor as to why the LCL is noticeably higher in these directions. • Summer /early Fall months: • A consistent high pressure system under the ridge over the SW USA remains in place. • Cooler and drier builds in from the WNW to ENE. Temperature and dew point observations compared to wind direction remain consistent through all seasons.

  16. Why this makes sense… • Looking closer at the winter months... • Temperature and dew point gradients are much larger in the winter months. • Observed differences in LCL height vs. wind direction are much more significant!

  17. Summer Example (July 11, 2011)

  18. Winter Example (January 11, 2011)

  19. Conclusions • Does the height of the PBL vary with synoptic conditions? • Yes. • Very strong evidence from this study to support this.

  20. Conclusions • Is wind direction a big factor in determining the LCL? • Yes. • Wind direction can be used as a proxy for the type of air to be advected (cold/dry vs. warm/moist). • The LCL is consistently higher (across all seasons) when the wind comes from the north – the air is often very dry from this direction.

  21. Conclusions • Does local topography influence the height of the PBL? • Inconclusive. • Given the industrial complex to the west, advected air from that direction should create a noticeably higher LCL (air is warmer and drier compared to the other local topography). • This is not the case, however; low LCLs are observed from the west.

  22. Next Area of Study • In order to study how different surfaces affect the LCL, an additional filter to the data must be applied: • Remove cloudy days from the data (where cloud base is within 1 standard deviation of the LCL height). • This will remove all large-scale weather events; remaining data will primarily be that of clear weather.

  23. Next Area of Study • Compare tower data to ceilometer data: • How close are the cloud base measurements? • Calculate the surface mixing ratio from the ceilometer data given the surface air temperature. • The water vapor deficit will indicate how close the measurements from each measuring device is.

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