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LCL, Cloud Base, and Wind Direction Analysis - PowerPoint PPT Presentation

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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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Things to look for
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?


  • 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.

Graphical notes
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.


  • 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.

Why this makes sense
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.

Why this makes sense1
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.

Why this makes sense2
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.

Why this makes sense3
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!


  • Does the height of the PBL vary with synoptic conditions?

    • Yes.

    • Very strong evidence from this study to support this.


  • 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.


  • 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.

Next area of study
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.

Next area of study1
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.