Best Way to Get Cloud Top Height from ISCCP HXG?

[gardne...@gmail.com]

The great folks at NOAA are hooking me up with HXG data so I can see historic cloud top height around the world. I was originally thinking that the variable "VPRS" (Vis-adjusted cloud top pressure) was the right one to use, but it seems that VPRS only contains data for daylight hours, leaving much of the globe missing. So is IPRS the right value for me to use?....should I try to blend it with VPRS data where it is available, or just use the basic IPRS data to calculate cloud top height?

Also, I use IPRS/VPRS data as index values into the pretab array to get pressure in hpa. Then I use the below formula to estimate cloud top height from cloud top pressure. The results seem reasonable, but I was hoping someone might confirm my math and approach....especially for high altitudes. 

height_ft = (Math.Pow(10, Math.Log10(pressure_hpa / 1013.25) / 5.2558797) - 1) / -6.8755856E-6

Thanks!

[Ken Knapp - NOAA Federal]

Hi

Thanks for the question.

The following are the pressure-related variables in HXG:

• iprs - "IR retrieved cloud top or surface pressure"
• vprs - "VIS adjusted cloud top pressure"
• vprsic - "VIS retrieved ice cloud top pressure"

(where the quotes are the long_name from the netCDF file.

iprs - retrieves cloud top pressure. It assumes that the cloud is opaque in the infrared. If this is wrong (that is the cloud is not opaque), then the value will be  too high (because the cloud top temperature will be contaminated by partly sensing the surface below, making the cloud warmer, placing the cloud too low, making the pressure too high). However, it is available both day and night and is consistently retrieved at both times.

vprs - Using visible data (reflected sunlight), the algorithm can determine how likely it is that the cloud is opaque in the infrared. If it is likely not opaque, then the optical depth in the IR is estimated from the visible optical depth. This new optical depth is used to calculated an adjusted cloud top pressure (where the calculation now includes contamination from the surface). This assumes that the cloud is liquid water.

vprsic - This is the same method/approach as vprs, but it assumes the cloud is sold water (i.e., ice).

So vprs and vprsic are only available during the day. They may, however, be more accurate than iprs since it uses some knowledge of the cloud to estimate the temperature (and hence the pressure). But while it may be more accurate, it is only available during the day. Also, there is some potential for other errors, e.g., when the assumed temperature of hte underlying surface is wrong, or when the cloud is very thin. 

What you use likely should depend on your application. Using this description, you should be able to determine which of the variables to use.

 I hope this helps-

-Ken

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Ken Knapp, Ph.D.

Meteorologist, Center for Weather and Climate

NOAA National Centers for Environmental Information

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