TITLE: CAPS - Cloud Aerosol and Precipitation Spectrometer

AUTHOR:

      Darrel Baumgardner
      UNAM/CCA, Ciudad Universitaria, Mexico City, Mexico, DF, 04510, (52) 555 622 4067
      darrel.baumgardner@gmail.com

1.0 DATA SET OVERVIEW:
	
      The DMT CAPS was located on the left pod of the C-130 in the 6 o'clock position
      Dates available: 	20071113, 20071116, 20071118, 20071120, 20071129, 20071130, 20071204,
				20071210, 20071211, 20071213, 20071216
	Filenames:	CAPSyyyymmdd.dat, where yyyymmdd is the day of the research flight

2.0 INSTRUMENT DESCRIPTION:

      The CAPS measures the size distribution of cloud particles from 0.5 to 1600 um using
      an optical particle spectrometer (CAS) that measures light scattering from individual particles
      that pass through a focused laser over the size range from 0.5 to 50 um and an optical array probe,
      a grayscale cloud imaging probe (CIP)with 15 um resolution that measures from 15-1500 um.  

3.0 DATA COLLECTION AND PROCESSING:

      The CAPS generates four serial data streams, two from the CAS and two from the CIP. The CAS and CIP
      generate histograms once per second that give the number of particles detected in difference size
      categories, 30 channels for the CAS and 62 for the CIP. In addition, the CAS transmits the scattering
      intensity and arrival time particle by particle each second and the CIP transmits the particle images
      that are packed in a data buffer that is transmitted asychronously.

	The four data streams are processed off line and from them a new histogram is constructed with
	29 size channels from 5 to 1500 um. Each channel represents the number concentration in units of
	number per liter for each size channel.

	In addition, bulk parameters are calculated, i.e. total concentration, ice water content, extinction
	coefficient and median volume diameter. These are all derived from the size distribution assuming
	spherical ice particle with a density of 0.9 gcm-3.


4.0 DATA FORMAT:

The Format of the ASCII (text)data files are:

UTCseconds IWC Concentration Extinction MVD PSD (29 channels)

UTCseconds:		Seconds from midnight
IWC:			Ice water content (mg/m3), assuming spherical particls, density of 0.9g/cm3, derived
			from CASand CIP
Concentration:	#/liter, derived from CAS and CIP
Extinction		km-1, derived from CAS and CIP
MVD			median volume diameter derived from combined CAS and CIP
PSD			Particle size distribution (#/L) where each column is labeled by the upper threshold
			of the size bin

The size derived from the CIP is the water equivalent diameter
Corrections have been made for out-of-focus particles using the Korolev algorithm
Corrections have been made for shatter in the CIP by filtering short arrival times.
No corrections have been made to the CAS.


5.0 DATA REMARKS:

      The estimated accuracy in derived number concentrations is +/- 15% and in size +/-20%. The derived
	IWC accuracy is +/-50%, where the largest uncertainty is a result of not knowing the volume of ice
	crystals as well as the 20% uncertainty in size. The uncertainty in the extinction coefficient is
	of the same order as the IWC.

	When the ice crystal concentrations in particles larger than 100 um exceed approximately 10/liter,
	the uncertainty in the number concentration for particles smaller than 50 um will increase; however,
	this has not be either qualified or quantified and work is in progress to decrease these uncertainties.

6.0 REFERENCES:

	Baumgardner, D., H. Jonsson, W. Dawson, D. O’Connor and R. Newton, 2001: The cloud, aerosol and precipitation spectrometer (CAPS):
	 A new instrument for cloud investigations, Atmos. Res., 59-60, 251-264.