ARM TR-020

> Vaisala Ceilometer (Model CT25K) Handbook
>
> November 2004
>
> C. Flynn
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 3](#general-overview)

[2. Contacts 3](#contacts)

[3. Deployment Locations and History
3](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 3](#near-real-time-data-plots)

[5. Data Description and Examples 3](#data-description-and-examples)

[6. Data Quality 3](#data-quality)

[7. Instrument Details 3](#instrument-details)

**Figures**

[1. 3](#__RefHeading___Toc86805311)

[2. 3](#__RefHeading___Toc86805312)

[3. 3](#__RefHeading___Toc86805313)

**Tables**

#  {#section .unnumbered}

[1. 3](#__RefHeading___Toc86805281)

[2. 3](#__RefHeading___Toc86805282)

# General Overview

The Vaisala ceilometer (VCEIL) is a self-contained, ground-based,
active, remote-sensing device designed to measure cloud-base height at
up to three levels and potential backscatter signals by aerosols. Model
CT25K has a maximum vertical range of 25,000. The ceilometer transmits
near-infrared pulses of light, and the receiver telescope detects the
light scattered back by clouds and precipitation.

# Contacts

## Mentor

Connor Flynn\
Pacific Northwest National Laboratory\
Phone: 509-375-2041

Fax: 509-375-4545\
<connor.flynn@pnl.gov>

## Instrument Developer

Vaisala Corporation\
Phone: 617-933-4500 (Distributor)

**Primary Contacts**:\
Selwyn Alpert\
Phone: 617-933-4500, ext. 228 (Sales)\
John Serveaies\
Phone: 617-933-4500, ext. 266 (Engineering)

**For instrument repair and maintenance**:\
Mike Mcgee\
Phone: 800-327-2967

[www.vaisala.com](http://www.vaisala.com/)

# Deployment Locations and History 

The instrument is used at all Tropical Western Pacific, North Slope of
Alaska sites and the boundary facilities at the Southern Great Plains
(SGP). In the summer of 2000, it replaced the [Belfort Laser
Ceilometer](http://www.arm.gov/instruments/static/blc.html) at the SGP
in the summer of 2000.

# Near-Real-Time Data Plots

See [General Quick
Looks](http://www.nsdl.arm.gov/Visualization/vceil/frame.htm).

# Data Description and Examples

The figure below shows an example of the output analysis software
package available for the Vaisala Ceilometers (CTVIEW). This particular
example shows a false color intensity depiction of the backscattering
profile for 28 September 1995. These data were taken at Sandia National
Laboratory in Albuquerque, New Mexico during the early phase testing for
the Atmospheric Radiation and Cloud Station (ARCS) for the Tropical
Western Pacific (TWP).

![](media/image1.png){width="3.4305555555555554in"
height="2.8159722222222223in"}

[]{#__RefHeading___Toc86805311 .anchor}Figure 1.

## Data File Contents

### Primary Variables and Expected Uncertainty

The Vaisala ceilometers measure the backscattered light intensity from a
pulsed InGaAs diode laser (905 nm) as a function of distance (15-m
resolution CT25K). These measurements are used to produce derived
products that are recorded. These products include:

1.  The backscatter profile with 15-m resolution.

2.  The cloud-bottom height determined with an algorithm to define cloud
    bottom as the height corresponding to a visibility reduction to 100
    m.

3.  Secondary cloud-bottom heights from a cloud above the lowest cloud.

4.  Tertiary cloud-bottom heights from an even higher cloud.

#### Definition of Uncertainty

The CT25K is recalibrated every year at the Tropical Western Pacific
site to ensure a range resolution of 15 m. The instrument derived
product of cloud-ceiling height may drift with time if the relative
sensitivity of the instrument degrades considerably. In the worst case,
this change would not be observed until differences with the Micro-Pulse
lidar (MPL) are observed. This would lead to an uncertainty equal to the
range resolution of the MPL (300 m for older systems, 75 m for newer).

A comparison was performed between the [Belfort Laser
Ceilometer](http://www.arm.gov/instruments/static/blc.html) (BLC) and
the Vaisala 25K at the SGP in September and October 1997. This
comparison showed a similar shape to cloud features observed by the two
systems with the BLC offset about 100 to 120 m higher than the Vaisala
25K. Below is an example of the comparison using ncexplorer of lowest
cloud heights from the two instruments on one of the 9 days.

![](media/image2.png){width="4.604166666666667in"
height="7.197916666666667in"}

[]{#__RefHeading___Toc86805312 .anchor}Figure 2.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

Cloud base heights above 7620 m or below 0 m.

Additional information may be found at [VCEIL25k Data Object Design
Changes](http://science.arm.gov/tool/dod/showdod.php?Inst=vceil25k) for
ARM netCDF file header descriptions.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

**What is the difference between cloud height determination algorithms
using the ceilometer and the Micro-Pulse Lidar (MPL)?**

The MPL uses a threshold variation to identify the cloud bottom, and the
ceilometers use a calculated vertical visibility threshold of 100 m.
This means that the ceilometer will not classify thin cloud regions that
the MPL would identify and usually give a slightly higher cloud bottom
height.

**Can direct sun damage the optics and detection systems of a
ceilometer?**

For the Tropical Western Pacific, because it is near the equator and
direct sun focusing is difficult to avoid, the CT25K ceilometer comes
with a narrow band window filter that excludes enough sunlight to avoid
optics damage. At the Southern Great Plains and NSA, this filter window
is not necessary as long as the instrument is pointed with the window
opening pointing north to avoid direct sun.

**Are the ceilometers eyesafe?**

Yes, however the outgoing beam must never be viewed through magnifying
optics such as binoculars or a camera.

**Can the CT25K ceilometer returns be used to detect aerosol mixed layer
depths in the absence of clouds?**

Not yet, the CT25K presently processes out the raw data that would be
needed to do this. We are working with the manufacturer to possibly make
hardware and software changes that would allow this.

**How does the CT25K compare with other ceilometers during low cloud
conditions such as are often observed in the Arctic?**

The CT25K uses overlapping transmitting and receiving optics so that
beam overlap occurs at shorter distances so that detection of thin
clouds only about 15-50 m above the ceilometer is improved.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by ARM\'s data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

QC frequency: Several days each month

QC delay: Not specified.

QC type: Comparison to MPL and MWR measurements; data flags; graphical
plots.

Inputs: Raw data files; maintenance logs

Outputs: Report to site scientists

Reference: N/A

VCEILs have been installed at the SGP, TWP and NSA for long-term,
continuous operation. The most useful quality control check for the
ceilometers is comparison with MPL measurements when one is located
nearby. The MPL uses photon counting rather than photocurrent detection,
and the ceilometer defines a cloud as a cloud droplet scattering that
reduces visibility to a pilot to less than 100 m rather than the MPL
procedure of identifying a sudden increase in backscatter. The increased
sensitivity and looser definition of a cloud causes the MPL to often
report clouds that are not reported by the ceilometer. However, both
instruments should normally report clouds and cloud bases that
correspond closely. Another quality check involves comparing clouds
detected by the ceilometer and liquid water measurement from the MWR.
This only works for relatively low clouds where one is certain that the
clouds are water and not ice clouds.

Instrument mentor Bill Porch recommends that these comparisons with MPL
and MWR data should be accomplished for sample periods of a few days for
each month. The rest of the data will be examined for internal
consistency, namely whether clouds are observed at heights up to the
limit of the system (7.62 km), the backscatter plot show expected
variability, data gaps are minimal, and warning flags such as window
dirt, dew, and frost minimal.

Once per 6 months or so, a calibration check will be performed that
requires the instrument to be manually tipped to near horizontal and
aimed at an object a know distance away. The results of this test will
be logged as a part of maintenance. A summary report will periodically
be provided to the SGP site scientist team.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into \"value-added\"
products or VAPs. Despite extensive instrumentation deployed at the ARM
CART sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not in order to fill unmet
measurement needs, but instead to improve the quality of existing
measurements. In addition, when more than one measurement is available,
ARM also produces \"best estimate\" VAPs. A special class of VAP called
a Quality Measurement Experiment (QME) does not output geophysical
parameters of scientific interest. Rather, a QME adds value to the input
datastreams by providing for continuous assessment of the quality of the
input data based on internal consistency checks, comparisons between
independent similar measurements, or comparisons between measurement
with modeled results, and so forth. For more information see [VAPs and
QMEs](http://www.arm.gov/data/vaps.stm).

The most useful quality measurement experiment for the ceilometer is
comparison of cloud heights detected with a micro-pulse lidar (MPL) and
the ceilometer. However, it must be kept in mind that the MPL can detect
backscatter from these thin clouds that is not discernible from the
processed ceilometer backscatter. One reason for this includes the fact
that the ceilometer is designed to detect clouds that obscure pilot
visibilities to less than 100 m and is just not as sensitive as the MPL
(photo-current vs. photon counting detection, respectively). Also, there
is a change in daytime/nighttime sensitivity in the data. The ceilometer
reports thin mid-level clouds at night that are not reported after
sunrise. We have taken a corrective action by recording the processed
data without the Vaisala noise reduction. However, the ceilometer will
still not report the daytime thin clouds as clouds. The prevalence of
thin scud type clouds at Manus, New Quinea makes this more of a problem
there than at other sites, but thin mid-level clouds can occur anywhere.
When there are both MPL and Ceilometer data the most sensitivity to thin
clouds will always come from the MPL. The combination of the MPL and the
ceilometer as a relatively low cost back-up is still valuable not only
in maintaining a continuous data stream, but also the higher resolution
of the ceilometer at Manus (TWP does not yet have the MPLHR) is useful
for lower level cloud detection and possibly cloud-free mixing-depth
estimates under some circumstances. 2. Another quality measurement
experiment can be performed by comparing the ceilometer cloud base
height and the calculated lifting condensation level from surface
humidity and temperature measurements. The lifting condensation level
temperature can be calculated as Tl = 1/\[(1/(Tk-55)) - ln(U/100)/2840\]
where Tk is the absolute surface temperature ( K ) and U is the surface
relative humidity (%). The height associated with the lifting
condensation level can be calculated from an adiabatic lapse rate
assumption.

# Instrument Details

## Detailed Description

### List of Components

The system components consist of the following line replaceable units:

[]{#__RefHeading___Toc86805281 .anchor}Table 1.

  ------------------------------------------- ---------------------------
  **Component**                               **Item \#**

  CPU                                         DMC 50A

  Transmitter                                 CTT21

  Assembly                                    \-

  Receiver Board                              CTR21

  Blower Assembly                             CT2688

  DC Converter                                DPS51

  Power Cable                                 CT3839

  Data Cable                                  CT3838

  Backup Battery                              4592
  ------------------------------------------- ---------------------------

The separate mechanical parts include the following:

1.  Ceilometer Measurement Unit

2.  Shield

3.  Pedestal

4.  Maintenance Cable (1 meter with 9 pin D connector)

5.  Data Cables (3 ft. with 9 pin D connector) CAB-000045, 50 ft.
    Twisted Pair RS422 CAB-000086, 50 ft. 12V power for RS422 Converter
    CAB-000087, and 10 ft. 9 pin RS232 cable to Computer in IVan
    CAB-000085)

6.  Power Cable (50 ft.) CA-000044

7.  Maintenance Terminal (Psion 3a)

8.  Computer (connection for data terminal running OS2)

9.  RS232 t0 RS422 and RS422 to RS232 converter boxes.

10. 4 Mounting Bolts and Hardware for Securing Pedestal to Cement Pad (4
    nuts M10, washers B10, and foundation screws M10).

11. Spare Parts ( spare connector cover, instrument door key, and
    manuals).

We do not presently stock individual system components as spares. This
may change when replacement technical manuals become available.
Presently, the only spare parts for the system include extra connectors
for external cable wiring, two Psion remote terminals for data capture
at the instrument, two null modem cables and two sets of keys for the
instrument door access.

### System Configuration and Measurement Methods

The CT25K Ceilometers measure cloud-bottom heights and vertical
visibilities. These instruments employ pulsed diode laser LIDAR (Light
Detection and Ranging) technology, where short, powerful laser pulses
are sent out in a vertical or slant direction. The directly
backscattered light caused by haze, fog, mist, virga, precipitation and
clouds is measured as the laser pulses traverse the sky. This is an
elastic backscatter system and the return signal is measured at the same
wavelength as the transmitted beam. The Raman Lidar looks at other
wavelengths than those transmitted. The ceilometer backscatter profile,
i.e., signal strength versus height, is stored and the cloud bases are
detected. Knowing the speed of light (3 x 10^8^ m/s it\'s not just a
good idea, it\'s the law), the time delay between the launch of the
laser pulse, and the detection of the backscatter signal gives the
cloud-base height (see Theory of Operation, Section 7.2).

The CT25K is able to detect three cloud layers simultaneously. Besides
cloud layers, it can detect whether there is precipitation or other
obstructions to vision. The embedded software includes several service
and maintenance functions and gives continuous status information from
internal monitoring.

The instruments are placed on a foundation. The foundation should be a
concrete pad at least 200-mm thick (the hole depth for the mounting
bolts is 160 mm). The width should be 500 mm or larger (the whole
spacing is a square pattern 283 mm on a side). The orientation of the
pad ordinarily is made so that the one side of the pad points in a
north-south direction. Because the Tropical Western Pacific site is near
the equator, this layout is not as important as orienting the pad so
that the instrument can be calibrated by tilting it near the horizon to
hit a large object at least 200 m away for calibration (the ceilometer
only tips to the horizon one way which is clockwise facing the tilting
flange). Also, the 50-ft cables require that the pad be placed less than
30 ft from the cable opening of the instrument van containing the
computer. The ceilometer must be at least 5 feet from the instrument van
to allow tipping on both sides.

### Specifications

Range: 7.5 km

Range resolution: 15 m

Wavelength: 905 nm @ 25 oC

Transmitter: Pulsed mode energy 1.6 microWatts +/- 5% Indium Gallium
Arsenide

Receiver: Silicon APD Response @ 905 nm = 65 Amps/Watt 50% Pass = 35 nm
@ 890 - 925 nm

Field of View: Divergence = +/- 0.66 mRadian

Optics: Focal length 377 mm, Lens diameter 145 mm, Transmittance 96%
lens, Window 98%

Size: 672 x 308 x 244 mm (without stand)

Weight: 16 kg (without stand)

Power input 100/115/230 VAC (about 430 Watts)

## Theory of Operation

**Basic Principle of Operation**

The operating principle of the CT25K ceilometer is based on measurement
of the time needed for a short pulse of light to traverse the atmosphere
from the transmitter of the ceilometer to a backscattering cloud base
and back to the receiver of the ceilometer. With the speed of light
being:

c = 2.9929 x 108 m/s (= 186,010 miles per second)

A reflection from 25,000 ft will be seen by the receiver after t = 50.9
us.

The general expression connecting time delay (t) and backscattering
height (h) is h = ct/2, where c is the speed of light.

**Practical Measurement Signal**

Generally, particles at all heights backscatter light, and so the actual
return signal may look like that shown in the figure below.

![](media/image3.png){width="4.489583333333333in"
height="1.4166666666666667in"}

[]{#__RefHeading___Toc86805313 .anchor}Figure 3.

The instantaneous magnitude of the return signal will provide
information on the backscatter properties of the atmosphere at a certain
height. From the return signal, information about fog and precipitation,
as well as cloud, can be derived. Since fog and precipitation attenuate
the light pulse, the cloud base signal will appear lower in magnitude in
the return echo. However, the fog and precipitation information also
provides data for estimating this attenuation and computing the
necessary compensation, up to a limit. In its normal full-range
operation, the CT25K ceilometer digitally samples the return signal
every 100 ns from O to 50 us, providing a spatial resolution of 50 feet
from ground to 25,000 feet distance. This resolution is adequate for
measuring the atmosphere, since visibility in the densest clouds is in
the order of 50 feet.

**Noise Cancellation**

For safety and economic reasons, the laser power used is so low that the
noise of the ambient light exceeds the backscattered signal. To overcome
this, a large number of laser pulses are used, and the return signals
are summed. The desired signal will be multiplied by the number of
pulses, whereas the noise, being random, will partially cancel itself.
The degree of cancellation for white (Gaussian) noise equals the square
root of the number of samples; thus, the resulting signal to noise ratio
improvement will be equal to the square root of the number of samples.
However, this processing gain cannot be extended ad infinitum since the
environment changes. For example, clouds move.

**Height Normalization**

Assuming a clear atmosphere, it can be seen that the power is inversely
proportional to the square of the distance or height i.e., the strength
of a signal from 10,000 ft is generally one hundredth of that from 1,000
ft. The height square dependence is eliminated by multiplying the value
measured with the square of the height (height normalization). However,
noise, being height independent from a measurement point of view, will
then be correspondingly accentuated with increasing height.

**The Backscatter Coefficient**

The volume backscatter coefficient, b(z), represents the portion of
light which is reflected back towards the ceilometer from a distance z
(e.g., by water droplets). It is obvious that the denser a cloud is, the
stronger the reflection will be. The relationship can be expressed as:
b(z) = k\*S(z)

where

k is a \"constant\" of proportionality.

S(z) is the extinction coefficient (i.e., the attenuation factor in a
forward direction).

The extinction coefficient relates to visibility in a straightforward
manner. If visibility is defined according to a 5 % contrast threshold
(World Meteorological Organization definition for Meteorological Optical
Range (MOR), equals daylight horizontal visibility), then S = 3 / V

where

S is the extinction coefficient\
V is MOR visibility (5 % contrast)

The \"constant\" of proportionality, k, also called the Lidar Ratio, has
been subjected to a lot of research. Although the Lidar Equation can be
solved without knowing its value, it must remain constant with height if
accurate estimates of the extinction (or visibility) profile are to be
made.

It has been found that in many cases, k can be assumed to equal 0.03,
tending to be lower in high humidities, to 0.02; and higher in low
humidities, to 0.05. However, in e.g. precipitation of various kinds, k
will have a wider range of values. Assuming a value 0.03 (srad-1) for k
and visibility in clouds being in the range 15\...150 m (50\...500 ft)
gives the range of value for B:

B = 0.0006\...0.006 m-1srad-1\' = 0.6\...6 km-1srad-1

**Extinction Normalization and Vertical Visibility**

Any fog, precipitation, or similar obstruction to vision between ground
and cloud base may attenuate the cloud base signal and produce
backscatter peaks that far exceed that from the cloud. Virtually any
backscatter height profile is possible, up to some physical limits. To
distinguish a significant cloud return signal, the attenuation of fog,
precipitation, etc., has to be taken into account by normalizing with
regard to extinction. The profile thus obtained is proportional to the
extinction coefficient at various heights, and enables the use of fairly
straightforward threshold criteria to determine what is cloud and what
is not.

By assuming a linear relationship between backscatter and extinction
coefficient according to the previous formula and that the ratio, k, is
constant over the range observed, it is possible to obtain an extinction
coefficient profile through a mathematical computation. This is also
called inverting the backscatter profile to obtain the extinction
coefficient profile, and answers the question, \"What kind of extinction
coefficient profile would produce the backscatter profile measured?\"

No assumption as to the absolute value of the ratio, k, needs to be made
if k is constant with height. The assumptions that have to be made are
fairly truthful, and in any case accurate enough for the purpose of
cloud detection. Likewise, the inversion is also independent of several
instrumental uncertainties including transmitted power and receiver
sensitivity.

An estimate of Vertical Visibility can easily be calculated from the
extinction coefficient profile because of the straightforward extinction
coefficient to visibility relationship, provided that a constant
contrast threshold is assumed. Visibility will simply be that height
where the integral of the extinction coefficient profile, starting from
ground, equals the natural logarithm of the contrast threshold, sign
disregarded.

Tests and research have, however, shown that the 5% contrast threshold
widely used for horizontal measurement is unsuitable for vertical
measurement if values close to those estimated by a ground-based
observer are to be obtained. The CT25K uses a contrast threshold value
which, through many tests, has been found to give Vertical Visibility
values closest to those reported by ground-based human observers. A wide
safety margin is obtained with regard to pilots looking down in the same
conditions since the contrast objects, especially runway lights, are
much more distinct on the ground.

## Calibration

### Theory

The calibration procedure is based on the ability of the instrument to
tip parallel to the ground. Comparison with MPL heights during low cloud
situations can also be informative. The calibration of the CT25K is
performed by tilting the instrument horizontally and detecting the
return from a solid object of known distance more than 100 m from the
ceilometer.

### Procedures

This section is not applicable to this instrument.

### History

[]{#__RefHeading___Toc86805282 .anchor}Table 2.

  ----------------- --------------- ------------- --------- ----------- ---------
  **ARCS            **Field         **CAL Record                        
  Calibration       Calibration**   CAL (CEIL)-                         
  Form**                            0001**                              

  Date:             GMT Begin Time: GMT End Time:                       

  9/12/95           21:00           23:00                               

  Instrument/       Ceilometer                                          
  System:                                                               

  Mfg or TWP Model  CT25K                                               
  or Part No.:                                                          

  Serial Number:    p0110009                                            

  Location (Example \-                                                  
  PNNL):                                                                

  Participants:     Sandia                                              

  Author            W. Porch                                            
                    (Originator)                                        

  Initials:         WMP                                                 

  II\. Initial                                                          
  Values                                                                

  Reference         Reading         Reference     Reading   Reference   Reading
  Distance m                                                            

  525               525             345           345       135         135

  III\. Final                                                           
  Values                                                                

  Reference         Reading         Reference     Reading   Reference   Reading
  Distance m                                                            

  525               525             345           345       135         135

  IV\. Calibration                                                      
  Change (if                                                            
  applicable)                                                           

  Original          None                                                
  Sensitivity                                                           
  (Distance                                                             
  Correction                                                            
  Factor):                                                              

  V. Documents      Calibration                                         
  Referenced:       Procedure                                           
                    PRO(CEI) -                                          
                    002.000                                             

  Documents                                                             
  Updated:                                                              

  Problems:         None                                                
  ----------------- --------------- ------------- --------- ----------- ---------

FM(CAL)001.00

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

This section is not applicable to this instrument.

### Software Documentation

ARM netCDF file header descriptions may be found at [VCEIL 25k Data
Object Design
Changes](http://science.arm.gov/tool/dod/showdod.php?Inst=vceil25k).

### Additional Documentation

A.  Calibrations and Related Performance Checks:

    1.  What are factory recommended calibration procedures? There are
        two calibration procedures used to verify and/or optimize
        performance of the CT25K ceilometers. Factory calibrations
        include testing output and received laser power, optical
        alignment, and pulse timing electronics. The results of these
        tests are available from the instrument microprocessor by typing
        \"GET FACTORY\" and \"GET STATUS\".

    2.  What are the factories recommended performance checks? All
        performance checks of this instrument are automatic. The
        \"instrument health\" data appears in the instrument output data
        files.

    3.  What are the mentor calibration procedures? The mentor (or the
        RESET team in the case of the TWP sites) calibrates the
        ceilometers by tipping the instrument to a near horizontal
        position and aims the beam at a known object more than 100 m
        distant.

    4.  What are the mentor performance checks? The instrument mentor or
        site operator checks the \"instrument health\" data in the
        output data files to diagnose a possible instrument malfunction.
        The \"instrument health\" data are of sufficient detail to
        pinpoint a failing or failed component or subsystem.

    5.  How are calibration and related performance checks documented?
        Performance checks of CT25K systems are documented in the TWP
        Site Data Log by the TWP weather observers. Field calibrations
        are logged on a ceilometer field calibration form (see example
        in Calibration History, Section 7.3.3).

        a.  Where are procedures documented? Performance checks and
            calibration of ceilometer operation are documented in TWP
            calibration procedures and a CT25K user's manual translation
            written for the TWP.

        b.  Have major changes to calibration procedures occurred? If
            so, for which components and when? To our knowledge, Vaisala
            has not issued any calibration procedure changes since the
            instrument was purchased and installed in 1996.

        c.  Are major changes to calibration procedures expected to
            occur? If so, for which components and when? No major
            changes to calibration procedures are anticipated.

    6.  Who implements (mentor) calibration and performance checks?
        Mentor: The mentor implemented field calibration at the TWP ARCS
        Integration Site at Sandia Natl\' Lab, Albuquerque, New Mexico.
        The RESET team will perform instrument performance checks and
        instrument calibration at the ARCS site in the TWP. Site Ops:
        Site ops personnel will support the instrument mentor in
        instrument performance checks and calibration procedures at the
        SGP.

    7.  What is standard schedule of calibrations and checks? The
        instrument mentor recommends instrument calibrations be
        performed upon delivery of the instrument and every 6-months
        thereafter.

    8.  How are the calibration and check procedures initiated
        (queued)?\
        Scheduled Calendar Event: These are to be performed at each
        RESET visit\
        Work Order: Routine by RESET\
        Data Inspection: Routine by Weather Observers at TWP and
        remotely by mentor and site scientist\
        Instrument Failure: CT25K performance checks are generally
        initiated upon observation of instrument malfunction or a
        perceived degradation in instrument performance.

    9.  How long does it take to perform calibration and performance
        check procedures? Performance checks require 1 data cycle (every
        15 seconds) CT25K calibration requires 2 hours in the field.

    10. Are any data affected or lost during calibration or performance
        check procedures? No data are lost during instrument performance
        checks because the performance data appear in each data packet.
        Since CT25K calibration requires placement of the tilting of the
        ceilometer, cloud base height data cannot be acquired during
        this procedure.

    11. What are corrective procedures when calibrations and or
        performance checks fall behind schedule? No corrective
        procedures have been implemented.

B.  Calibration Data:

    1.  Where are calibration data documented?\
        Site Data System: No\
        Site Ops Data base - Hard Copy: No\
        Site Ops Data base - Electronic Copy: No\
        Instrument Mentor - Hard Copy: Yes\
        Instrument Mentor - Electronic Copy: Yes\
        Data Logger: No\
        netCDF file: No\
        Special Archive Database: Yes (TWP calibration records)\
        Special Databases accessible via the WWW: No\
        Other: In the CT25K lidar unit (EEPROM) which is accessible by
        the portable maintenance terminal (PSION handheld or laptop
        computer).

    2.  Where are calibration coefficients and algorithms applied to
        convert data to geophysical units? Conversion of raw lidar data
        to cloud base height and visibility occurs in the CT25K
        transmitter/receiver module. These data may be accessed via
        modem or local area network.

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C.  Maintenance Procedures:

    1.  What are the factory recommended maintenance procedures:
        (preventive and corrective)? The ceilometer transmitter and
        receiver windows should be cleaned at least every three weeks
        using Windex or Glass-Plus.

    2.  What are the mentor preventative and corrective maintenance
        procedures? There are no documented preventative maintenance
        procedures. Corrective maintenance is performed at the
        component-level (i.e., by replacing the circuit board or
        component containing the defective component(s)).

    3.  How are maintenance procedures documented?

        a.  Where are procedures documented? Maintenance procedures are
            documented in the CT25K Users guide manual and Manual
            translation for the TWP.

        b.  Have major changes to maintenance procedures occurred? If
            so, for which components and when? No significant changes
            have been made to instrument maintenance procedures since
            the CT25K was purchased and delivered in 1996.

        c.  Are major changes to maintenance procedures expected to
            occur? If so, for which components and when? No major
            changes to maintenance procedures are anticipated.

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4.  What is the procedure schedule? Part of ARCS daily rounds.

5.  How are the procedures initiated (queued)?\
    Scheduled Calendar Event: Maintenance schedule and RESET visits\
    Work Order: No\
    Data Inspection: Yes\
    Instrument Failure: Instrument maintenance procedures are initiated
    either by instrument failure or by a perceived degradation in
    instrument performance.

6.  How long does it take to perform maintenance procedure? Variable,
    1-8 hours.

7.  Are any data affected or lost during maintenance procedure? CT25K
    data are unavailable during performance of maintenance procedures.

8.  How are potential affects to data documented? The time period during
    which the instrument was shut down for maintenance will be
    documented in the Site Operations Log.

9.  What are corrective procedures when maintenance falls behind
    schedule? The only regular, periodic maintenance required for the
    CT25K is cleaning of the transmitter and receiver windows. If the
    windows are overly dirty, this will be noticed by one of the Weather
    Observers and corrected on the spot.

10. Where is actual maintenance work documented? In the Site Operations
    Log.

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D.  Data Integrity and Quality Inspections:

    1.  What nodes or activities along the data pipeline affect (or can
        potentially affect) the data stream? Controller Boxes: No\
        Microprocessors: Yes\
        Data Logger: No\
        Communication lines/links: Yes\
        Calibration Data files: No\
        Ingest Modules: Yes.

    2.  What are current difficulties? We have found that the CT25K is
        having difficulty detecting thin clouds during the day between 2
        and 7 km because of the bright shy in the tropics and Vaisala\'s
        nise reduction algorithm at the microprocessor level. We are
        turning off the noise reduction algorithm.

    3.  List and describe any standard or non-standard data inspections
        (active or planned) under each of the following categories:\
        Data Existence check: This check is performed during and after
        ingest by the Experiment Center at PNNL. This check is also
        performed periodically by Jim Mather at the Pennsylvania State
        University.\
        Mentor QC checks (during ingest): This check is the
        responsibility of the instrument mentor but has not yet been
        implemented on a regular basis.\
        Mentor QC checks (outside of ingest): Yes\
        Within Platform Check: No\
        Multiple Platform Check: No\
        Other automated netCDF file checks: No\
        Other analytic tools or algorithms: No.

    4.  Does storage media exist on the instrument system to back up
        data and store it for delayed data ingest? Please identify media
        and the maximum period of time that the data can be backed up on
        the media: Data may be temporarily stored on the hard disk of
        the CT25K Linebacker Computer located in the TWP instrument van
        Trailer. The maximum amount of data which can be backed up on
        this laptop computer is about 1 month.

## Glossary

Ceilometer - an instrument for observing the height of cloud bottoms
that degrade horizontal visibility for aircraft to less than 100 m.

Lidar - an instrument that uses laser ranging and detection to determine
the direct backscattering profile in the atmosphere

um - a micrometer (10-6 m)

Also see the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

LIDAR: Light Detection and Ranging

MPL: Micro-Pulse Lidar

MWR: MicroWave Radiometer

SGP: Southern Great Plains

TWP: Tropical Western Pacific

WSI: Whole-Sky Imager

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Lonnqvist, J. 1995: Experiences with a Novel Single-lens Cloud Height
Lidar. *Preprints, Ninth Symposium on Meteorological Observations and
Instrumentation*, March 27-31, 1995, Charlotte, NC, Amer. Meteorol.
Soc., Boston, MA, pp. 106-109.