CAMEX-4 CVI Cloud Condensed Water Content

Table of Contents

Introduction

The counterflow virtual impactor (CVI) was used to measure condensed water content (liquid water or ice in particles about 8 microns in diameter and up) and cloud condensation nuclei (CCN) on the DC-8 during CAMEX-4. Water content was measured after evaporation with a dual-wavelength Lyman-alpha hygrometer.

The CVI is designed to separate cloud droplets or ice crystals and other non-volatile particles larger than about 8 mm in aerodynamic diameter from smaller particles. Once these selected larger particles enter the instrument, water is evaporated from the condensation nuclei. Residual water vapor and non-volatile then pass into sampling instruments. Water vapor is measured with a Lyman-a hygrometer (or similar instrument) and non-volatiles are examined with optical particle counters, condensation nuclei counters and/or filters for chemical analyses.

As shown in the cartoon to the right, air flows into the probe from the right side of the image (Vr). Inside the probe is a porous liner depicted by the stippled area. Around this liner is injected warmed, dry nitrogen gas (F1). This gas permeates through the porous liner and some is forced out of the tip of the probe (F3) while some is drawn through the center of the probe (F2) into the sensor instruments. The dry nitrogen gas that exits the tip of the probe (F3) displaces small particles and droplets away from the inlet. Heavier particles pass into the tip of the probe down the middle to the sensor area.

Data are in files of the form:

c4dcvi_2001.ddd_hhmmss.asc

where c4dcvi represents CAMEX4 and the CVI instrument, 2001.ddd is the four digit year and day of year, asc indicates that the file is in ASCII format.

An example of the CVI data is shown below:

Column headers are described as follows:

TIME: Time in UTC seconds (e.g., 07:00 = 25200). Times have been corrected for an approximate 2 sec lag time in the instrument, but since times were recorded on an independent laptop computer, small additional time variances may be observed relative to other DC-8 variables.

INFLAG: Flag to warn if CVI was being used as a whole-air inlet; 0 = normal, 1 = whole-air mode. Since whole-air mode typically was used only outside of cloud, condensed water content (CWCCORR) was set to 0 when INFLAG = 1. Some clear-air aerosol data may be available during these times, but they are not included in this data set.

SATFLAG: Flag to warn of saturation conditions; 0 = normal, 9 = saturated. This occurred when the inlet system became saturated with water vapor due to high condensed water contents. This manifests as a flattening of the CWCCORR signal at some upper value. Under extreme conditions, saturation sometimes extended into clear-air conditions until the system had time to dry out.

CWCCORR: Condensed water content in g m-3 at ambient conditions. Non-zero out-of-cloud baselines have been removed (to the extent possible) with a routine that slaves the CWCCORR baseline to the signal from a particle counter behind the CVI. Bad data is flagged with a 9999.

The data producer is:

Cynthia H. Twohy
College of Oceanography and Atmospheric Sciences
Oceanography Admin 104
Oregon State University
Corvallis, OR 97331-5503
twohy@oce.orst.edu

To order these data or for further information, please contact:

Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/