NAMMA Counterflow Virtual Impactor (CVI)

Table of Contents

Introduction

The counterflow virtual impactor (CVI) was used to measure condensed water content (liquid water and/or ice) on the DC-8 during NAMMA. The cloud particle size range sampled was about 5 microns aerodynamic diameter and up at high altitude and 8 microns and up at low altitude.

These data files were generated during support of the NASA African Monsoon Multidisciplinary Analyses (NAMMA) campaign, a field research investigation sponsored by the Science Mission Directorate of the National Aeronautics and Space Administration (NASA). This mission was based in the Cape Verde Islands, 350 miles off the coast of Senegal in west Africa. Commencing in August 2006, NASA scientists employed surface observation networks and aircraft to characterize the evolution and structure of African Easterly Waves (AEWs) and Mesoscale Convective Systems over continental western Africa, and their associated impacts on regional water and energy budgets. For more information about the NAMMA Campaign, go to the NAMMA web site: http://namma.nsstc.nasa.gov/

The CVI is designed to separate cloud droplets or ice crystals from smaller particles. Once these selected larger particles enter the instrument, water is evaporated from the condensation nuclei. Residual water vapor and non-volatile particles then pass into sampling instruments. Water vapor is measured with a MayComm Tunable Diode Laser (TDL) hygrometer and non-volatile particles are examined with an optical particle counter, a condensation nuclei counter and an impactor for subsequent chemical analyses.

Data are in files of the form:

NAMMA_CVI_yyyymmdd_CWC.txt

where,

NAMMA_CVI  - Represents the NAMMA project and the CVI instrument
yyyymmdd  - Is the year, month and date of the flight
CWC - Indicates a condensed water content file
txt  - Indicates that the file is in tab-delimited text format

An example of the CVI data is shown in figure1 below:

Figure 1

Column headers are described as follows:

TIME: 

Time in UTC seconds (e.g., 07:00 = 25200).

CVCWC: 

Condensed water content in g m-3 at ambient conditions. Pressure-dependent calibrations with N2 have been applied. CVCWC missing data flag of -9999 is used whenever sampling ambient air (INLET=1). Ambient sampling occurred often but with rare exceptions, was only used in clear-air (CVCWC ~0.0). In heavy cloud, CVI occasionally became saturated, which is manifested by a flattening of the signal at high values (usually >1.5 g m-3), with a subsequent large "hysteresis" signal.

INLET:

Flag to warn if CVI was being used as a whole-air inlet; 0 = normal CVI mode, 1= whole-air, ambient mode. Some aerosol data may be available during these times, but they are not included in this data set.

CVRAD:

CVI 50ut size (aerodynamic) in microns radius.

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.orstcoas.oregonstate.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/