Field Evaluation of Real-time Cloud OD Sensor TWST during the DOE-ARM-TCAP Campaign, 2013

Description

The objective of this IRAD funded project by Aerodyne Research, Inc. was to demonstrate the field-worthiness and assess the performance of a real-time Cloud OD sensor (dubbed TWST for Three-Waveband Spectrally-agile Technique) through a side-by-side comparison with proven, ground-based operational sensors already deployed at the DOE ARM Mobile Facility (ARM) site on the Cape Cod National Seashore for the Two-Column Aerosol Project (TCAP). We anticipated direct comparisons with the AERONET (when in Cloud Mode) and SAS instruments and expected ancillary data from other sensors such as the Total Sky Imager, the Scanning Cloud Radar and the Microwave Radiometer to facilitate and validate these comparisons. Since the cloud optical depth retrieval algorithms used by AERONET, SAS and TWST are totally independent, this deployment provided a unique opportunity to evaluate the field performance of TWST. This paper reports the results of a successful deployment of TWST near the end of the TCAP campaign. TWST data were collected on thirty-seven days from 17 May to 27 June 2013. The TWST results are compared to AERONET spectral radiances and to the cloud optical depth measurements from both the AERONET Cloud Mode and Microwave Radiometer (MWR) sensors. Although TWST is based upon the same principle of operation as the Cloud Mode AERONET sensor, it offers certain key advantages in the measurement of cloud optical depth (COD). (1) High temporal resolution, 1 sec or less versus 90 sec. Although the AERONET Cloud Mode collects a full sample in 9 sec, the Cloud Mode averaging technique (referred to as a ‘trimmed mean’) requires 10 full samples to generate one data point. The TWST spectrometer which sees all spectral resolution elements simultaneously provides a valuable multiplex advantage. As a result TWST can collect samples at least one to two orders of magnitude faster without sacrificing signal-to-noise-ratio (SNR) and thereby better capture the fast evolution of cloud properties. (2) High spatial resolution, 0.5 deg versus 1.2 deg. This reduces clear sky background contamination and promotes the study of cloud edges where cloud-aerosol interactions are an important effect. (3) High spectral resolution, approximately 300 spectral resolution elements versus 6 or 8 spectral bands. The spectrometer enables the application of the oxygen A-band Equivalent Width to resolve the thick/thin cloud ambiguity posed by all radiance based cloud OD measurements.