Preview of TWP MFRSR Data

Chuang, C., Lawrence Livermore National Laboratory

Radiation Processes

Radiative Processes

N/A


Figure 1. MFRSR data from the TWP site (97/09/10)


Figure 2. Aerosol optical depth and Angstrom exponent


Figure 3. Water vapor derived from MFRSR and MWR


Figure 1. MFRSR data from the TWP site (97/09/10)

Figure 2. Aerosol optical depth and Angstrom exponent

Figure 3. Water vapor derived from MFRSR and MWR

Key contributors to this work are Jim Barnard and Will Shaw.

The Multi-Filter Rotating Shadowband Radiometer (MFRSR) is a ground-based radiometer that uses interference-filter-photodiode detectors to measure narrowband radiation at six discrete wavelengths in the shortwave spectrum. (The nominal wavelengths at which the measurements are made are 415 nm, 500 nm, 615 nm , 673 nm, 869 nm, 940 nm; the filter width is 10 nm [FWHM]). Recently, (97/08/27) the MFRSR at ARM's Tropical Western Pacific (TWP) site was refurbished by the installation of a new MFRSR head. (The MFRSR head is part of the MFRSR that actually measures the incoming radiation). This short communication highlights the capabilities of a well-calibrated MFRSR by providing a sample of data obtained from this instrument as well as products derived from these data.

Figure 1 shows MFRSR data taken on 97/09/10. These data consist of all three components of the incoming radiation—the direct beam, the diffuse downwelling radiation, and the total downwelling radiation. Before deployment, the MFRSR head was field tested in the United States for about four months. During this time, the calibration of each wavelength channel was checked using Langley regressions to insure that the calibration was accurate and stable. After the head was installed at the TWP site, the calibration was again checked by the Langley method and it was confirmed that the calibration was still accurate. (Clear skies are required to perform a Langley regression and we were fortunate to have several days of clear skies at the usually cloudy TWP site).

A well-calibrated MFRSR can provide modelers of shortwave radiative transfer with a lot of useful information. The upper panel of Figure 2 shows a time series of aerosol optical depths (AOD) at wavelengths of 415 nm and 869 nm for 97/09/10. The angstrom exponent associated with the aerosol distribution is shown in the bottom panel of this figure. The excursions of the Angstrom exponent to zero are indicative cloud droplets; these excursions are correlated with the increase in the diffuse radiation depicted in Figure 1. Figure 3 shows a time series of water vapor derived from the MFRSR (red line) and the TWP microwave radiometer (MWR, blue line). The two instruments track each other quite well. It s surprising that the columnar vapor at this usually steamy site is as low as about 2 cm.

A well-calibrated MFRSR is capable of providing much information to those who seek to understand shortwave radiative transfer. Data from the instrument can be processed to give AOD and columnar water vapor unless the sun is blocked by clouds. Even though the TWP site is often cloudy, it is still possible to calculate values for AOD and water vapor during the times that the sun pokes though the cloud field. Using these values one can then infer the overall trends in AOD and water vapor. For example, as shown in Figure 1 between the hours of 9 AM and 11 AM, the sun was occasionally obscured by clouds, yet during this time the trends in AOD and water vapor are easily discernible in Figure 2. MFRSR data can also be used to calculate cloud optical depth over the site if the clouds are stratiform.