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NSA Radiation Data-

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NSA Cloud Occurrence Frequency from the Active Remote Sensing of Clouds (ARSCL) Value-Added Product

Observations from the 35 GHz Millimeter Cloud Radar (MMCR), Micropulse Lidar (MPL), and ceilometer have been combined using the Active Remote Sensing of Clouds (ARSCL; Clothiaux et al. 2000) to produce time series of the heights of cloud boundaries, cloud location, and profiles of radar moments. For the purposes of this metric, this data product has been produced for the year 2008, a portion of the International Polar Year.

At each permanent ACRF site, data from the MMCR and MPL, as well as ceilometer and surface precipitation measurements have been synthesized to produce best-estimate time-height profiles of hydrometeor locations, radar reflectivities, mean Doppler velocities and Doppler spectral widths using the Active Remote Sensing of CLouds (ARSCL) value-added products (Clothiaux et al. 2000). None of the instruments alone can see the entire vertical cloud profile at all times. Cloud radars can miss thin clouds, particularly cirrus clouds, and cannot clearly distinguish cloud boundaries from precipitation and drizzle. Lidars cannot penetrate thick low-level cloud to see higher cloud layers that may lie above. The MMCR has several distinct operating modes, each optimized for specific types and locations of clouds and precipitation. The ARSCL software incorporates the different radar observing modes while correcting them for possible artifacts, such as velocity aliasing or pulse-coding effects. The resulting best-estimate reflectivity observations are merged with MPL and ceilometer-determined cloud bases to separate cloud from precipitation returns and to help in the identification and removal of insect and other non-hydrometeor .clutter. radar returns. In addition, lidar observations of thin cirrus clouds, which the radar alone can miss due to incomplete radar beam filling or insufficient sensitivity, are incorporated into the product.s results.

Daily data files include time sequences of cloud boundaries, specifically ceilometer cloud base, MPL/ceilometer best-estimate cloud base, radar-derived first cloud top, combined radar-MPL cloud base and top for up to 10 cloud layers for each time, the MPL derived cloud mask, original and masked MMCR reflectivity, and masked mean Doppler velocity and spectral width at a temporal resolution of 10 seconds and a vertical resolution of approximately 45 m. These daily files (arscl1cloth) are available in the ARM data archive (

From these daily files, statistics of cloud occurrence frequency are compiled based on the merged radar-MPL cloud boundaries. The hourly cloud occurrence frequency for each height bin is calculated as the ratio of the number of positive cloud detections and the number of total observations. A single annual file has been produced that reports this hourly cloud frequency over the Barrow, Alaska site as a function of time and altitude.


Clothiaux, E. E., T. P. Ackerman, G. G. Mace, K. P. Moran, R. T. Marchand. M. A. Miller, B. E. Martner, 2000: Objective Determination of Cloud Heights and Radar Reflectivities Using a Combination of Active Remote Sensors at the ARM CART Sites., J. Appl. Meteor., 39, 645-665.

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Eugene Clothiaux