Accuracy of cloud liquid water path from ground‐based microwave radiometry 2. Sensor accuracy and synergy

Abstract

[1] The influence of microwave radiometer accuracy on retrieved cloud liquid water path (LWP) was investigated. Sensor accuracy was assumed to be the sum of the relative (i.e., Gaussian noise) and the absolute accuracies of brightness temperatures. When statistical algorithms are developed the assumed noise should be as close as possible to the real measurements in order to avoid artifacts in the retrieved LWP distribution. Typical offset errors of 1 K in brightness temperatures can produce mean LWP errors of more than 30 g m À2 for a two-channel radiometer retrieval, although positively correlated brightness temperature offsets in both channels reduce this error to 16 g m À2 . Large improvements in LWP retrieval accuracy of about 50% can be achieved by adding a 90-GHz channel to the two-channel retrieval. The inclusion of additional measurements, like cloud base height from a lidar ceilometer and cloud base temperature from an infrared radiometer, is invaluable in detecting cloud free scenes allowing an indirect evaluation of LWP accuracy in clear sky cases. This method was used to evaluate LWP retrieval algorithms based on different gas absorption models. Using two months of measurements, the Liebe 93 model provided the best results when the 90-GHz channel was incorporated into the standard two-channel retrievals.INDEX TERMS: 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; 6969 Radio Science: Remote sensing; 1640 Global Change: Remote sensing; 1655 Global Change: Water cycles (1836); KEYWORDS: microwave radiometer, cloud liquid water, sensor synergy, ground-based remote sensing Citation: Crewell, S., and U. Löhnert, Accuracy of cloud liquid water path from ground-based microwave radiometry, 2, Sensor accuracy and synergy,

The Convective and Orographically‐induced Precipitation Study (COPS): the scientific strategy, the field phase, and research highlights

“…Turner et al, 2007). The sensitivity to small amounts of liquid water is much larger at higher frequencies, and thus incorporating measurements at 90 or 150 GHz into the LWP retrievals would reduce the random errors by a factor of 2-3 (Crewell and Löhnert, 2003). Evaluation of microwave absorption models resulted in improvements for both precipitable WV and LWP retrievals with respect to the WV absorption in all of these models .…”

G band atmospheric radars: new frontiers in cloud physics

“…While remote sensing of the column integrated amount of cloud liquid water (LWP) using passive microwave radiometry can be achieved with sufficient accuracy (Crewell and Löhnert, 2003;van Meijgaard and Crewell, 2005), only limited information can be extracted on the vertical profile using the cloud radar backscatter signal. This is mainly because the liquid water content (LWC, proportional to the third moment of the droplet spectrum) of BL clouds is dominated by small (diameter < 40 µm) cloud droplets, whereas the radar backscatter signal (Z, proportional to the sixth moment of the droplet spectrum) is dominated by drizzle when present (Fox and Illingworth, 1997).…”

Detection and characterization of heavy drizzle cells within subtropical marine stratocumulus using AMSR-E 89-GHz passive microwave measurements

“…A similar study by Kubar et al (2009) . Although our algorithm identifies only the larger, heavily drizzling cells, it is an important contribution in mapping the global occurrence of drizzle within marine stratocumulus and its interrelations with changes in cloud fraction (de Szoeke et al, 2010).…”

“…The variances in observed brightness temperature are primarily a function of the background ocean-surface emission (primarily related to sea surface temperature -SST -and wind speed), gas absorption, water vapor profile, beam filling, and LWP, including cloud and precipitable water (Westwater et al, 2001;Crewell and Lǒhnert, 2003;Greenwald et al, 2007;Horváth and Gentemann, 2007;Greenwald, 2009). Of these sources of emission, we expect that LWP will have the largest variance at spatial scales on the order of individual drizzle cells within marine stratocumulus.…”

“…The calculation of clear-sky values can be challenging for regions of broad cloudiness, like stratocumulus regions, without relying on climatology data or data from multiple sensor platforms. Crewell and Lǒhnert (2003) used vertically pointing (upward) 90 GHz radiometer data to improve the accuracy of retrieved LWP of cloud liquid water and drizzle.…”