by Nicolae Ajtai, Ruben Barragan, Xinxin Busch-Li, Maria Jose Granados, Lev Labzovskii
Motivation:
Studying cloud cover is important since it plays an important role in the radiative balance of the atmosphere and its dynamics are significant in climate change studies. Cloud fraction (CF), a common quantity to measure cloud coverage, is defined as the portion of sky that is covered by clouds. In our study CF is measured by several ground-based remote sensors, Total sky imager (TSI), MIRA-36 Cloud Radar, and CT-25K Ceilometer. Comparing the CF observations from those three instruments is our major goal to evaluate the CF estimation difference.
Methodology:
We develop a strategy for cloud fraction (CF) estimation from multiple sensor analysis (total sky imager, radar, ceilometer). We analyzed spatial CF for hemispherical viewing total sky camera (blue to red ratio of individual pixels of the imagery), and temporal CF for zenith narrow-field observations from radar and ceilometer (frequency of cloud occurrence within an interval, no. of profiles with clouds / total no. of profiles), respectively. The strategy we developed to estimate CF based on three instruments is based on one day case study. Specifically, in TSI CF is estimated by the percentage of opaque cloud pixels in an image of one day observations; CF is detected by the profiles of ceilometer with at least one cloud layer divided by the total number of profiles. When profiles of radar reflectivity with a minimum of 4 range gates per profile is measured by radar, CF is obtained by a minimum of 4 range gates per profile dived by the total number of profiles. Furthermore, a one year long-term ground based remote sensing observations was analyzed based on the strategy developed from the synergy of multiple remote sensors.
Main results:
We found reasonable correlations between the 3 instruments CF data (all correlation coefficients R between two data sets are all larger than 0.6). Radar overestimates CF at 90% CF compared to the other two instruments, because the high altitude ice clouds can be seen by radar, but ice clouds are invisible for TSI and ceilometer. Besides, when we compare day and night observations, there is no significant difference between day and nighttime CF estimations for radar and ceilometer. According to our study, for the low or mid-level clouds the most reliable instrument to determine CF is the TSI, followed by the ceilometer and the radar respectively. However, for the high altitude clouds radar has the best ability to detect CF.
Yearly CF estimates for teh JOYCE site are also estimated. The mean CF at Juelich, Germany is 61.8% observed by TSI, and 58.1% by ceilometer, and 77.5% by radar. We suggest a weighthing function to combine CF measurements to overcome single instrument limitation.
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(b)
Figure 1. Cloud fraction (CF) measurements on (a) the one day case study on 5th June (from top to bottom): Radar, ceilometer and TSI CF for the 5th of June 2013. Cloud base height as determined by the ceilometer. Radar reflectivity after filtering profiles with less than 4 range gates per profile. (b) the monthly average of CF observed by radar, ceilometer, and TSI respectively from March 2012 to March 2013.