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Effects of precipitation on the relationships between cloud optical thickness and drop size derived from space‐borne measurements
Author(s) -
Kobayashi Takahisa,
Masuda Kazuhiko
Publication year - 2008
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2008gl036140
Subject(s) - precipitation , effective radius , cloud computing , atmospheric sciences , environmental science , radius , cloud fraction , liquid water content , radar , infrared , satellite , meteorology , cloud cover , physics , astrophysics , optics , telecommunications , computer science , operating system , computer security , astronomy , galaxy
Cloud optical thickness and the effective radius of cloud droplets were derived by a combined use of the precipitation radar and visible and infrared scanner onboard the Tropical Rainfall Measuring Mission satellite to study the effect of precipitation on the relationship between cloud optical thickness ( τ ) and the effective radius (r e ) on a global scale. Whether τ and r e correlate positively or negatively is a key to finding evidence of the indirect effects of aerosols, but up to now this has been a subject of considerable debate. No clear correlation has been reported on a global scale because τ is affected by many factors, among which the effect of precipitation is one of the most uncertain. To study the effect of precipitation, we assessed the relationship between τ and r e in terms of the critical radius of cloud droplets (r c ), below which precipitation hardly forms. In our analysis, there was no clear correlation between τ and r e in water clouds which is consistent with many previous studies. However, interesting features of the relationship were revealed that are related to r c . Cloud optical thickness is a maximum for clouds with r e around the value of r c . On a global average, cloud optical thickness tends to increase with r e (positive correlation) for r e < r c and to decrease (negative correlation) for r e > r c . A change in the sign of the relationship was clearly observed at r e ∼ r c on a global scale. These features were observed typically for non‐precipitating clouds and clouds with weak rain. The relationship is strongly affected by precipitation which should be carefully considered when using it to find evidence of the indirect effects of aerosols.