3/21/2024 0 Comments Keep calm and carry on rgb red2a shows 1.6 micrometer simulated thick cloud reflectivity as a function of effective cloud particle size for water (red curve) and ice (blue curve) elements. (Note that the difference in absorption between ice and water is not small: it is shown on a logarithmic scale.) For the NIR0.87 channel the index is much lower and about the same for ice and water particles.įigure 1: Absorption spectra of water (blue curve) and ice (red curve)įig. Due to ice's higher absorption in the NIR1.61 channel their reflection is lower than that of water clouds. A cloud layer's absorption also depends on many other things (like the number of droplets/particles etc.), but the absorption coefficient of the material has a dominant role. This index characterizes the absorption of the material, not the absorption of a cloud layer. 1 presents the imaginary refraction index of water and ice. Ice crystals absorb radiation more strongly than water particles in the NIR1.6 channel. The NIR1.61 channel helps to distinguish water clouds from ice clouds. (Effective particle size is the size of an 'averaged' particle, calculated by dividing total particle volume by total particle surface area.) NIR1.61 channel data contains information on cloud top microphysics: the phase and the effective particle size of the cloud top elements. Distinguishing ice and water clouds and clouds with small and large cloud top particles For semi-transparent clouds the interpretation of the measured radiation is more complicated: besides the cloud's temperature, it also depends on its transparency and the temperature of the underlying surface. For opaque clouds the measured signal depends mainly on cloud top temperature. IR10.8 is an atmospheric window channel (where the absorption of gas molecules is low). The IR10.8 channel distinguishes thick clouds by their cloud top temperatures. NIR0.87 reflectivity values are high for opaque and lower for semi-transparent clouds. In cloudy areas the NIR0.87 channel values depend mainly on cloud thickness. Physical Basis Distinguishing clouds according to their optical thickness and cloud top temperature It contains information on cloud thickness, cloud top phase, particle size and cloud top temperature. The aim of this RGB type is to provide general cloud analysis with good color contrast between different cloud types and between clouds and the surface. It is a daytime RGB since it uses shortwave channels. The 0.87 micrometer channel (NIR0.87) is visualized in red, the 1.61 micrometer channel (NIR1.61) in green and the 10.8 micrometer channel (IR10.8) in blue. METOP AVHRR Day Microphysics RGB images are created by combining data from three different channels. The SEVIRI standard Day Microphysics RGB uses IR3.9 channel data since it is more sensitive to cloud top particle size than NIR1.6 data. The Meteosat SEVIRI instrument uses two channels that provide microphysical information during daytime: NIR1.6 and IR3.9. The METOP AVHRR instrument only uses one microphysical channel during daytime (NIR1.61), which means it can only be used to create Day Microphysics RGBs. EUMETSAT's recommendation for Microphysics RGBs is that the red component should be used for information on cloud thickness, the green one for microphysical information, and the blue one for temperature.
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