Atomic and molecular vibrations correspond to excited ：原子和分子的振动对应激.pptVIP

Radar observations of precipitation Radar backscatter efficiency (Qback) for water and ice spheres at the wavelength of the WSR-88D operational weather radar (wavelength = 10.71 cm) Up to Diameters of ~6 mm, the Rayleigh relationship (Qback proportional to r4) holds 6 mm is the rough upper limit of the size of raindrops observed in heavy rain Note that Rayleigh relationship holds up to D = ~3 cm for ice (i.e. hailstones) Radar observations of precipitation Because of these relationships: The backscattered power measured by the radar receiver is actually proportional to a reflectivity factor, Z: where D is droplet diameter and n(D) is the droplet size distribution function Hence the reflectivity factor is equal to the sum of the sixth powers of the diameters of all the drops in a unit volume of air. Most weather radars record and display estimates of Z at each range d. Standard units of Z are mm6 m-3 (D in mm), but due to the enormous range of observed values of Z, a non-dimensional logarithmic unit dBZ is used: Z [dBZ] = 10 log (Z) Radar observations of precipitation A typical weather radar measures reflectivities ranging from -20 to 70 dBZ Because of the D6 dependence in Z, reflectivity is strongly influenced by the few largest drops in a volume of air – a single drop of diameter 5 mm reflects more microwave radiation than 15,000 drops of 1 mm diameter Clouds (D of ~20 μm) are invisible to most radars, despite large droplet concentrations Radar reflectivity example Take a cloud containing 100 cloud droplets per cm3 with diameter 20 μm What is the radar reflectivity factor? Hence we have N = 100×106 m-3 and D = 20×10-3 mm, so Z = 0.0064 mm6m-3 So Z [dBZ] = -22 dBZ (i.e. very low reflectivity) Radar observations of volcanic clouds Eruption of Redoubt volcano, Alaska on March 22, 2009 (continuing) /weather/radar/ - Real-time weather radar for US Direct and diffuse radiation Random paths of 100 photons in a plane-parallel, isotropicall