radice −− compute cloud physics ice parameters from radar data
radice reflname wtname wname rhoname [Cd A alpha]
Radice expects synthesized Doppler radar data from radvert or a similar program on the standard input and adds the fields dbn (precipitation particles per cubic meter, decibel measure), a (particle radius in millimeters), rhoice (density in grams per cubic centimeter of precipitation particles), aequiv (radius of the equivalent water drop, = a*pow(rhoice,0.33333)), dbqp (mixing ratio of precipitation in grams per kilogram, decibel measure), and dbprate (precipitation rate in millimeters per hour, decibel measure). The result is sent to the standard output.
Fields needed for the computation are the radar reflectivity (reflname), particle terminal velocity (wtname, normally negative), particle vertical velocity (wname), and air density as a function of height (rho). All input fields are expected to be 3−D variable fields except the density, which is assumed to be a 1−D static field.
Optional command line arguments are Cd, the drag coefficient assumed for precipitation particles, and A and alpha, parameters for an empirical power law relationship between particle terminal velocity, wt, and particle radius, a: wt = A*pow(a,alpha)*pow(rho,−0.5) (Pruppacher and Klett, 1978). If these aren’t specified, the default values Cd = 0.5, A = 2.5, and alpha = 0.8 are used.
A unique relationship between particle radius and particle density is assumed: rhoice = (3/(8*g))*Cd*pow(A,2)*pow(a,2*alpha − 1), where g is the acceleration of gravity in meters per second. This comes from equating the empirical wt equation above with the theoretical equation wt = pow(8*g*rhoice*a/(3*Cd*rho),0.5) and solving for rhoice.
The reflectivity of n ice particles per cubic meter of radius a and density rhoice is Z = 14.5 pow(rhoice,2)*n*pow(a,6) (Mason, 1971). This is used to find n and dbn. Precipitation mixing ratio is given by qp = 4.19e−3*pow(a,3)*rhoice*n/rho. Finally, the precipitation rate is prate = 3.6*qp*rho*w, where w is the vertical particle velocity.
Mason, B. J., 1971: The physics of clouds, 2nd ed., Oxford University Press, London.
Pruppacher, H. R., and J. D. Klett, 1978: Microphysics of clouds and precipitation, D. Reidel Publishing Company, Dordrecht.
Radvert(1).
The precipitation mixing ratio and the precipitation rate are very sensitive to particle terminal velocity. For this reason, the calculations are only performed at a grid point if the terminal velocity exceeds 2 m/s.
The calculations are sensitive to the empirical relationship between terminal velocity and particle radius.
The assumption of constant drag coefficient may not be justified.
The assumption of a monodisperse size distribution for precipitation may also not be justified.