Radar Reflectivity Observations Research Articles

The temporal and spatial variability of rainfall power

AbstractIn this paper a statistical model of raindrop size distributions is used to examine the temporal variability of rainfall power and to develop techniques for estimating rainfall power from observations of rainfall rate and radar reflectivity factor. The model of raindrop size distributions is based on the assumptions that: 1) raindrop arrival rate at the ground can be represented by a Poisson process with randomly varying rate of occurrence and 2) diameters of raindrops have a lognormal distribution with parameters that are time‐varying random processes. The lognormal distribution plays a central role in both characterization of the temporal variability of rainfall power and in state estimation of rainfall power from rainfall rate and radar reflectivity observations. Empirical results are presented using drop‐size data from a number of sites in the United States. Spatial variability of rainfall power is examined empirically using the rainfall power‐reflectivity relationships developed from drop‐size data and radar data from the southern plains region of the United States.

One-Dimensional Time-Dependent Modeling of GATE Cumulonimbus Convection

A one-dimensional time-dependent cumulonimbus model is designed that, unlike in previous one-dimensional models, simulates cloud-top heights, vertical velocities, and water contents that are reasonably consistent with those observed in real convective cores. The model successfully simulates deep tropical oceanic cumulonimbus with results that are in agreement with aircraft observations of vertical velocity, observations of radar reflectivity, and three-dimensional model simulations. These results are achieved by improving the parameterizations of the following physical processes: vertical mixing through the inclusion of an overturning thermal circulation near cloud top, lateral entrainment by modifying the assumed shape of the cloud, initiating convection with sustained boundary-layer forcing that resembles the lifting by gust fronts associated with tropical oceanic cumulonimbus, and making the pressure perturbation internally consistent with the horizontal distribution of vertical velocity in the cloud. The effect of a tilted updraft on precipitation fallout and enhanced cloud growth are also examined.

The formation of rain in convective clouds

One of the chief problems in cloud physics1 is to explain how, in the absence of ice, small cloud droplets can coalesce to form raindrops. Cloud droplets are produced in concentrations of ∼108 m−3 by condensation on sub-micrometre atmospheric nuclei, but, because they are so small, collisions between droplets are rare events. In mature rainclouds there are2 usually about 1,000 drops per m3; here, however, I present differential radar reflectivity observations which indicate that, when ice is not present, first echoes of convective showers consist of large (>4 mm) raindrops present in much lower concentrations ( 30μm radius) which are present in similarly low background concentrations3. In some clouds this initial raindrop spectrum persists, whereas in other cases it changes rapidly and irreversibly to the raindrop distribution normally observed in mature clouds with many more smaller raindrops. The latter may occur when the break-up of the large raindrops becomes more common and many small satellite drops are produced.

Small cumuli observed with a 3 mm wavelength Doppler radar

The paper presents quantitative millimeter wave Doppler radar observations of radar reflectivity and air vertical velocity inside shallow cumuli whose vertical development is restricted by a capping inversion layer. The maximum observed radar reflectivity is −34 to −30 dBZ, which is equivalent to a cloud liquid water content of 0.1 to 0.25 g/m³ depending on cloud droplet size. The main updraft region, which coincides roughly with the maximum reflectivity region, occupies the center of the cloud and extends throughout the radar‐visible cloud depth. Downdrafts exhibit more complex structure and are found on the cloud edges.