Assessment of variability in continental low stratiform clouds based on observations of radar reflectivity

Abstract

[1] The variability of overcast low stratiform clouds observed over the ARM Climate Research Facility Southern Great Plains (ACRF SGP) site is analyzed, and an approach to characterizing subgrid variability based on assumed statistical distributions is evaluated. The analysis is based on a vast (>1000 hours) radar reflectivity database collected by the Millimeter-Wave Cloud Radar at ACRF SGP site. The radar data are classified into two low cloud categories and stratified by scale and the presence of precipitation. Cloud variability is analyzed by studying statistical distributions for the first two moments of the probability distribution functions (PDF) of radar reflectivity. Results indicate that variability for a broadly defined low-altitude stratiform cloud type exhibits on average 40% greater standard deviation than canonical boundary layer clouds topped by an inversion. Cloud variability also dramatically depends on microphysical processes (as manifested in radar reflectivity) and increases by 2-5 times within a typical reflectivity range. Finally, variability is a strong function of scale and almost doubles in the 20-100 min temporal scale range. Formulations of subgrid variability, based on PDFs of reflectivity, are evaluated for the two cloud types and two scales of 10 and 30 km, taken to be representative of mesoscale and NWP model grid sizes. The results show that for these cloud types and scales the PDF of reflectivity can be reasonably well approximated by a truncated Gaussian function, specified by mean and standard deviation with the latter parameterized as a linear function of the mean.

Threshold radar reflectivity for drizzling clouds

“… Matrosov et al [2004] found a gradual deterioration of the liquid water content (L) retrieved from radar reflectivity when reflectivity threshold set for the retrieval increases, and at high reflectivity values an ambiguity exists between clouds with high L and those with drizzle. Kogan et al [2005] used −17 dBZ as the reflectivity threshold to partition their observations into non‐precipitating and precipitating clouds. They also examined the influence of varying the threshold between −20 and −15 dBZ on their results.…”

Detecting snowfall over land by satellite high‐frequency microwave observations: The lack of scattering signature and a statistical approach

“…For snowfall detection, we need to define a threshold by the CPR radar reflectivity for snowfall onset. Several investigators have so far proposed "precipitation threshold" [e.g., Haynes et al, 2009;Liu, 2008a;Sauvageot and Omar, 1987;Matrosov et al, 2004;Frisch et al, 1995;Baedi et al, 2002;Mace and Sassen, 2000;Kato et al, 2001;Kogan et al, 2005;Wang and Geerts, 2003], mostly for drizzle, with radar reflectivity generally between -20 and -10 dBZ. To be consistent with these previous studies, we define -15 dBZ as the snowfall threshold, which corresponds to about 0.01 mm h -1 snowfall rate based on the Liu [2008a] conversion formula.…”

Large-Eddy Observation of Post-Cold-Frontal Continental Stratocumulus

“…This ensures that the Doppler velocities represent the air motion and not a combination of air motion and precipitation sedimentation. A number of thresholds have been used previously to delineate clouds from precipitation (Chin et al 2000;Kato et al 2001;Kogan et al 2005;Liu et al 2008), and we chose a value of 220 dBZ to screen out most precipitation-sized particles. Only a small fraction of scans were filtered out during the 0600-1200 UTC period of interest.…”