Raindrop Size Distributions and Rain Characteristics in California Coastal Rainfall for Periods with and without a Radar Bright Band

Abstract

Recent studies using vertically pointing S-band profiling radars showed that coastal winter storms in California and Oregon frequently do not display a melting-layer radar bright band and inferred that these nonbrightband (NBB) periods are characterized by raindrop size spectra that differ markedly from those of brightband (BB) periods. Two coastal sites in northern California were revisited in the winter of 2003/04 in this study, which extends the earlier work by augmenting the profiling radar observations with collocated raindrop disdrometers to measure drop size distributions (DSD) at the surface. The disdrometer observations are analyzed for more than 320 h of nonconvective rainfall. The new measurements confirm the earlier inferences that NBB rainfall periods are characterized by greater concentrations of small drops and smaller concentrations of large drops than BB periods. Compared with their BB counterparts, NBB periods had mean values that were 40% smaller for mean-volume diameter, 32% smaller for rain intensity, 87% larger for total drop concentration, and 81% larger (steeper) for slope of the exponential DSDs. The differences are statistically significant. Liquid water contents differ very little, however, for the two rain types. Disdrometer-based relations between radar reflectivity (Z ) and rainfall intensity (R) at the site in the Coast Range Mountains were Z ϭ 168R 1.58 for BB periods and Z ϭ 44R 1.91 for NBB. The much lower coefficient, which is characteristic of NBB rainfall, is poorly represented by the Z-R equations most commonly applied to data from the operational network of Weather Surveillance Radar-1988 Doppler (WSR-88D) units, which underestimate rain accumulations by a factor of 2 or more when applied to nonconvective NBB situations. Based on the observed DSDs, it is also concluded that polarimetric scanning radars may have some limited ability to distinguish between regions of BB and NBB rainfall using differential reflectivity. However, differential-phase estimations of rain intensity are not useful for NBB rain, because the drops are too small and nearly spherical. On average, the profiler-measured echo tops were 3.2 km lower in NBB periods than during BB periods, and they extended only about 1 km above the 0°C altitude. The findings are consistent with the concept that precipitation processes during BB periods are dominated by ice processes in deep cloud layers associated with synoptic-scale forcing, whereas the more restrained growth of hydrometeors in NBB periods is primarily the result of orographically forced condensation and coalescence processes in much shallower clouds.

Categorisation of northern California rainfall for periods with and without a radar brightband using stable isotopes and a novel automated precipitation collector

“…Echo-top heights derived from S-PROF radar reflectivity profiles are available for selected CZD and ATA events and are provided in the Supplementary File. Previous work (Martner et al, 2008) indicated that the mean echo-top height was higher (6.8 km, corresponding to (26.3 8C) for BB periods and lower (3.6 km, corresponding to (5.6 8C) for NBB periods, in accord with HR periods being colder with lower d . HRperiod echo-top heights are greater on average than those of LR periods, and the average BB height (height of maximum radar reflectivity) represents the freezing level and was 2.31 km above mean sea level.…”

“…Figure 18 shows the strong correlation between mean echo-top heights of HR and LR periods and minimum d 2 H VSMOW of values of rainfall for 23 events having echotop data. Because BB rainfall has higher mean echo-top heights than NBB rainfall (Martner et al, 2008), we anticipate that there may be a correlation between intervals having precipitation categorised by S-Prof radar as BB rain and intervals categorised as HR periods using algorithmicisotopic categorisation of precipitation. Similarly, we anticipate that there may be a correlation between intervals having precipitation objectively categorised as NBB rain and periods categorised as LH periods.…”

“…Organised ascent is often associated with specific regions within extratropical cyclones, including storms that impact the western coast of North America during the winter wet season. Studies using meteorological arrays have provided insights into precipitation-generating processes Kingsmill et al, 2006;Ralph et al, 2006Ralph et al, , 2013aSmith et al, 2010) and precipitation characteristics during such storm events (White et al, 2003;Martner et al, 2008).…”

“…Because such precipitation occurs initially at higher (and much colder) altitudes, it would be expected to have an isotopic composition depleted in 2 H and 18 O (Dansgaard, 1953) and rainout is relatively high. In contrast, a shallow rainfall process occurring without a BB, that is, non-brightband (NBB) rain, forms primarily at low altitudes (White et al, 2003;Neiman et al, 2005;Martner et al, 2008) and is expected to exhibit an isotopic composition consistent with a relatively warm marine source (more enriched in 2 H and 18 O), and rainout is substantially lower. Many of the events in this study are atmospheric rivers (ARs), which are characterised by strong horizontal watervapour fluxes concentrated in a narrow band in the lower troposphere immediately ahead of the polar cold fronts.…”

Skewness as measure of the invariance of instantaneous renormalized drop diameter distributions – Part 2: Orographic precipitation

“…This data set is the same one used in the companion paper "Skewness as measure of the invariance of instantaneous renormalized drop diameter distributions -Part 1: Convective vs. stratiform precipitation" (Ignaccolo and De Michele, 2012). Bodega Bay is located on the coast of California, for extensive period of times (White at al., 2003;Martner et al, 2008). During these time intervals, precipitation "is primarily the result of orographically forced condensation and coalescence processes" (Martner et al, 2008).…”