Abstract Radar reflectivity factors determined from disdrometer measurements of drop spectra are compared with simultaneous WSR-57 radar measurements in two Oklahoma thunderstorms. The possibility of using a disdrometer for an in-field calibration check of a radar is examined and found to have limited usefulness for convective precipitation sampled at long ranges.

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (KDP and do, respectively) in rain are discussed for X- ( l ; 3 cm) and Ka-band (l ; 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between KDP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h21 is generally small and should not pose a serious problem when estimating KDP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (l ; 10-11 cm) wavelengths is an increase in KDP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at K a band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase. Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4-5 mm h21. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A KDP-R relation derived for the mean drop aspect ratio (R 5 20.5 ) provided a satisfactory agreement between rain accumulations derived from radar 0.80 KDP measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

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 = 168R(1.58) for BB periods and Z = 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 degrees 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.

Abstract The maritime mountain ranges of western North America span a wide range of elevations and are extremely sensitive to flooding from warm winter storms, primarily because rain falls at higher elevations and over a much greater fraction of a basin’s contributing area than during a typical storm. Accurate predictions of this rain–snow line are crucial to hydrologic forecasting. This study examines how remotely sensed atmospheric snow levels measured upstream of a mountain range (specifically, the bright band measured above radar wind profilers) can be used to accurately portray the altitude of the surface transition from snow to rain along the mountain’s windward slopes, focusing on measurements in the Sierra Nevada, California, from 2001 to 2005. Snow accumulation varies with respect to surface temperature, diurnal cycles in solar radiation, and fluctuations in the free-tropospheric melting level. At 1.5°C, 50% of precipitation events fall as rain and 50% as snow, and on average, 50% of measured pre...

A strong elevated temperature inversion in a landfalling winter storm in northern California produced two simultaneous melting layers with associated radar bright bands. The storm was observed with scanning and profiling radars. Serial radiosonde launches from the scanning radar site precisely documented the evolving temperature structure of the air mass that produced the double bright band. The radiosonde and radar observations, which were coincident in location and time, clearly illustrate the cause (two melting layers) and effect (two bright bands) of this unusual phenomenon. An automated algorithm for determining the melting-layer height from profiling radar data was tested on this situation. In its operational form, the algorithm detects only the lower melting layer, but in modified form it is capable of detecting both melting layers simultaneously.

A comparative study of the use of X- and S-band polarimetric radars for rainfall parameter retrievals is presented. The main advantage of X-band polarimetric measurements is the availability of reliable specific differential phase shift estimates, K-DP, for lighter rainfalls when phase measurements at the S band are too noisy to produce usable K-DP. Theoretical modeling with experimental raindrop size distributions indicates that due to some non-Rayleigh resonant effects, K-DP values at a 3.2-cm wavelength (X band) are on average a factor of 3.7 greater than at 11 cm (S band), which is a somewhat larger difference than simple frequency scaling predicts. The non-Rayleigh effects also cause X- band horizontal polarization reflectivity, Z(eh), and differential reflectivity, Z(DR), to be larger than those at the S band. The differences between X- and S-band reflectivities can exceed measurement uncertainties for Z(eh) starting approximately at Z(eh) > 40 dBZ, and for Z(DR) when the mass-weighted drop diameter, D-m, exceeds about 2 mm. Simultaneous X- and S-band radar measurements of rainfall showed that consistent KDP estimates exceeding about 0.1 degrees km(-1) began to be possible at reflectivities greater than similar to 26-30 dBZ while at the S band such estimates can generally be made if Z(eh) > similar to 35-39 dBZ. Experimental radar data taken in light-to-moderate stratiform rainfalls with rain rates R in an interval from 2.5 to 15 mm h(-1) showed availability of the K-DP-based estimates of R for most of the data points at the X band while at the S band such estimates were available only for R greater than about 8-10 mm h(-1). After correcting X- band differential reflectivity measurements for differential attenuation, Z(DR) measurements at both radar frequency bands were in good agreement with each other for D-m < 2 mm, which approximately corresponds to Z(DR) approximate to 1.6 dB. The Z(DR)-based retrievals of characteristic raindrop sizes also agreed well with in situ disdrometer measurements.

Most radar estimations of rainfall intensity and accumulations are still based on empirical relations between rainfall rate R (in mm/h) and reflectivity factor Z (in mmm). In many cases these Z-R relations are derived from drop size distributions (DSDs) measured by raindrop disdrometers. Regressions are fit to the scatter of (Z,R) points to determine the relation, which is almost always stated as Z = aR. The wellknown table by Battan (1973) lists dozens of examples of early Z-R equations from researchers in various parts of the world, with the implication that the equation differences are attributable to different types of rainfall processes. Scores of additional relations have been published since then.

An objective algorithm presented in White et al. was applied to vertically pointing S-band (S-PROF) radar data recorded at four sites in northern California and western Oregon during four winters to assess the geographic, interannual, and synoptic variability of stratiform nonbrightband (NBB) rain in landfalling winter storms. NBB rain typically fell in a shallow layer residing beneath the melting level (similar to 6 km MSL) The shallow NBB echo tops often resided beneath the coverage of the operational Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning radars yet were still capable of producing flooding rains.NBB rain contributed significantly to the total winter-season rainfall at each of the four geographically distinct sites (i.e., 18%-35% of the winter-season rain totals). In addition, the rainfall observed at the coastal mountain site near Cazadero, California (CZD), during each of four winters was composed of a significant percentage of NBB rain (18%-50%); substantial NBB rainfall occurred regardless of the phase of the El Nino-Southern Oscillation (which ranged from strong El Nino to moderate La Nina conditions). Clearly, NBB rain occurs more widely and commonly in California and Oregon than can be inferred from the single-winter, single-site study of White et al.Composite NCEP-NCAR reanalysis maps and Geostationary Operational Environment Satellite (GOES) cloud-top temperature data were examined to evaluate the synoptic conditions that characterize periods of NBB precipitation observed at CZD and how they differ from periods with bright bands. The composites indicate that both rain types were tied generally to landfalling polar-cold-frontal systems. However, synoptic conditions favoring BB rain exhibited notable distinctions from those characterizing NBB periods. This included key differences in the position of the composite 300-mb jet stream and underlying cold front with respect to CZD, as well as notable differences in the intensity of the 500-mb shortwave trough offshore of CZD. The suite of BB composites exhibited dynamically consistent synoptic-scale characteristics that yielded stronger and deeper ascent over CZD than for the typically shallower NBB rain, consistent with the GOES satellite composites that showed 20-K warmer (2.3-km shallower) cloud tops for NBB rain. Composite soundings for both rain types possessed low-level potential instability, but the NBB sounding was warmer and moister with stronger low-level upslope flow, thus implying that orographically forced rainfall is enhanced during NBB conditions.

The utility of X-band polarimetric radar for quantitative retrievals of rainfall parameters is analyzed using observations collected along the U.S. west coast near the mouth of the Russian River during the Hydrometeorological Testbed project conducted by NOAA's Environmental Technology and National Severe Storms Laboratories in December 2003 through March 2004. It is demonstrated that the rain attenuation effects in measurements of reflectivity (Z(c)) and differential attenuation effects in measurements of differential reffectivity (Z(DR)) can be efficiently corrected in near-real time using differential phase shift data. A scheme for correcting gaseous attenuation effects that are important at longer ranges is introduced. The use of polarimetric rainfall estimators that utilize specific differential phase and differential reflectivity data often provides results that are superior to estimators that use fixed reflectivity-based relations, even if these relations were derived from the ensemble of drop size distributions collected in a given geographical region. Comparisons of polarimetrically derived rainfall accumulations with data from the high-resolution rain gauges located along the coast indicated deviation between radar and gauge estimates of about 25%. The ZDR measurements corrected for differential attenuation were also used to retrieve median raindrop sizes, D, Because of uncertainties in differential reflectivity measurements, these retrievals are typically performed only for D-0 > 0.75 mm. The D-0 estimates from an impact disdrometer located at 25 kin from the radar were in good agreement with the radar retrievals. The experience of operating the transportable polarimetric X-band radar in the coastal area that does not have good coverage by the National Weather Service radar network showed the value of such radar in filling the gaps in the network coverage. The NOAA X-band radar was effective in covering an area up to 40-50 km in radius offshore adjacent to a region that is prone to flooding during wintertime landfalling Pacific storms.

Many useful methods for calibrating weather radars in the field and laboratory are summarized by Joe and Smith (2001) from presentations at the 2001 AMS Radar Calibration Workshop in Albuquerque, NM . Additional methods are described by Atlas (2002). Common external (full-system) calibration methods range from using metal spheres suspended from balloons to the use of solar radio emissions. Although the task is seemingly straightforward, it is challenging to devise methods that are accurate but not so difficult to conduct that they are impractical. The method used by NOAA’s Environmental Technology Laboratory (ETL) for calibrating its scanning X-band and Ka-band radars is described in this article. The radars are described by Martner et al. (2001, 2002).

Profiling radar observations of precipitation in northern California’s coastal mountains by the CALJET experiment during the strong El Niño winter of 1997-98 revealed new insights about microphysical properties of the region’s orographically-forced precipitation. Data from Sband precipitation profilers showed that, although these storms extended above and below the freezing level, they often did not exhibit the radar melting layer bright band (BB) that is usually characteristic of midlatitude winter storms. Yet these non-bright band (NBB) situations contributed substantially to the region’s record-breaking winter precipitation that year, even though they were generally shallow and commonly passed beneath the coverage of the nearest NEXRAD radars. PACJET, the ongoing follow-on to CALJET, collected new S-band profiler data in 2003 in the same area, this time augmented with raindrop disdrometer and polarimetric scanning radar measurements. This article presents preliminary analyses of the 2003 observations, which corroborate and clarify some of the earlier findings.