Abstract

Nonbrightband (NBB) rain is a shallow, orographic precipitation that does not produce a radar bright band as a result of melting ice crystals. However, NBB rain is not the same as warm rain, which excludes ice from being involved in the microphysical growth of precipitation. Despite this difference, NBB rain is often treated as warm rain in the literature, and past studies have mostly ignored the role of ice. Here, we use two wet seasons (2015/16 and 2016/17) at four precipitation-observing sites in the Northern Coast Ranges of California to show the role of echo top height and ice in determining NBB rain intensity. It was found that NBB rain was only absent of bright bands 32%–46% of the time depending on location of the site. Additionally, all NBB rain rates that exceeded 10 mm h⁻¹ exhibited observable bright bands during the hour period. We also define growth efficiency (GE) as the ability of shallow rain clouds to produce raindrops larger than drizzle size (D > 0.5 mm). High-GE rain drop size distributions were composed of fewer small drops and more large drops than low-GE rain, which was mostly drizzle. High-GE rain occurred with echo top heights above the freezing level where rapid growth of precipitation was observed by radar. Echo tops that only extended 1 km or less above the freezing level suggested hydrometeor growth from mixed-phase processes, indicating that ice may be present in coastal precipitation at warmer temperatures than previously considered.

Abstract

This study examines the impact of microphysics regime on the relationship between orographic forcing and orographic rain in the coastal mountains of Northern California using >4000 h of data from profiling Doppler radars, rain gauges, and a GPS receiver collected over 10 cool seasons. Orographic forcing is documented by hourly upslope flow, integrated water vapor (IWV), and IWV flux observed along the coast at Bodega Bay (BBY; 15 m MSL). Microphysics regime is inferred in the coastal mountains at Cazadero (CZC; 478 m MSL), where hourly periods of brightband (BB) and nonbrightband (NBB) rain are designated. BB rain is associated with a microphysics regime dominated by the seeder–feeder process while NBB rain is associated with a microphysics regime dominated by the warm-rain process. Mean BBY upslope flow, IWV, and IWV flux are ~16%, ~5%, and ~19% larger, respectively, for NBB rain compared to BB rain, while mean CZC rain rate is ~33% larger for BB rain compared to NBB rain. The orographic enhancement ratio of CZC to BBY rain rate is 3.7 during NBB rain and 2.7 during BB rain. Rain rate at CZC increases as orographic forcing at BBY increases. For a given amount of BBY orographic forcing, mean CZC rain rates are larger for BB rain compared to NBB rain. Correlation coefficients associated with the relationship between CZC rain rate and BBY orographic forcing are smaller for NBB rain relative to BB rain, but these differences are not statistically significant.

Abstract

Two vertically pointing S-band radars (coastal and inland) were operated in western Washington during two winters to monitor brightband snow-level altitudes. Similar snow-level characteristics existed at both sites, although the inland site exhibited lower snow levels by ~70 m because of proximity to cold continental air, and snow-level altitude changes were delayed there by several hours owing to onshore translation of weather systems. The largest precipitation accumulations and rates occurred when the snow level was largely higher than the adjacent terrain. A comparison of these observations with long-term operational radiosonde data reveals that the radar snow levels mirrored climatological conditions. The inland radar data were used to assess the performance of nearby operational freezing-level forecasts. The forecasts possessed a lower-than-observed bias of 100–250 m because of a combination of forecast error and imperfect representativeness between the forecast and observing points. These forecast discrepancies increased in magnitude with higher observed freezing levels, thus representing the hydrologically impactful situations where a greater fraction of mountain basins receive rain rather than snow and generate more runoff than anticipated. Vertical directional wind shear calculations derived from wind-profiler data, and concurrent surface temperature data, reveal that most snow-level forecast discrepancies occurred with warm advection aloft and low-level cold advection through the Stampede Gap. With warm advection, forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL. An analysis of sea level pressure differences across the Cascades indicated that mean forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL when higher pressure was west (east) of the range.

Abstract

An evaluation of Weather Surveillance Radar-1988 Doppler (WSR-88D) KMUX and KDAX radar quantitative precipitation estimation (QPE) over a site in California’s northern Sonoma County is performed and rain type climatology is presented. This site is next to the flood-prone Russian River basin and, because of the mountainous terrain and remoteness from operational radars, is generally believed to lack adequate coverage. QPE comparisons were conducted for multiyear observations with concurrent classification of rainfall structure using measurements from a gauge and an S-band profiler deployed at the location of interest. The radars were able to detect most of the brightband (BB) rain, which contributed over half of the total precipitation. For this rain type hourly radar-based QPE obtained with a default vertical profile of reflectivity correction provided results with errors of about 50%–60%. The operational radars did not detect precipitation during about 30% of the total rainy hours with mostly shallow nonbrightband (NBB) rain, which, depending on the radar, provided ~(12%–15%) of the total precipitation. The accuracy of radar-based QPE for the detected fraction of NBB rain was rather poor with large negative biases and characteristic errors of around 80%. On some occasions, radars falsely detected precipitation when observing high clouds, which did not precipitate or coexisted with shallow rain (less than 10% of total accumulation). For heavier rain with a significant fraction of BB hourly periods, radar QPE for event totals showed relatively good agreement with gauge data. Cancelation of errors of opposite signs contributed, in part, to such agreement. On average, KDAX-based QPE was biased low compared to KMUX.

Abstract

Radar and rain gauge observations collected in coastal mountains during the California Land-Falling Jets Experiment (CALJET) are used to diagnose the bulk physical properties of rainfall during a wet winter season (January–March 1998). Three rainfall types were clearly distinguishable by differences in their vertical profiles of radar reflectivity and Doppler vertical velocity: nonbright band, bright band, and hybrid (seeder–feeder). The contribution of each rainfall type to the total rainfall observed at the radar site (1841 mm) was determined by a new, objective algorithm. While hybrid rain occurred most often, nonbrightband rain (NBB rain) contributed significantly (28%) to the total. This paper focuses on characterizing NBB rain because of the need to document this key physical process and because of its impact on Weather Surveillance Radar-1988 Doppler (WSR-88D) precipitation surveillance capabilities.

NBB rain is a quasi-steady, shallow rain process that does not exhibit a radar bright band, that occurs largely beneath the melting level, and that can produce rain rates exceeding 20 mm h⁻¹. Composite vertical profiles were produced for NBB rain using 1417 samples and brightband rain using 5061 samples. Although the mean rain rate for each composite was 3.95 mm h⁻¹, at all altitudes NBB rain had systematically weaker equivalent radar reflectivity (e.g., 20.5 dBZ _e vs 28.5 dBZ _e at 263 m above ground level) and much smaller Doppler vertical fall velocities (e.g., 2.25 m s⁻¹ vs 6.25 m s⁻¹ at 263 m) than did brightband rain. The reflectivity–rain-rate (Z–R) relationship for NBB rain (Z = 1.2R ^1.8) differs significantly from that of brightband/hybrid rain (Z = 207R ^1.1).

The meteorological context in which NBB rain occurred is described through case studies and seasonal statistics. NBB rain occurred in a wide variety of positions relative to frontal zones within land-falling storms, but three-quarters of it fell when the layer-mean, profiler-observed wind direction at 1250 m MSL (the altitude of the composite low-level jet) was between 190° and 220°. The importance of orographic forcing during NBB rain, relative to all rain events, was indicated by a stronger correlation between upslope wind speed and coastal rain rates at 1250 m MSL (r = 0.74 vs r = 0.54), stronger low-level wind speeds, and wind directions more orthogonal to the mean terrain orientation.

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 = 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°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 precipitation contributes to increases in snow water equivalent (SWE). Between 2.5° and 3°C, snow is equally likely to melt or accumulate, with most cases resulting in no change to SWE. Qualitatively, brightband heights (BBHs) detected by 915-MHz profiling radars up to 300 km away from the American River study basin agree well with surface melting patterns. Quantitatively, this agreement can be improved by adjusting the melting elevation based on the spatial location of the profiler relative to the basin: BBHs decrease with increasing latitude and decreasing distance to the windward slope of the Sierra Nevada. Because of diurnal heating and cooling by radiation at the mountain surface, BBHs should also be adjusted to higher surface elevations near midday and lower elevations near midnight.

Abstract

Numerical prediction of precipitation associated with five cool-season atmospheric river events in northern California was analyzed and compared to observations. The model simulations were performed by using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) with four different microphysical parameterizations. This was done as a part of the 2005–06 field phase of the Hydrometeorological Test Bed project, for which special profilers, soundings, and surface observations were implemented. Using these unique datasets, the meteorology of atmospheric river events was described in terms of dynamical processes and the microphysical structure of the cloud systems that produced most of the surface precipitation. Events were categorized as “bright band” (BB) or “nonbright band” (NBB), the differences being the presence of significant amounts of ice aloft (or lack thereof) and a signature of higher reflectivity collocated with the melting layer produced by frozen precipitating particles descending through the 0°C isotherm.

The model was reasonably successful at predicting the timing of surface fronts, the development and evolution of low-level jets associated with latent heating processes and terrain interaction, and wind flow signatures consistent with deep-layer thermal advection. However, the model showed the tendency to overestimate the duration and intensity of the impinging low-level winds. In general, all model configurations overestimated precipitation, especially in the case of BB events. Nonetheless, large differences in precipitation distribution and cloud structure among model runs using various microphysical parameterization schemes were noted.

Abstract

The snow level, or altitude in the atmosphere where snow melts to rain, is an important variable for hydrometeorological prediction in mountainous watersheds; yet, there is no operational performance measure associated with snow-level forecasts in the United States. To establish a performance measure, it is first necessary to establish the baseline performance associated with snow-level forecasts. Using data collected by vertically pointing Doppler radars, an automated algorithm has been developed to detect the altitude of maximum radar reflectivity in the radar bright band that results from the precipitation melting process. This altitude can be used as a proxy for the snow level, partly because it always exists below the freezing level, which is defined as the altitude of the 0°C isotherm. The skill of freezing-level forecasts produced by the California–Nevada River Forecast Center (CNRFC) is evaluated by comparing spatially interpolated and forecaster-adjusted numerical model freezing-level forecasts with observed freezing levels estimated by radars operating at 2875 MHz (S band). The freezing level was chosen instead of the snow level as the comparison parameter because the radar algorithm and the CNRFC have different interpretations of the snow level. The evaluation occurred at two sites: one in the coastal mountains north of San Francisco and the other in the Sierra Nevada. The evaluation was conducted for forecasts made during the winter wet season of 2005/06. Although the overall mean freezing-level forecast bias is small enough not to be hydrologically significant, about 15% of the forecasts had biases greater than 300 m (forecast too low). The largest forecast biases were associated with freezing levels above 2.3 km that were underforecasted by as much as 900 m. These high freezing-level events were accompanied by the heaviest precipitation intensities, exacerbating the flood threat and making the forecast even more challenging.

Abstract

Atmospheric rivers (ARs) are narrow regions of enhanced water vapor transport, usually found on the warm-sector side of the polar cold front in many midlatitude storms formed primarily over the oceans. Nonbrightband (NBB) rain is a shallow orographic rainfall process driven by collision and coalescence that has been observed in some of these storms. NBB rain accounts for about one-third, on average, of the total winter season rainfall occurring at a coastal mountain site in Northern California. During the California Energy Commission’s CalWater project, nearly the same fraction of NBB rain was observed at a northern Sierra Nevada foothills site as compared to the coastal mountains, whereas less than half of the fractional amount of NBB rain was observed at a southern Sierra Nevada foothills site. Both Sierra Nevada sites often experience terrain-induced blocked flow, that is, Sierra barrier jet (SBJ) during landfalling winter storms. However, the northern Sierra Nevada site often is oriented geographically downwind of a gap in the coastal terrain near San Francisco during AR landfall. This gap allows maritime air in the AR to arrive at the northern site and enhance the collision–coalescence process in orographic feeder clouds as compared with the southern site. As a result, a greater amount and intensity of NBB rain and overall precipitation was produced at the northern site. This study uses a variety of observations collected in the coastal and Sierra Nevada ranges from the Hydrometeorology Testbed and CalWater field campaigns to document this behavior. A detailed case study provides additional context on the interaction between AR flow, the SBJ, and precipitation processes.