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Robert Conrick
,
Clifford F. Mass
, and
Lynn McMurdie

Abstract

Current bulk microphysical parameterization schemes underpredict precipitation intensities and drop size distributions (DSDs) during warm rain periods, particularly upwind of coastal terrain. To help address this deficiency, this study introduces a set of modifications, called RCON, to the liquid-phase (warm rain) parameterization currently used in the Thompson–Eidhammer microphysical parameterization scheme. RCON introduces several model modifications, motivated by evaluating simulations from a bin scheme, which together result in more accurate precipitation simulations during periods of warm rain. Among the most significant changes are 1) the use of a wider cloud water DSD of lognormal shape instead of the gamma DSD used by the Thompson–Eidhammer parameterization and 2) enhancement of the cloud-to-rain autoconversion parameterization. Evaluation of RCON is performed for two warm rain events and an extended period during the Olympic Mountains Experiment (OLYMPEX) field campaign of winter 2015/16. We show that RCON modifications produce more realistic precipitation distributions and rain DSDs than the default Thompson–Eidhammer configuration. For the multimonth OLYMPEX period, we show that rain rates, rainwater mixing ratios, and raindrop number concentrations were increased relative to the Thompson–Eidhammer microphysical parameterization, while concurrently decreasing raindrop diameters in liquid-phase clouds. These changes are consistent with an increase in simulated warm rain. Finally, real-time evaluation of the scheme from August 2021 to August 2022 demonstrated improved precipitation prediction over coastal areas of the Pacific Northwest.

Significance Statement

Although the accurate simulation of warm rain is critical to forecasting the hydrology of coastal areas and windward slopes, many warm rain parameterizations underpredict precipitation in these locations. This study introduces and evaluates modifications to the Thompson–Eidhammer microphysics parameterization scheme that significantly improve the accuracy of rainfall prediction in those regions.

Restricted access
Robert Conrick
,
Joseph P. Boomgard-Zagrodnik
, and
Lynn A. McMurdie

Abstract

Midlatitude cyclones approaching coastal mountain ranges experience flow modifications on a variety of scales including orographic lift, blocking, mountain waves, and valley flows. During the 2015/16 Olympic Mountain Experiment (OLYMPEX), a pair of scanning ground radars observed precipitating clouds as they were modified by these orographically induced flows. The DOW radar, positioned to scan up the windward Quinault Valley, conducted RHI scans during 285 h of precipitation, 80% of which contained reversed, down-valley flow at lower levels. The existence of down-valley flow in the Quinault Valley was found to be well correlated with upstream flow blocking and the large-scale sea level pressure gradient orientated down the valley. Deep down-valley flow occurred in environments with high moist static stability and southerly winds, conditions that are common in prefrontal sectors of midlatitude cyclones in the coastal Pacific Northwest. Finally, a case study of prolonged down-valley flow in a prefrontal storm sector was simulated to investigate whether latent heat absorption (cooling) contributed to the event. Three experiments were conducted: a Control simulation and two simulations where the temperature tendencies from melting and evaporation were separately turned off. Results indicated that evaporative cooling had a stronger impact on the event’s down-valley flow than melting, likely because evaporation occurred within the low-level down-valley flow layer. Through these experiments, we show that evaporation helped prolong down-valley flow for several hours past the time of the event’s warm frontal passage.

Significance Statement

This paper analyzes the characteristics of down-valley flow over the windward Quinault Valley on the Olympic Peninsula of Washington State using data from OLYMPEX, with an emphasis on regional pressure differences and blocking metrics. Results demonstrate that the location of precipitation over the Olympic Peninsula is shifted upstream during events with deep down-valley flow, consistent with blocked upstream airflow. A case study of down-valley flow highlights the role of evaporative cooling to prolong the flow reversal.

Open access
Brenda Dolan
,
Steven A. Rutledge
, and
Kristen L. Rasmussen

Abstract

Orographic precipitation results from complex interactions between terrain, large-scale flow, turbulent motions, and microphysical processes. This study appeals to polarimetric radar data in conjunction with surface-based disdrometer observations, airborne particle probes, and reanalysis data to study these processes and their interactions as observed during the Olympic Mountain Experiment (OLYMPEX). Radar and disdrometer observations from OLYMPEX, which was conducted over the Olympic Peninsula in the winter of 2015, revealed 3 times as much rain fell over elevated sites compared to those along the ocean and coast. Several events were marked by significant water vapor transport and strong onshore flow. Detailed analysis of four cases demonstrated that the warm sector, which previous authors noted to be a period of strong orographic enhancement over the terrain, is associated not only with deeper warm cloud regions, but also deeper cold cloud regions, with the latter supporting the growth of dendritic ice crystals between 4 and 6 km. This dendritic growth promotes enhanced aggregation just above the melting layer, which then seeds the warm cloud layer below, allowing additional drop growth via coalescence. Periods of subsynoptic variability associated with mesoscale boundaries and low-level jets are shown to locally modify both the ice microphysics as well as surface drop-size distributions. This study explores the spatial and temporal variability of precipitation, cloud microphysics, and their relationship over the complex terrain of the Olympic Peninsula.

Significance Statement

This study appeals to polarimetric radar, aircraft particle probes, disdrometer data, and reanalysis to investigate the complex interactions between large frontal systems, terrain, and microphysical processes contributing to precipitation characteristics at the surface over the Olympic Peninsula. The study finds that the precipitation is a complex function of the synoptic regime, distance inland, and terrain height. Ice microphysical processes aloft act to modulate the surface rain drop size distributions, and are more important in contributing to higher rain accumulations inland during the later phases of the warm sector, particularly over the middle terrain heights (100–500 m).

Full access
Andrew DeLaFrance
,
Lynn McMurdie
, and
Angela Rowe

Abstract

Over mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (Z H) above the melting level bright band (i.e., a secondary Z H maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher Z H values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 1–2 and 3 December 2015 orographically enhanced precipitation events. Above the secondary Z H maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary Z H maximum, there are high concentrations of large (>~2-mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low, mid-, and high elevations, secondary Z H maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary Z H maximum. This result suggests that precipitation originating from secondary Z H maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout.

Full access
Joseph P. Zagrodnik
,
Lynn McMurdie
, and
Robert Conrick

Abstract

High-resolution numerical model simulations of six different cases during the 2015/16 Olympic Mountains Experiment (OLYMPEX) are used to examine dynamic and microphysical precipitation processes on both the full barrier-scale and smaller sub-barrier-scale ridges and valleys. The degree to which stratiform precipitation within midlatitude cyclones is modified over the coastal Olympic Mountains range was found to be strongly dependent on the synoptic environment within a cyclone’s prefrontal and warm sectors. In prefrontal sectors, barrier-scale ascent over stably stratified flow resulted in enhanced ice production aloft at the coast and generally upstream of higher terrain. At low levels, stable flow orientated transverse to sub-barrier-scale windward ridges generated small-scale mountain waves, which failed to produce enough cloud water to appreciably enhance precipitation on the scale of the windward ridges. In moist-neutral warm sectors, the upstream side of the barrier exhibited broad ascent oriented along the windward ridges with lesser regions of adjacent downward motion. Significant quantities of cloud water were produced over coastal foothills with further production of cloud water on the lower-windward slopes. Ice production above the melting layer occurred directly over the barrier where the ice particles were further advected downstream by cross-barrier winds and spilled over into the lee. The coastal foothills were found to be essential for the production and maintenance of cloud water upstream of the primary topographic barrier, allowing additional time for hydrometeors to grow to precipitation size by autoconversion and collection before falling out on the lower-windward slopes.

Open access
Ousmane O. Sy
,
Simone Tanelli
,
Stephen L. Durden
,
Andrew Heymsfield
,
Aaron Bansemer
,
Kwo-Sen Kuo
,
Noppasin Niamsuwan
,
Robert M. Beauchamp
,
V. Chandrasekar
,
Manuel Vega
, and
Michael P. Johnson

Abstract

This article illustrates how multifrequency radar observations can refine the mass–size parameterization of frozen hydrometeors in scattering models and improve the correlation between the radar observations and in situ measurements of microphysical properties of ice and snow. The data presented in this article were collected during the GPM Cold Season Precipitation Experiment (GCPEx) (2012) and Olympic Mountain Experiment (OLYMPEx) (2015) field campaigns, where the true mass–size relationship was not measured. Starting from size and shape distributions of ice particles measured in situ, scattering models are used to simulate an ensemble of reflectivity factors for various assumed mass–size parameterizations (MSP) of the power-law type. This ensemble is then collocated to airborne and ground-based radar observations, and the MSPs are refined by retaining only those that reproduce the radar observations to a prescribed level of accuracy. A versatile “retrieval dashboard” is built to jointly analyze the optimal MSPs and associated retrievals. The analysis shows that the optimality of an MSP depends on the physical assumptions made in the scattering simulators. This work confirms also the existence of a relationship between parameters of the optimal MSPs. Through the MSP optimization, the retrievals of ice water content M and mean diameter D m seem robust to the change in meteorological regime (between GCPEx and OLYMPEx); whereas the retrieval of the diameter spread S m seems more campaign dependent.

Free access
Aaron R. Naeger
,
Brian A. Colle
,
Na Zhou
, and
Andrew Molthan

Abstract

Field observations from the Olympic Mountain Experiment (OLYMPEX) around western Washington State during two atmospheric river (AR) events in November 2015 were used to evaluate several bulk microphysical parameterizations (BMPs) within the Weather Research and Forecasting (WRF) Model. These AR events were characterized by a prefrontal period of stable, terrain-blocked flow with an abundance of cold rain over the lowland region followed by less stable, unblocked flow with more warm rain, and a shift in the largest precipitation amounts to over the windward Olympic slopes. Our WRF simulations underpredicted the precipitation by 19%–36% in the Morrison (MORR) and Thompson (THOM) BMPs and 10%–23% in the predicted particle properties (P3) BMP, with the largest underpredictions over the windward slopes during the more convective, unblocked flow conditions. Several important processes related to the BMPs led to the differences in simulated precipitation. First, the prognostic single ice category parameterization in the P3 scheme promoted a more realistic evolution of rimed particles and larger cold rain production, which led to the lowest underpredictions in precipitation among the schemes. Second, efficient melting processes associated with the production of nonspherical ice and snow in the P3 and THOM BMPs, respectively, promoted a more realistic transition to rain fall speeds within the warm layer compared to the spherical snow assumption in MORR. Last, all BMPs underpredict the contribution of warm rain processes to the surface precipitation, particularly during the unblocked flow period, which may be partly explained by too weak condensational and collisional growth processes due to the neglect of turbulence parameterizations within the schemes.

Free access
Ali Tokay
,
Leo Pio D’Adderio
,
David A. Marks
,
Jason L. Pippitt
,
David B. Wolff
, and
Walter A. Petersen

Abstract

The ground-based-radar-derived raindrop size distribution (DSD) parameters—mass-weighted drop diameter Dmass and normalized intercept parameter NW—are the sole resource for direct validation of the National Aeronautics and Space Administration (NASA) Global Precipitation Measurement (GPM) mission Core Observatory satellite-based retrieved DSD. Both Dmass and NW are obtained from radar-measured reflectivity ZH and differential reflectivity ZDR through empirical relationships. This study uses existing relationships that were determined for the GPM ground validation (GV) program and directly compares the NASA S-band polarimetric radar (NPOL) observables of ZH and ZDR and derived Dmass and NW with those calculated by two-dimensional video disdrometer (2DVD). The joint NPOL and 2DVD datasets were acquired during three GPM GV field campaigns conducted in eastern Iowa, southern Appalachia, and western Washington State. The comparative study quantifies the level of agreement for ZH, ZDR, Dmass, and log(NW) at an optimum distance (15–40 km) from the radar as well as at distances greater than 60 km from radar and over mountainous terrain. Interestingly, roughly 10%–15% of the NPOL ZHZDR pairs were well outside the envelope of 2DVD-estimated ZHZDR pairs. The exclusion of these pairs improved the comparisons noticeably.

Free access
Robert Conrick
,
Joseph P. Zagrodnik
, and
Clifford F. Mass

Abstract

Radar retrievals of drop size distribution (DSD) parameters are developed and evaluated over the mountainous Olympic Peninsula of Washington State. The observations used to develop retrievals were collected during the 2015/16 Olympic Mountain Experiment (OLYMPEX) and included the NASA S-band dual-polarimetric (NPOL) radar and a collection of second-generation Particle Size and Velocity (PARSIVEL2) disdrometers over the windward slopes of the barrier. Nonlinear and random forest regressions are applied to the PARSIVEL2 data to develop retrievals for median volume diameter, liquid water content, and rain rate. Improvement in DSD retrieval accuracy, defined by the mean error of the retrieval relative to PARSIVEL2 observations, was achieved when using the random forest model when compared with nonlinear regression. Evaluation of disdrometer observations and the retrievals from NPOL indicate that the radar retrievals can accurately reproduce observed DSDs in this region, including the common wintertime regime of small but numerous raindrops that is important there. NPOL retrievals during the OLYMPEX period are further evaluated using two-dimensional video disdrometers (2DVD) and vertically pointing Micro Rain Radars. Results indicate that radar retrievals using random forests may be skillful in capturing DSD characteristics in the lowest portions of the atmosphere.

Free access
Yagmur Derin
,
Emmanouil Anagnostou
,
Marios Anagnostou
, and
John Kalogiros

Abstract

The difficulty of representing high rainfall variability over mountainous areas using ground-based sensors is an open problem in hydrometeorology. Observations from locally deployed dual-polarization X-band radar have the advantage of providing multiparameter measurements near ground that carry significant information useful for estimating drop size distribution (DSD) and surface rainfall rate. Although these measurements are at fine spatiotemporal scale and are less inhibited by complex topography than operational radar network observations, uncertainties in their estimates necessitate error characterization based upon in situ measurements. During November 2015–February 2016, a dual-polarized Doppler on Wheels (DOW) X-band radar was deployed on the Olympic Peninsula of Washington State as part of NASA’s Olympic Mountain Experiment (OLYMPEX). In this study, rain gauges and disdrometers from a dense network positioned within 40 km of DOW are used to evaluate the self-consistency and accuracy of the attenuation and brightband/vertical profile corrections, and rain microphysics estimation by SCOP-ME, an algorithm that uses optimal parameterization and best-fitted functions of specific attenuation coefficients and DSD parameters with radar polarimetric measurements. In addition, the SCOP-ME precipitation microphysical retrievals of median volume diameter D 0 and normalized intercept parameter N W are evaluated against corresponding parameters derived from the in situ disdrometer spectra observations.

Full access