Abstract Orography has a dramatic influence on cool-season precipitation, building an upper-elevation snowpack for regional water resources and winter tourism and modulating winter storms that impact transportation and public safety. Using K-band profiling Micro Rain Radar (MRR) observations from the Wasatch Mountains (2682 m MSL) and the adjoining Salt Lake Valley (1372 m MSL), this paper investigates the characteristics of cool-season storms in a continental mountain environment of the eastern Great Basin of North America. Compared to the valley site, the mountain site observed a higher frequency of low-level reflectivity ≥ 5 dB Z e , consistent with greater precipitation frequency due to orographic enhancement. Echoes were deepest during southerly or southwesterly flow with high integrated vapor transport and the passage of cold fronts or baroclinic troughs and shallowest during northwesterly flow postcold-frontal periods when median −10-dB Z e echo tops were only 1260 m AGL. During postcold-frontal periods, reflectivities most often increased with decreasing height near the ground at the mountain site, whereas other storm types featured nearly equal frequencies of increasing and decreasing reflectivity with decreasing height. Due to presumed subcloud sublimation and evaporation, postcold-frontal periods at the valley site featured decreasing modal reflectivity with decreasing height near the ground. These results illustrate important contrasts in precipitation growth and loss processes between mountain and lowland sites, where subcloud sublimation and evaporation represent underappreciated mechanisms for reducing lowland precipitation and enhancing the orographic precipitation gradient. Operational and spaceborne precipitation radars likely inadequately sample these near-surface processes in continental mountain environments. Significance Statement Mountains strongly affect winter storms, complicating forecasting and enabling the development of an upper-elevation snowpack that is vital for water resources and winter tourism. This research shows that contrasts in valley and mountain precipitation in drier, continental mountain environments are strongly influenced by shallow processes near the ground, which are poorly sampled by conventional weather radars and satellites. In the mountains, precipitation growth can occur near the ground, especially in postcold-frontal environments, whereas valley precipitation can decrease as it falls through a dry layer. The latter is an underappreciated contributor to the mean increase of precipitation with elevation found in continental mountain environments.

Predicting the accurate type of precipitation during winter storms is crucial for the implementation of mitigation measures such as aircraft deicing in commercial aviation or the spreading of salt and abrasives on roads. For this reason, a better understanding of the microphysical processes leading to winter precipitation types is essential. During freezing rain events, secondary ice produced by the freezing of supercooled raindrops via the fragmentation of freezing drops (FFD) process can initiate a chain reaction, potentially transitioning freezing rain into ice pellets. However, including this process in numerical weather prediction models is challenging due to the uncertainty in the efficiency of this mechanism. To bridge this gap, this study aims to evaluate the efficiency of the FFD process during ice pellet precipitation using measurements collected onboard the NRC Convair-580 research aircraft during the WINTRE-MIX field campaign, in February 2022. Specifically, measurements from two missed-approaches conducted in the Saint Lawrence Valley, Quebec, Canada during an ice pellet storm are analyzed. These missed-approaches provide unique datasets collected above, within, and below the ice pellet freezing altitude using in-situ and remote sensing instruments. In the region characterized by completely frozen ice pellets, a bi-modal particle size distribution, indicative of secondary ice production, was measured. Observations from imaging and optical-array probes suggest that particles smaller than 200 µm in diameter were, likely, non-spherical ice crystals, whereas the particle size mode with the larger diameters was associated with ice pellets. The observations of fractured ice pellets and ice pellets with bulges and spicules on most large particles suggested the occurrence of the FFD process. Subsequently, the measured number concentration of small ice particles, which was of the order of 500 L-1, was compared with the number concentration of ice particles simulated through existing parametrizations of secondary ice production. This analysis will be valuable for selecting the most accurate FFD process parametrization to use for freezing rain and ice pellets simulation.

This study characterizes the size and shape distributions of 10 µm to 6 mm diameter particles observed during six penetrations of wildfire-induced pyroconvection near Boise, Idaho, USA, by a research aircraft over the period 29–30 August 2016. In situ measurements by the aircraft include winds, atmospheric state, and bulk water content and particle concentration, size, and shape. These observations are complemented by data from airborne and ground-based radars. One of the penetrations is through a subsaturated smoke–ash plume with negligible cloud liquid water content that is characterized by an updraft of almost 36 m s−1. The size distribution of number concentration is very similar to that documented previously for a smoke plume from a prescribed fire, and particle shapes exhibit qualitative and quantitative attributes comparable to ash particles created in a burn chamber. Particles sampled during this penetration are most likely pyrometeors composed of ash. Pyrocumulus clouds are probed in the other penetrations where values of relative humidity and cloud liquid water content are larger, but updrafts are weaker. Compared to the smoke-plume penetration, size distributions are mostly characterized by larger concentration, and particle shapes exhibit a higher degree of circular symmetry. Particle composition in these pyrocumulus penetrations is most likely a combination of hydrometeors (ice particles) and pyrometeors (ash).

During near-0 & DEG;C surface conditions, diverse precipitation types (p-types) are possible, including rain, drizzle, freezing rain, freezing drizzle, ice pellets, wet snow, snow, and snow pellets. Near-0 & DEG;C precipitation affects wide swaths of the United States and Canada, impacting aviation, road transportation, power generation and distribution, winter recreation, ecology, and hydrology. Fundamental challenges remain in observing, diagnosing, simulating, and forecasting near-0 & DEG;C p-types, particularly during transitions and within complex terrain. Motivated by these challenges, the field phase of the Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX) was conducted from 1 February to 15 March 2022 to better understand how multiscale processes influence the variability and predictability of p-type and amount under near-0 & DEG;C surface conditions. WINTRE-MIX took place near the U.S.-Canadian border, in northern New York and southern Quebec, a region with plentiful near-0 & DEG;C precipitation influenced by terrain. During WINTRE-MIX, existing advanced mesonets in New York and Quebec were complemented by deployment of 1) surface instruments, 2) the National Research Council Convair-580 research aircraft with W-and X-band Doppler radars and in situ cloud and aerosol instrumentation, 3) two X-band dual-polarization Doppler radars and a C-band dual-polarization Doppler radar from the University of Illinois, and 4) teams collecting manual hydrometeor observations and radiosonde measurements. Eleven intensive observing periods (IOPs) were coordinated. Analysis of these WINTRE-MIX IOPs is illuminating how synoptic dynamics, mesoscale dynamics, and microscale processes combine to determine p-type and its predictability under near-0 & DEG;C conditions. WINTRE-MIX research will contribute to improving nowcasts and forecasts of near-0 & DEG;C precipitation through evaluation and refinement of observational diagnostics and numerical forecast models.

Surface, upper-air, and radar observations are used to assess the performance of the Antarctic Mesoscale Prediction System (AMPS) in simulating the mesoscale aspects of a wind and precipitation event over the Ross Island region of Antarctica that spanned January 16-20, 2016. The observations, collected during the Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE), provide a unique dataset for evaluating AMPS, especially the radar observations that facilitate a three-dimensional depiction of winds and precipitation. Comparisons of AMPS forecast data with surface meteorology, balloon-sounding, and profiling radar observations at and above sites near McMurdo Station reveal a mixture of similarities and differences. A generally southerly flow is evident at low levels in both the AMPS simulations and observed Doppler radial velocities. AMPS winds are comparable to those observed at the surface and aloft in terms of magnitude, direction, and timing but the strongest simulated southerly flow is displaced eastward relative to the observations. AMPS-simulated reflectivity over the broader Ross Island region is more limited in areal extent and smaller in magnitude than observed by a scanning Doppler radar. Three episodes of surface precipitation are observed near McMurdo Station over the five-day event with peak rates of similar to 3 mm h(-1) and a total accumulation of similar to 22 mm. However, AMPS produces no surface precipitation at that location over the five-day event due to a low-level dry bias in the forecasts. The results show the first observationally based three-dimensional understanding of meteorology in the Ross Island region.

Airborne cloud radar reveals extreme wildfire updrafts (~60 m s −1 ) and downdrafts (~30 m s −1 ) rivaling those in supercell thunderstorms. These extreme vertical velocities occur through a 3‐km‐deep layer and below the base of a developing pyrocumulonimbus (pyroCb) cloud, which extends to the tropopause at 12 km. In situ aircraft sampling shows updrafts are linked to large temperature and moisture excesses but remain subsaturated at flight level (i.e., below cloud base). Parcel estimates using the in situ data help explain how these “hot‐moist” updrafts trigger the overlying pyroCb. The extreme vertical motions observed also pose a previously undocumented aviation hazard.

The Chilean Coastal Orographic Precipitation Experiment (CCOPE) was a 3-month field campaign (June, July and August 2015) that investigated wintertime coastal rain events. Reported here are analyses of aerosol measurements made at a coastal site during CCOPE. The aerosol monitoring site was located near Arauco, Chile. Aerosol number concentrations and aerosol size distributions were acquired with a condensation particle counter (CPC) and an ultra high sensitivity aerosol spectrometer (UHSAS). Arauco CPC data were compared to values measured at the NOAA observatory Trinidad Head (THD) on the northern Pacific coast of California. The winter-averaged CPC concentration at Arauco is 2971 ± 1802 cm−3; at THD the average is 1059 ± 855 cm−3. Despite the typically more pristine South Pacific region, the Arauco average is larger than at THD (p<0.01). Aerosol size distributions acquired during episodes of onshore flow were analyzed with Köhler theory and used to parameterize cloud condensation nuclei activation spectra. In addition, sea salt aerosol (SSA) concentration was parameterized as a function of sea surface wind speed. It is anticipated these parameterizations will be applied in modeling of wintertime Chilean coastal precipitation.

This study documents the mean properties and variability of kinematic and precipitation structures associated with orographic precipitation along the coast of Northern California in the context of terrain-trapped airflows (TTAs). TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. Seven land-falling winter storms are examined with observations from a scanning X-band Doppler radar deployed on the coast at Fort Ross, California. Additional information is provided by a 915-MHz wind-profiling radar, surface meteorology, a GPS receiver, and balloon soundings. The composite kinematic structure during TTA conditions exhibits a significant horizontal gradient of wind direction from the coast to approximately 50 km offshore and a low-level jet (LLJ) that surmounts a weaker airflow offshore corresponding to the TTA, with a zone of enhanced precipitation evident between ~5 and 25 km offshore and oriented nearly parallel to the coastline. Conversely, the composite kinematic structure during NO-TTA conditions exhibits a smaller offshore horizontal gradient of wind direction and precipitation structures are generally enhanced within km of the coastline. Interstorm variability analysis reveals significant variations in kinematic structures during both TTA and NO-TTA conditions, whereas significant variations in precipitation structures are only evident during TTA conditions. The interstorm analysis also illustrates more clearly how LLJ vertical structures evident during NO-TTA conditions exhibit ascent along the coast and over the coastal mountains, which is in contrast to TTA conditions where the ascent occurs offshore and over the TTA.

Analysis of WRF Model output from experiments using two double-moment microphysics schemes is carried out to demonstrate that there can be an inconsistency between the predicted mass and number concentrations when a single-moment convective parameterization is used together with a double-moment microphysics scheme. This inconsistency may arise because the grid-scale and subgrid-scale cloud schemes generally apply different levels of complexity to the parameterized microphysical processes. In particular, when a multimoment formulation is used in the microphysics scheme and other physical parameterizations modify only the mass-related moment while the values of the second (or higher) moment for individual hydrometeors remain unchanged, an unintended modification of the particle size distribution occurs. Simulated radar reflectivity is shown to be a valuable tool in diagnosing this inconsistency. In addition, potential ways to minimize the problem are explored by including number concentration calculations in the cumulus parameterization that are consistent with the assumptions of hydrometeor sizes in the microphysics parameterization. The results of this study indicate that it is physically preferable to unify microphysical assumptions between the grid-resolved and subgrid cloud parameterization schemes in weather and climate simulation models.

The Rapid Deployments to Wildfires Experiment (RaDFIRE) was a meteorological field campaign aimed at observing fire–atmosphere interactions during active wildfires. Using a rapidly deployable scanning Doppler lidar, airborne Doppler radar, and a suite of other instruments, the field campaign sampled 21 wildfires from 2013 to 2016 in the western United States. Observations include rotating convective plumes, plume interactions with stable layers and multilayered smoke detrainment, convective plume entrainment processes, smoke-induced density currents, and aircraft in situ observations of developing pyrocumulus. Collectively, these RaDFIRE observations highlight the range of meteorological phenomena associated with wildfires, especially plume dynamics, and will provide a valuable dataset for the modeling community.

This study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320 degrees-140 degrees) and altitude of nearby topography, mean wind direction in the lowest 500m MSL ((WDIR) over bar (500)) between 0 degrees-140 degrees is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between (WDIR) over bar (500) and orographic rainfall reveals that an upper threshold of 150 degrees more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to 0 degrees <= (WDIR) over bar (500), 150 degrees with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.

The Chilean Coastal Orographic Precipitation Experiment (CCOPE) was conducted during the austral winter of 2015 (May-August) in the Nahuelbuta Mountains (peak elevation 1.3 km MSL) of southern Chile (388S). CCOPE used soundings, two profiling Micro Rain Radars, a Parsivel disdrometer, and a rain gauge network to characterize warm and ice-initiated rain regimes and explore their consequences for orographic precipitation. Thirty-three percent of foothill rainfall fell during warm rain periods, while 50% of rainfall fell during ice-initiated periods. Warmrain drop size distributions were characterized by many more and relatively smaller drops than ice-initiated drop size distributions. Both the portion and properties of warm and iceinitiated rainfall compare favorably with observations of coastal mountain rainfall at a similar latitude in California. Orographic enhancement is consistently strong for rain of both types, suggesting that seeding from ice aloft is not a requisite for large orographic enhancement. While the data suggest that orographic enhancement may be greater during warm rain regimes, the difference in orographic enhancement between regimes is not significant. Sounding launches indicate that differences in orographic enhancement are not easily explainable by differences in low-level moisture flux or nondimensional mountain height between the regimes.

Dual-polarization scanning radar measurements, air temperature soundings, and a polarimetric radarbased particle identification scheme are used to generate maps and probability density functions (PDFs) of the ice water path (IWP) in Hurricanes Arthur (2014) and Irene (2011) at landfall. The IWP is separated into the contribution from small ice (i. e., ice crystals), termed small-particle IWP, and large ice (i. e., graupel and snow), termed large-particle IWP. Vertically profiling radar data from Hurricane Arthur suggest that the small ice particles detected by the scanning radar have fall velocities mostly greater than 0.25ms 21 and that the particle identification scheme is capable of distinguishing between small and large ice particles in a mean sense. The IWP maps and PDFs reveal that the total and large-particle IWPs range up to 10 kgm 22, with the largest values confined to intense convective precipitation within the rainbands and eyewall. Small-particle IWP remains mostly,4 kgm 22, with the largest small-particle IWP values collocated with maxima in the total IWP. PDFs of the small-to-total IWP ratio have shapes that depend on the precipitation type (i. e., intense convective, stratiform, or weak-echo precipitation). The IWP ratio distribution is narrowest (broadest) in intense convective (weak echo) precipitation and peaks at a ratio of about 0.1 (0.3).

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 similar to 16%, similar to 5%, and similar to 19% larger, respectively, for NBB rain compared to BB rain, while mean CZC rain rate is similar to 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.

This study characterizes mountain waves and orographic precipitation associated with a winter storm passing over the ∼3.5 km above mean sea level (MSL) Park Range of northern Colorado on 15 December 2010. Observations from an airborne vertically pointing Doppler radar are used to document reflectivity and horizontal and vertical velocity in 13 two‐dimensional vertical planes extending across the Park Range from upstream of the windward slope, over the crest and downstream of the lee slope. The winter storm investigated in this study is associated with a general zonal flow over the western continental USA and significant vertical wind shear between 700 and 500 hPa. A vertically propagating wave forced by the Park Range is most evident above 4 km MSL and associated with relatively wide, upstream‐tilted updraughts and downdraughts located above the Park Range windward and lee slopes, respectively. The Park Range also forces a trapped lee wave that manifests itself as a relatively erect updraught ∼15–20 km east of the crest. Smaller‐scale trapped lee waves forced by terrain upstream of the Park Range are evident below 4 km MSL and associated with rotor circulations composed of relatively narrow updraughts and downdraughts located above the Yampa Valley and the Park Range windward slope. A ∼1 km thick layer of strong vertical shear exists between the mountain waves forced by the Park Range and those forced by upstream terrain. This shear layer exhibits a large vertical displacement over the Park Range, with relatively strong westerly winds plunging to low levels over the lee slope. While precipitation on the Park Range windward slope is generally enhanced for the event, data analysed for this case surprisingly does not show a spatially and temporally consistent correlation between mountain‐wave kinematic structures and orographic precipitation. Transient processes such as wave‐regime interactions may have masked this correlation.

This study documents orographic precipitation forcing along the coastal mountains of Northern California during the landfall of a significant winter storm over the period 16-18 February 2004. The primary observing asset is a scanning X-band Doppler radar deployed on the coast at Fort Ross, California, which provides low-level (e.g., below 1 km MSL) horizontal and vertical scans of radial velocity and reflectivity to characterize airflow and precipitation structures. Further context is provided by a wind-profiling radar, a radio acoustic sounding system (RASS), balloon soundings, buoys, a GPS receiver, and surface meteorological sensors. The winter storm is divided into two episodes, each having pre-cold-frontal low-level jet (LLJ) structures and atmospheric river characteristics. Episode 1 has a corridor of terrain-trapped airflow (TTA) that forms an interface with the LLJ. The interface extends similar to 25 km offshore in a similar to 0.5-km vertical layer, and the western edge of this interface near the ocean surface advances toward the coast over the course of similar to 5 h. The TTA acts as a dynamically driven barrier, so that the incoming LLJ slopes upward offshore below 1.5 km MSL and precipitation is enhanced over the ocean and near the coast. The absence of a TTA in episode 2 allows the cross-barrier flow to slope upward and enhance precipitation directly over the coastal mountains. A theoretical analysis favors the hypothesis that a gap flow exiting the Petaluma Gap forces the TTA.