Cold-frontal Rainband Research Articles

Application of TRMM PR and TMI Measurements to Assess Cloud Microphysical Schemes in the MM5 for a Winter Storm

AbstractThis paper uses observations from Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and microwave imager (TMI) to evaluate the cloud microphysical schemes in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5; version 3.7.4) for a wintertime frontal precipitation system over the eastern Pacific Ocean. By incorporating a forward radiative transfer model, the radar reflectivity and brightness temperatures are simulated and compared with the observations at PR and TMI frequencies. The main purpose of this study is to identify key differences among the five schemes [including Simple ice, Reisner1, Reisner2, Schultz, and Goddard Space Flight Center (GSFC) microphysics scheme] in the MM5 that may lead to significant departures of simulated precipitation properties from both active (PR) and passive (TMI) microwave observations. Radiative properties, including radar reflectivity, attenuation, and scattering in precipitation liquid and ice layers are investigated. In the rain layer, most schemes are capable of reproducing the observed radiative properties to a reasonable degree; the Reisner2 simulation, however, produces weaker reflectivity and stronger attenuation than the observations, which is possibly attributable to the larger intercept parameter (N0r) applied in this run. In the precipitation ice layer, strong evidence regarding the differences in the microphysical and radiative properties between a narrow cold-frontal rainband (NCFR) and a wide cold-frontal rainband (WCFR) within this frontal precipitation system is found. The performances of these schemes vary significantly on simulating the microphysical and radiative properties of the frontal rainband. The GSFC scheme shows the least bias, while the Reisner1 scheme has the largest bias in the reflectivity comparison. It appears more challenging for the model to replicate the scattering signatures obtained by the passive sensor (TMI). Despite the common problem of excessive scattering in the WCFR (stratiform precipitation) region in every simulation, the magnitude of the scattering maximum seems better represented in the Reisner2 scheme. The different types of precipitation ice, snow, and graupel are found to behave differently in the relationship of scattering versus reflectivity. The determinative role of the precipitation ice particle size distribution (intercept parameters) is extensively discussed through sensitivity tests and a single-layer radiative transfer model.

Alongfront Variability of Precipitation Associated with a Midlatitude Frontal Zone: TRMM Observations and MM5 Simulation

Abstract On 19 February 2001, the Tropical Rainfall Measuring Mission (TRMM) satellite observed complex alongfront variability in the precipitation structure of an intense cold-frontal rainband. The TRMM Microwave Imager brightness temperatures suggested that, compared to the northern and southern ends of the rainband, a greater amount of precipitation ice was concentrated in the middle portion of the rainband where the front bowed out. A model simulation conducted using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is examined to explain the distribution of precipitation associated with the cold-frontal rainband. The simulation reveals that the enhanced precipitation ice production and the implied mean ascent along the central part of the front were associated with a synergistic interaction between a low-level front and an upper-level front associated with an intrusion of high-PV stratospheric air. The low-level front contributed to an intense bow-shaped narrow cold-frontal rainband (NCFR). The upper-level front was dynamically active only along the central to northern portion of the NCFR, where the upper-level PV advection and Q-vector convergence were most prominent. The enhanced mean ascent associated with the upper-level front contributed to a wide cold-frontal rainband (WCFR) that trailed or overlapped with the NCFR along its central to northern segments. Because of the combination of the forcing from both lower- and upper-level fronts, the ascent was deepest and most intense along the central portion of the front. Thus, a large concentration of precipitation ice, attributed to both the NCFR and WCFR, was produced.

Morphology and evolution of cold‐frontal misocyclones

AbstractA numerical simulation of an intense, narrow cold‐frontal rain band (NCFR) is presented. This front was associated with localized wind damage as it moved eastwards across the United Kingdom on 24 September 2007. The model used was the operational and research Weather Research and Forecasting‐‐Advanced Research (WRF‐‐ARW) mesoscale model, initialized with Global Forecast System (GFS) operational model output data. The simulation produced a front strongly resembling that seen in radar imagery and conforming to previous studies and conceptual models of the ana‐cold front. In the simulation, a strong surface horizontal wind‐speed maximum is located along the southern flank of a misocyclone; this is shown to be typical of a number that form along the front. Using trajectory analysis, descending cold air parcels, which originate in the rear inflow jet and accelerate within the circulation of the misocyclone, are identified as the origin of the strongest winds. In addition to the localized strong straight‐line winds, circumstantial evidence is presented for the possible occurrence of weak tornadogenesis. The results are discussed in relation to recent studies of NCFRs, squall lines and misocyclones. Copyright © 2009 Royal Meteorological Society

Vertical Structure of Misocyclones along a Narrow Cold Frontal Rainband

Five misocyclones occurring along a narrow cold frontal rainband (NCFR) were detected by single, X-band Doppler radar at Yokosuka, Japan on April 20, 2006. Each of the misocyclones, which formed in succession in the core and gap regions of the meandering NCFR over a period of 30 minutes, had a short lifetime. Three of the five misocyclones generated near the surface and reached altitudes of up to 4 km ASL. The diameters of the lower-level misocyclones increased with altitude and vorticity in the order of 10 -2 s -1 was observed near the surface. Two of the five misocyclones had similar diameters and vorticity, and at more than 6 km ASL. One of the observed misocyclones was related to the tornado in Fujisawa and as a non-supercell type tornado within the NCFR. This tornado-related misocyclone had the largest vorticity (5 x 10- 2 s -1 ) near the surface (300 m ASL) and was characterized as having the reduced diameter below the cloud base, which is considered typical of a tornado.

The IMPROVE-1 Storm of 1–2 February 2001. Part IV: Precipitation Enhancement across the Melting Layer

Abstract Previous model simulations indicate that in stratiform precipitation, the precipitation rate can increase by 7% in the melting layer through direct condensation onto melting snow and the resultant cooled rain. In the present study, a model simulation of stratiform precipitation in a wide cold frontal rainband indicates that the precipitation rate can also increase by 5% in the melting layer through accretion, by melting snow and rain, of additional cloud water produced by the latent cooling of the ambient air associated with melting snow. The contribution of the combined processes, and therefore the additional precipitation gained through the latent cooling of melting snow within the melting layer, may contribute as much as 10% to the precipitation rate in stratiform precipitation.

Effects of Radar Beam Shielding on Rainfall Estimation for the Polarimetric C-Band Radar

Abstract Radar reflectivity (Zh), differential reflectivity (Zdr), and specific differential phase (Kdp) measured from the operational, polarimetric weather radar located in Trappes, France, were used to examine the effects of radar beam shielding on rainfall estimation. The objective of this study is to investigate the degree of immunity of Kdp-based rainfall estimates to beam shielding for C-band radar data during four typical rain events encountered in Europe. The rain events include two cold frontal rainbands with average rainfall rates of 7 and 17 mm h−1, respectively, and two summertime convective rain events with average rainfall rates of 11 and 22 mm h−1. The large effects of beam shielding on rainfall accumulation were observed for algorithms using Zh and Zdr with differences of up to ∼2 dB (40%) compared to a Kdp-based algorithm over a power loss range of 0–8 dB. This analysis reveals that Zdr and Kdp are not affected by partial beam shielding. Standard reflectivity corrections based on the degree of beam shielding would have overestimated rainfall rates by up to 1.5 dB for less than 40% beam shielding and up to 3 dB for beam shielding less than 75%. The investigation also examined the sensitivity of beam shielding effects on rainfall rate estimation to (i) axis–ratio parameterization and drop size distribution, (ii) methods used to smooth profiles of differential propagation phase (ϕdp) and estimate Kdp, and (iii) event-to-event variability. Although rainfall estimates were sensitive to drop size distribution and axis–ratio parameterization, differences between Zh- and Kdp-based rainfall rates increased independently from those parameters with amount of shielding. Different approaches to smoothing ϕdp profiles and estimating Kdp were examined and showed little impact on results.

The IMPROVE-1 Storm of 1–2 February 2001. Part III: Sensitivity of a Mesoscale Model Simulation to the Representation of Snow Particle Types and Testing of a Bulk Microphysical Scheme with Snow Habit Prediction

Abstract A mesoscale model simulation of a wide cold-frontal rainband observed in the Pacific Northwest during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-1) field study was used to test the sensitivity of the model-produced precipitation to varied representations of snow particles in a bulk microphysical scheme. Tests of sensitivity to snow habit type, by using empirical relationships for mass and velocity versus diameter, demonstrated the defectiveness of the conventional assumption of snow particles as constant density spheres. More realistic empirical mass–diameter relationships result in increased numbers of particles and shift the snow size distribution toward larger particles, leading to increased depositional growth of snow and decreased cloud water production. Use of realistic empirical mass–diameter relationships generally increased precipitation at the surface as the rainband interacted with the orography, with more limited increases occurring offshore. Changes in both the mass–diameter and velocity–diameter relationships significantly redistributed precipitation either windward or leeward when the rainband interacted with the mountain barrier. A method of predicting snow particle habit in a bulk microphysical scheme, and using predicted habit to dynamically determine snow properties in the scheme, was developed and tested. The scheme performed well at predicting the habits present (or not present) in aircraft observations of the rainband. Use of the scheme resulted in little change in the precipitation rate at the ground for the rainband offshore, but significantly increased precipitation when the rainband interacted with the windward slope of the Olympic Mountains. The study demonstrates the promise of the habit prediction approach to treating snow in bulk microphysical schemes.

Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

Abstract The mechanism responsible for the core-gap structure of precipitation along narrow cold-frontal rainbands (NCFRs) is investigated through analyses of idealized cloud-resolving simulations of cold fronts. The control simulation, in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy (CAPE) of ∼60 J kg−1, reproduces the typical alongfront variability of observed NCFRs. The simulated NCFR is broken up into regularly spaced, ellipsoidal cores oriented at a clockwise angle to the cold front. While horizontal-shear instability (HSI) has frequently been proposed as a mechanism leading to the alongfront variability of NCFRs, no characteristic features of HSI are recognized in the simulated vertical vorticity field at the leading edge of the cold front. The alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front. The wave phase lines are oriented nearly perpendicular to the direction of mean vertical shear, with enhanced (suppressed) precipitation occurring at the wave updrafts (downdrafts). An analysis of the eddy kinetic energy budget indicates that the wavelike disturbance derives most of its energy from the mean vertical shear and the buoyancy. Sensitivity experiments reveal a systematic relationship between the alongfront variability of NCFRs and the stability of the prefrontal thermal stratification. Simulated precipitation cores remain essentially parallel to the cold front when the prefrontal environment is absolutely stable or almost neutral to surface parcel ascent. The typical alongfront variability of NCFRs is reproduced for weakly unstable environments with small amounts of CAPE (≤140 J kg−1). On the other hand, simulations with sufficiently unstable environments produce precipitation cores oriented counterclockwise to cold fronts.

Characteristics of tornado damage in Fujisawa on April 20, 2006

At noon of April 20, 2006, wind disaster occurred in Fujisawa City, Kanagawa Prefecture. We surveyed the characteristics of this disaster during two days. Damage to over 40 residential buildings was observed and damage area extended a line of 2 km by 50 m. According to the feature of damage, we presumed that Fujita-Pearson-scale (FPP-scale) was F0-P1-P1. The reasons why the gust in the area was a tornado were as follows. 1) The damaged area lies in a narrow line. 2) Indication of rotatory (cyclonic) wind was observed. 3) The vortex with 2 km in diameter (misocyclone) was observed by a Doppler radar. 4) Pressure drop of 1.5 hPa was recorded near the damage area. The wind disaster in Fujisawa was caused by a tornado, which formed in a narrow cold frontal rainband.

Tornado Generation in a Narrow Cold Frontal Rainband -Fujisawa Tornado on April 20, 2006-

At noon on April 20, 2006, a wind storm occurred in Fujisawa City, Kanagawa Prefecture, Japan with a F0-P1-P1 rank on the Fujita-Pearson-scale (FPP-scale). Damage to over 40 residential buildings was observed. The damage covered an area 2 km by 50 m. The strong winds were accompanied by the passage of a cold front. Eyewitness accounts of a dust column and the recording of a pressure drop (1.5 hPa) indicate the existence of a tornado vortex. A hook-shaped radar echo (5 km in diameter) was observed in the narrow cold frontal rainband, with strong horizontal wind shear above the damage area. The Doppler velocity pattern indicated a misocyclone, which had a diameter of 2 km and a vorticity on the order of 10-2 s-1. The wind storm may have caused the tornado, which formed in the cold frontal wind shear zone.

The IMPROVE-1 Storm of 1–2 February 2001. Part II: Cloud Structures and the Growth of Precipitation

Abstract On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells. A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations. The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

The IMPROVE-1 Storm of 1–2 February 2001. Part I: Development of a Forward-Tilted Cold Front and a Warm Occlusion

Abstract Analysis of observations and the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to study the development of a forward-tilted cold front off the coast of Washington State. The vertical velocity associated with the cold front produced a wide cold-frontal rainband. In the early stage of development the midtropospheric baroclinic zone (or upper cold front) moved forward with time over the warm sector to produce a structure similar to a split front. The movement of the upper cold front was due to horizontal transport and frontogenetical propagation. The frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis, which resulted in a negative/positive couplet of frontogenesis straddling the baroclinic zone. The lower-tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion. In the initial occluding process the converging frontal zones tilted into a warm-type occlusion configuration due to the presence of a background vertical shear of the horizontal wind component perpendicular to the occluded front. Consequently, as the storm moved over the observing network, the occluded front had the structure of a warm occlusion (tilted forward) in the lower levels. Above the occlusion, the cold front was also tilted forward because it retained its split-front-like structure. Thus, the development of the split front and the warm occlusion were separate processes that occurred in sequence. Although the MM5 captured the basic forward tilt with time of the cold front, some key aspects of the midtropospheric frontal structure were not well simulated. Because diabatic heating was an important contributor to the maintenance and movement of the upper cold front, it is hypothesized that discrepancies in diabatic heating associated with deficiencies in the model’s explicit microphysical scheme may be responsible for deficiencies in reproducing the structure of the upper cold front.

Evolution of a Cold Front Encountering Steep Quasi-2D Terrain: Coordinated Aircraft Observations on 8–9 December 2001 during IMPROVE-2

Abstract Research aircraft observations from the 8–9 December 2001 case of the second phase of the Improvement of Microphysical Parameterization through Operational Verification Experiment (IMPROVE-2) describe the evolution of a wide cold-frontal rainband (WCFR) during its eastward advance from the Pacific coastline to a point 200 km inland over the Cascade Mountains of Oregon. This analysis has two objectives: first, to illustrate the rapid weakening of the circulation associated with a landfalling WCFR and the relationship of these changes to terrain-induced airflow modifications, and second, to quantify the degree to which this weakening impacted cloud microphysical properties such as liquid water content, ice particle concentrations, and precipitation rate. The kinematic structure of the WCFR is detailed using Doppler radar observations from a NOAA P-3 aircraft, while some concomitant cloud microphysical properties are documented using flight-level measurements from the University of Washington Convair-580 aircraft. An accompanying the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) control simulation (nested to a horizontal resolution of 4 km over the IMPROVE-2 domain) provides a mesosynoptic context and thermodynamic information to complement the aircraft observations. To the authors’ knowledge, this case study represents the most complete documentation obtained to date of the rapid modifications that may occur when a frontal rainband progresses from coastal waters into a region of prominent terrain.

Microphysical Processes and Synergistic Interaction between Frontal and Orographic Forcing of Precipitation during the 13 December 2001 IMPROVE-2 Event over the Oregon Cascades

Abstract On 13–14 December 2001 a vigorous cyclonic storm passed over the Pacific Northwest, producing heavy orographic precipitation over the Cascade Mountains. This storm was one of several studied during the second field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). A wide variety of in situ and remotely sensed measurements were obtained as this storm passed over the Oregon Cascades. These measurements provided a comprehensive dataset of meteorological state parameters (temperature, pressure, humidity, winds, and vertical air velocity), polarization Doppler radar measurements, and cloud microphysical parameters (cloud liquid water, particle concentrations, size spectra, and imagery). The 13–14 December case was characterized by the passage of a tipped-forward lower-tropospheric front that extended upward to a preceding vigorous upper cold-frontal rainband, which produced clouds up to ∼8–9 km. An important difference between this storm and those studied previously over the Washington Cascades was that the prefrontal low-level airflow over the Oregon Cascades was characterized by strong westerly (as opposed to weak easterly) cross-barrier flow. Consequently, as the upper cold-frontal band passed over the Oregon Cascades there was both strong ice particle production aloft and significant production of liquid water at lower levels in the orographic lifting zone. Airborne in situ measurements, ground-based microwave radiometer measurements, and observations of snow crystals showed the simultaneous presence of high ice crystal concentrations and relatively large values of cloud liquid water aloft, and heavily rimed particles reaching the ground. Analyses indicate that a synergistic interaction occurred between the frontal and orographic precipitation.

Variations Associated with Cores and Gaps of a Pacific Narrow Cold Frontal Rainband

Abstract A NOAA P-3 instrumented aircraft observed an intense, fast-moving narrow cold frontal rainband (NCFR) as it approached the California coast on 19 February 2001 during the Pacific Coastal Jets Experiment. Airborne Doppler radar data obtained while the frontal system was well offshore indicated that a narrow ribbon of very high radar reflectivity convective cores characterized the rainband at low levels with echo tops to ∼4–5 km, and pseudo-dual-Doppler analyses showed the low-level convergence of the prefrontal air. The NCFR consisted of gaps of weaker reflectivity and cores of stronger reflectivity along its length, perhaps as a result of hydrodynamic instability along its advancing leading edge. In contrast to some earlier studies of cold frontal rainbands, density-current theory described well the motion of the overall front. The character of the updraft structure along the NCFR varied systematically along the length of the precipitation cores and in the gap regions. The vertical shear of the c...

Numerical Simulations of a Landfalling Cold Front Observed during COAST: Rapid Evolution and Responsible Mechanisms*

This paper identifies mechanisms that led to the observed rapid evolution of a landfalling weak cold front along the steep mountainous northern California coast on 1 December 1995. This event was simulated down to 3-km horizontal grid spacing using the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5). The MM5 simulation reproduced the basic features such as the timing, location, and orientation of the cold front and associated precipitation evolution, as well as the tendency for enhanced precipitation to extend ∼50–100 km upwind of the coastal barrier, with the heaviest amounts occurring over the windward slopes (0–20 km inland); locally, however, the model underestimated the magnitude of the prefrontal terrain-enhanced flow by as much as 30% since the simulated low-level static stability was weaker than observed. The MM5 simulations illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and prefrontal terrain-enhanced winds exceeding 25 m s−1. Because of the irregular coastline and highly three-dimensional terrain, the low-level winds were not uniform along the coast. Rather, prefrontal southerly flow was significantly reduced downwind of the major capes (viz. Mendocino and Blanco), while there were localized downgradient accelerations adjacent to regions of higher topography along uninterrupted stretches of coastline. Terrain–front interactions resulted in a slowing of the front as the system made landfall, and blocking contributed to a “tipped forward” baroclinic structure below 800 mb. The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and banded precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist convection, and a simulation without coastal topography illustrated rapid development of coastal precipitation even in the absence of local terrain influences. The coastal topography helped to further enhance and collapse the thermal gradient and associated cold-frontal rainband through enhanced deformation frontogenesis associated with the prefrontal terrain-enhanced flow. Diabatic effects from precipitation are also shown to have been important in organizing the precipitation in the coastal zone and further enhancing the frontal temperature gradient.

Interaction of diabatic frontogenesis and moisture processes in cold-frontal rain-band

Three-dimensional simulation of cold-front rain-band (NCFR) associated with a straight cold front has been studied by use of a non-hydrostatic, full compressible storm-scale model (ARPS) including multi-phase microphysical parameterization. The dynamical and physical features of the frontal cloud de-velopment have been well simulated and analyzed. It is in evidence that the frontal cloud is triggered by the updraft of the secondary frontal circulation. However, the long persistence of diabatic frontogenesis only can be attributed to positive feedback between the frontal baroclinicity and the prefrontal latent heat release. The simulations indeed demonstrate that the potential temperature gradient enhancement in front zone is strongly related with the re-distribution of cloud moisture, by the action of tilted updraft. In conse-quence, the splice of cooling and heating pool that is respectively created from the evaporation of cloud wat-er and condensing J freezing of water vapor J rain droplet, wich is in favor of the strong contrast of cool and warm air mass across the frontal zone to diabatic frontogenesis.

Tropospheric wind and humidity derived from spaceborne radar intensity and phase observations

Causal connections between mesoscale wind patterns and moisture distribution in the lower boundary layer can be studied using satellite synthetic aperture radar (SAR) observations. Wind information is retrieved from radar backscatter intensity over water areas, whereas the moisture distribution is reflected in the radar signal delay. The latter can only be observed over land areas, using the interferometric combination of two radar images. This paper reports on the relation between wind and moisture associated with boundary layer rolls and with a cold‐frontal rain band. The combination of both signals, originating from one sensor, is used to infer a quantitative description of the underlying dynamical processes.

Airborne Doppler Observations of a Landfalling Cold Front Upstream of Steep Coastal Orography

This study uses airborne Doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of Oregon and northern California on 1 December 1995 during the Coastal Observations and Simulations with Topography experiment. This section of coastline constitutes a steep, approximately two-dimensional north‐south-oriented orographic barrier. The front exhibited a northeast‐southwest orientation and thus intersected the axis of high terrain at an acute angle. The alongbarrier pressure gradient and low-level winds increased with time along the coastal zone and reached a maximum as the front made landfall. Stably stratified prefrontal flow was strongly blocked by the orography, resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated southsouthwesterly flow near the coast, where wind speeds approached 30 m s 21 at a height of 750 m above mean sea level. Postfrontal flow was much less affected by the topography, probably because of its weaker static stability. Upstream blocking by the steep coastal terrain also evidently led to modifications of precipitation in the vicinity of the front, including the rapid genesis of a narrow cold-frontal rainband (NCFR) and nearshore enhancement of two prefrontal precipitation bands. This evolution of the NCFR is interpreted in conjunction with changes in prefrontal vertical wind shear, which favored more upright convective ascent as the front neared shore and encountered accelerated along-barrier flow adjacent to the steep terrain. In addition, a statistical examination of observed radar reflectivity patterns shows that the intensity of frontal precipitation systematically decreased with upstream distance away from the orographic barrier.

Numerical Modeling of Precipitation Cores on Cold Fronts

Abstract A nonhydrostatic, three-dimensional, mesoscale model, including cloud physics, is used to simulate the structure of a narrow cold-frontal rainband (NCFR). The model simulations reproduce the observed “core–gap” structure of the NCFR. Trapped gravity waves, triggered by regions of stronger convection on the cold front, induce subsidence and regions of warming aloft. In these regions, precipitation is suppressed, thereby creating precipitation gaps along the front separated by precipitation cores. The advection of hydrometeors is responsible for the parallel orientation and the elliptical shapes of the precipitation cores. Gravity waves produce pressure perturbations just behind the cold front, which modify the wind and thermal structure. Parts of the front behave locally like a gravity current, traveling at the theoretical gravity current speed in a direction perpendicular to the local orientation of the front, but the motion of the front as a whole is not well described by the gravity current spe...