Short-wave Trough Research Articles

Wintertime Nonbrightband Rain in California and Oregon during CALJET and PACJET: Geographic, Interannual, and Synoptic Variability

Abstract An objective algorithm presented in White et al. was applied to vertically pointing S-band (S-PROF) radar data recorded at four sites in northern California and western Oregon during four winters to assess the geographic, interannual, and synoptic variability of stratiform nonbrightband (NBB) rain in landfalling winter storms. NBB rain typically fell in a shallow layer residing beneath the melting level (<∼3.5 km MSL), whereas rainfall possessing a brightband (BB) was usually associated with deeper echoes (>∼6 km MSL). The shallow NBB echo tops often resided beneath the coverage of the operational Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning radars yet were still capable of producing flooding rains. NBB rain contributed significantly to the total winter-season rainfall at each of the four geographically distinct sites (i.e., 18%–35% of the winter-season rain totals). In addition, the rainfall observed at the coastal mountain site near Cazadero, California (CZD), during each of four winters was composed of a significant percentage of NBB rain (18%–50%); substantial NBB rainfall occurred regardless of the phase of the El Niño–Southern Oscillation (which ranged from strong El Niño to moderate La Niña conditions). Clearly, NBB rain occurs more widely and commonly in California and Oregon than can be inferred from the single-winter, single-site study of White et al. Composite NCEP–NCAR reanalysis maps and Geostationary Operational Environment Satellite (GOES) cloud-top temperature data were examined to evaluate the synoptic conditions that characterize periods of NBB precipitation observed at CZD and how they differ from periods with bright bands. The composites indicate that both rain types were tied generally to landfalling polar-cold-frontal systems. However, synoptic conditions favoring BB rain exhibited notable distinctions from those characterizing NBB periods. This included key differences in the position of the composite 300-mb jet stream and underlying cold front with respect to CZD, as well as notable differences in the intensity of the 500-mb shortwave trough offshore of CZD. The suite of BB composites exhibited dynamically consistent synoptic-scale characteristics that yielded stronger and deeper ascent over CZD than for the typically shallower NBB rain, consistent with the GOES satellite composites that showed 20-K warmer (2.3-km shallower) cloud tops for NBB rain. Composite soundings for both rain types possessed low-level potential instability, but the NBB sounding was warmer and moister with stronger low-level upslope flow, thus implying that orographically forced rainfall is enhanced during NBB conditions.

Evolution of a Typhoon-Like Subtropical Low Causing Severe Weather over the Kanto Area on 13 October 2003

The purpose of this article is to document genesis, development and dissipation of a subsynoptic-scale typhoon-like cyclone, that caused severe weather on 13 October 2003, over the eastern part of the Kanto area. In this documentation, practically all available data obtained operationally by the Japan Meteorological Agency are used.The subtropical low of interest formed initially as a mesoscale vortex at 0000UTC 8 October over the ocean near Okinawa Island (∼26°N, 128°E), where the western end of a quasi-stationary polar front was located. The genesis region is characterized by a weak baroclinicity, and nearly saturated low-level atmosphere, with a negative Showalter Stability Index. Several mesoscale convective systems developed prior to the formation of the subtropical low in the genesis region. It is inferred that diabatic heating, by the release of latent heat in deep convection helped spin up pre-existing relative vorticity associated with the quasi-stationary polar front. In addition, an upper-level short wave trough spawned a cut-off low over the region, helping the vortex to develop into the subtropical low. When matured ∼50 h later, the subtropical low exhibited remarkable axisymmetry in the wind and thermal structures, with the diameter of l00∼200 km. The cloud pattern also displayed a cloud-free eye-like structure. After it traveled slowly northwards for four days, the subtropical low was overtaken by a synoptic scale trough, and underwent the processes known as the extratropical transition of a typhoon, while it moved towards the east.Finally, briefly discussed are similarities and dissimilarities between the present subtropical low, and hurricane-like subtropical cyclones that develop in other regions of the globe, such as the Mediterranean Sea and the central/eastern Pacific Ocean.

A Comparison of Two Banded, Heavy Snowstorms with Very Different Synoptic Settings

Abstract Two banded, heavy snowstorms that occurred over the northern mid-Atlantic region are compared and contrasted. On 6–7 January 2002, a narrow, intense band of heavy snow was observed, along with several other weaker bands, embedded within a large area of moderate snow. On 19–20 January 2002, a single, broader band of heavy snow was observed, embedded within a broken area of light snow. The synoptic-scale settings associated with these two storms were strikingly dissimilar. In the first case, strong quasigeostrophic (QG) forcing for ascent was present just to the south of the heavy snowfall area. A highly amplified longwave trough was located over the Mississippi River valley, while a compact shortwave trough moved northward, up the east side of the longwave trough. The result was robust cyclogenesis off of the mid-Atlantic coast. In the second case, the relatively weaker QG forcing for ascent was located much farther southwest of the snowband. The flow aloft was much less amplified, with weaker cyclogenesis occurring off of the mid-Atlantic coast. Analysis of the frontal scale environments for both cases indicated that the snowbands were each associated with the collocation of midtropospheric frontogenesis and reduced stability. In the first case, evidence is shown that a layer of potential symmetric instability (PSI) was located just above a deep, sloping zone of frontogenesis, in the presence of deep near-saturated conditions. In the second case, evidence is shown that a layer of potential instability (PI), associated with rapidly decreasing relative humidity with height, was located just above a shallow, sloping zone of frontogenesis. In addition, it is shown that a particularly favorable thermal environment for snowflake growth and accumulation became collocated with the heavy snowband. It is hypothesized that the differences in the intensity and horizontal extent of the bands observed with these two events resulted from differing atmospheric responses associated with the areal extent of large-scale and frontogenetical forcing, moisture availability, degree of instability, and specific thermal profiles.

Generation and Development of a Polar Mesoscale Cyclone over the East Coast of Asia as Simulated in an AGCM

A polar mesoscale cyclone (PMC) developed over the east coast of Asia in an atmospheric general circulation model (AGCM) that used seasonally varying climatological sea surface temperatures (SSTs). The AGCM had 52 layers and triangular spectral truncation at wavenumber 106 (T106L52). This study compares the model PMC, which formed in February, 8 years after model spin-up, with observational studies. The simulated PMC formed over the western Sea of Japan under the influence of a short-wave trough propagating around a cold upper low. PMC genesis occurred as a surface trough over the Sea of Japan. The surface trough extended northward from a major cyclone that developed over the western North Pacific south of Japan. Large sensible heat fluxes from the sea surface facilitated PMC genesis by decreasing the vertical stability and maintaining a thermal gradient in the lower troposphere. Significant weakening of the PMC as it passed over Japan suggests that energy supplied from the surface played an important role in the evolution of the PMC. The PMC subsequently developed rapidly over the western North Pacific east of Japan, under the influence of the upper cold low and a short-wave trough. Precipitation, diabatic heating, and energy from the sea surface all increased as the PMC developed. The evolution of the simulated PMC matched the evolution of observed PMCs. Realistic formation of synoptic-scale circulation systems, such as the upper cold low, short-wave troughs, and a major cyclone in the model, are crucial if PMCs are to be simulated realistically in the AGCM.

Case Study of an Unusually Heavy Rain Event over Eastern Taiwan during the Mei-Yu Season

Abstract A heavy rain event during the mei-yu season was observed over eastern Taiwan on 28 May 2001 under a potentially unstable environment with high CAPE and copious moisture. This event, with an unusually high daily rainfall maximum of 307 mm, was observed in the Ilan area over northeastern Taiwan. The retrogression of a mei-yu front over the nearby ocean collocated with a short-wave trough provided favorable large-scale conditions for the production of localized heavy rainfall. Latent heating was suggested to be responsible for the retrogression and the development of the mei-yu front over the nearby ocean to the east of Taiwan. In addition to the large-scale forcings, the local circulations and local topography modulated the rainfall distribution over the Ilan area. During the daytime hours, the strengthened onshore wind component due to the intensified mei-yu frontal circulation and orographic lifting provided enhanced rising motion over the windward slopes of the mountain to produce the significan...

Hailstorms in Northern Greece: synoptic patterns and thermodynamic environment

¶The synoptic scale atmospheric circulation prevailing during hail days in Northern Greece is examined for a period of 26 years (1976–2001) during the warm season. The initial objective of this classification scheme is to serve as a predictive tool for hail forecasting and permit a specification of greater or lesser hail risk, within the forecast activities of the Greek National Hail Suppression Program. The atmospheric circulation was classified into seven synoptic types and their frequency distribution was examined on an annual and monthly basis. Southwesterly flow (SW) and Shortwave trough (SWT) appear to be the most conducive synoptic types for hailstorm activity. Furthermore, the thermodynamic environment favoring the hailstorm development and evolution was examined using selected instability indices. The frequency distribution of the instability indices for a period of 11 years (1991–2001) in association with the synoptic types demonstrated their performance as a forecasting aid for thunderstorm activity to occur in a given synoptic environment.

Polar Low Genesis over the East Coast of the Asian Continent Simulated in an AGCM

This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies. In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows. It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.

An East Asian Cold Surge: Case Study

Since a cold surge is a hazardous weather phenomenon in east Asia, the rapid population increase and economic growth over the past two decades require improvement in forecasting cold surges and their related weather events over this region. However, without a better understanding of these events, this task cannot be accomplished. A cold surge with a well-defined cold front passing through Taiwan was selected to illustrate its impact on the east Asian weather system. This case is typical of a large portion of surges occurring in the region. Major findings of this study are as follows. Coupling with the upper ridge–trough structures of the wave train straddling the eastern seaboard of northeast Asia, cold surges occur sequentially. A cold front with a prefront high pressure zone is formed by the new surge outflow interacting with the anticyclone of the aging surge. The warm moist air advected northeastward along the cold front assists the development of the new surge's low center, while the prefront high pressure zone facilitates the formation of a double-cell structure in the local Hadley circulation. The southeastward propagation of the cold front is driven by the eastward-propagating short-wave trough through the couplets of both new and aging cold surges. The surface weather conditions in the low-elevation zones of Taiwan are modulated by the cold surge flow, but the high-elevation areas may be affected instead by the tropical southeast Asian high. Despite the success of the prior and post-WMONEX (Winter Monsoon Experiment) research in exploring the tropical–midlatitude interaction, the close interaction of cold surges with local weather systems and the planetary-scale circulation in east Asia, illustrated by the case study presented, provides another dimension of cold surge research.

Some Common Ingredients for Heavy Orographic Rainfall

Abstract The purpose of this paper is to synthesize some common synoptic and mesoscale environments conducive to heavy orographic rainfall. Previous studies of U.S. and Alpine cases and new analyses of some Alpine and east Asian cases have shown the following common synoptic and mesoscale environments are conducive to heavy orographic rainfall: 1) a conditionally or potentially unstable airstream impinging on the mountains, 2) a very moist low-level jet (LLJ), 3) a steep mountain, and 4) a quasi-stationary synoptic system to slow the convective system over the threat area. A deep short-wave trough is found to approach the threat area in the U.S. and European cases, but is not found in the east Asian cases. On the other hand, a high convective available potential energy (CAPE) value is observed in east Asian cases, but is not consistently observed in the U.S. and European cases. The enhancement of low-level upward motion and the increase of instability below the trough by the approaching deep short-wave tr...

Large .LAMBDA.-shaped Cloud Zone Formed around July 6, 1991 with Pole-ward Moisture Transport from Intense Rainfall Area in Meiyu-Baiu Front.

Many previous papers stressed that the Meiyu-Baiu rain zone exhibits the quasi-stationary nature, except the abrupt transitional period. This is true only for the 10-day averaged large-scale field. The actual Meiyu-Baiu front indicates complicated variations even in “the quasi-stationary period”. The present paper will study a large λ-shaped cloud zone seen around July 6, 1991 as one of the cases of significant variations of the Meiyu-Baiu front. This λ-shaped cloud zone is formed when a north-south oriented cloud zone elongated from the intense rainfall area of Meiyu-Baiu front with pole-ward moisture transport. This process is quite different from “the abrupt transition of the Meiyu-Baiu front”, since the Meiyu-Baiu frontal zone itself remains in almost the same latitude belt. The λ-shaped cloud zone is associated with a north-south extending trough, which resulted from coupling of the short-wave troughs in northern (40-50°N) and southern (30-40°N) latitude. When the extending trough passed over the Tibetan Plateau and approached a quasi-stationary ridge over ∼120°E, the narrow north-south elongated cloud zone, which is associated with southerly wind and ascending motion, is formed in front of the trough. Convective precipitation occurs within the north-south elongated cloud zone. However, subsidence to the west of the trough and that to the east of the ridge block the northward shift of the Meiyu-Baiu front. The north-south elongating cloud zone disappears when the eastward propagating southern short-wave trough is de-coupled from the northern short-wave trough by the blocking ridge. The λ-shaped cloud zone occurs only under the specific phase relations among the short-wave troughs in northern and southern latitudes, quasi-stationary ridge and intense rainfall area in the Meiyu-Baiu front. Typical large λ-shaped cloud zone appears only a few times in a Meiyu-Baiu season.

A Diagnostic Study of Cold-Air Outbreaks over South America

Midlatitude disturbances such as intense cold fronts and cutoff lows are a very important cause of severe-weather events over the southern part of South America, particularly during the austral winter months. Behind cold fronts, cold air from higher latitudes is forced to flow equatorward to the east of the Andes. Occasionally fronts might produce cold surges in tropical latitudes, with freezing temperatures (referred as to GEADAS in Portuguese) over crop-growing areas of southern, southeastern, and central Brazil. The structure and dynamical processes of winter synoptic-scale waves associated with cold surges over South America are documented in this paper. It is shown that the rotated extended empirical orthogonal function technique is capable of extracting well-defined regional patterns from 850-hPa meridional-wind perturbation fields. Over South America, waves behave very differently in the lower and middle troposphere: while upper-level waves propagate northeastward as they cross the Andes, evolving in a manner consistent with the concept of Rossby wave dispersion, at lower levels, waves tend to conform to the shape of the mountain range, in agreement with the theory of topographic Rossby waves. However, it was found that cold surges do not seem to result from the generation of rotationally trapped waves. Significant differences have been found between synoptic-scale patterns associated with cold surges over extratropical South America (NO GEADA composites) and synoptic-scale patterns associated with major cold-air outbreaks that produce extreme low temperatures at tropical regions (GEADA composites). While both surges are characterized by a long-wave pattern consisting of a cyclonic perturbation over South America and anticyclonic perturbation behind over the southern Pacific Ocean, at the early stages of GEADA composites, two additional features were identified: a subpolar and subtropical short-wave pattern. The presence of the upper-level subtropical cyclonic perturbation is associated with the subtropical jetstream location farther north, which would facilitate the equatorward penetration of frontal systems. In addition, this cyclonic perturbation enhances rising motion at subtropical and tropical latitudes that is associated with a well-defined secondary circulation with its descending branch over central Argentina. It is shown that this configuration favors the rapid and equatorward penetration of the subpolar short-wave trough, the temperature decrease over southern Brazil, and the anticyclone intensification over central Argentina. Thus the presence of the subtropical upper-level feature plays a key role on the cold-surge occurrence over tropical regions of South America.

Using Piecewise Potential Vorticity Inversion to Diagnose Frontogenesis. Part I: A Partitioning of theQVector Applied to Diagnosing Surface Frontogenesis and Vertical Motion

The technique of piecewise potential vorticity (PV) inversion is used to identify the nondivergent wind fields attributed to upper-, middle-, and lower-tropospheric PV anomalies in addition to the irrotational wind with the goal of diagnosing the respective wind fields’ frontogenetic potentialities. Frontogenesis is diagnosed using a piecewise separation of the Q vector into parts associated with the partitioned wind field. Partitioned geostrophic Q vectors are used to diagnose the vertical motion attributed to the upper-, middle-, and lower-tropospheric PV anomalies. Insight gained from this new diagnostic technique is demonstrated by examining a particular case of extratropical marine cyclogenesis resulting from the interaction of an upper-tropospheric short-wave trough with a surface thermal wave. In the early stages of development, the largest contributor to surface frontogenesis was associated with winds attributed to the lower-tropospheric thermal wave. As the cyclone matured, the contributions of the upper-tropospheric PV and near-surface potential temperature anomalies to surface frontogenesis increased. Winds attributed to the upper-tropospheric PV were frontogenetical north of the thermal ridge axis and frontolytical south of the thermal ridge in the warm sector. The upper-tropospheric PV acted to amplify the thermal ridge while simultaneously narrowing the warm sector. The patterns of geostrophic Q vectors associated with the upper-tropospheric PV suggest that ascent should be favored in the narrowing surface thermal ridge. The contribution to surface frontogenesis due to lower- and middle-tropospheric PV, whose increase is imputed to latent heat release, was variable during the evolution of the cyclone—suggesting that the location of diabatically generated PV anomalies relative to frontal zones can have a significant impact on frontogenesis and associated frontal precipitation distribution. Throughout the evolution of the cyclone, the irrotational wind was frontogenetical along the warm and cold fronts with the magnitude of the irrotational frontogenesis increasing as the surface cyclone amplified. For the case considered, partitioned geostrophic Q vector “forcing” for vertical motion revealed approximately equal contributions from the upper-tropospheric PV anomalies and the near-surface thermal perturbations. Characteristic patterns of Q vectors and Q-vector divergence are identified and presented for cases including an upper trough interacting with a surface baroclinic zone and a propagating surface edge wave.

Interaction of Potential Vorticity Anomalies in Extratropical Cyclogenesis. Part II: Sensitivity to Initial Perturbations

In Part I of this series of papers, the static potential vorticity (PV) inversion diagnostics are performed to examine the lateral and vertical interactions between two upper-level (northern and southern) troughs and their possible influences on the development of the March 1993 superstorm. This study continues the investigation of the relative importance of two PV anomalies associated with the short-wave troughs and their interaction in the surface cyclogenesis by considering their individual roles as an initial-value problem. The piecewise PV inversion technique is first used to isolate the upper-level disturbances with a balanced vortex for each. The vortex is then either removed or its intensity doubled by subtracting the balanced vortex from or adding to the control initial conditions. With the modified initial conditions, a mesoscale model is integrated for 36 h to examine how each of the upper-level disturbances contributes to the surface development. It is found that the northern and southern troughs play different roles in the surface cyclogenesis. Without the northern trough, the eastward propagation of the southern trough slows down and the surface development is severely impeded. More rapid cyclogenesis occurs when the northern trough’s intensity is doubled. On the other hand, the deepening rate increases considerably when the southern trough is removed. Doubling the southern trough intensity slows the eastward progression of the northern trough and leads to a reduced deepening rate. In conclusion, the northern trough is crucial for the rapid development of the superstorm, whereas the southern trough is important only during the incipient stage of the cyclone. In particular, a much stronger surface cyclone could be spawned in the absence of the southern trough. This finding appears to contradict the previous work on the roles of trough mergers in extratropical cyclogenesis.

A Flash Flood from a Lake-Enhanced Rainband

Abstract An analysis of a flash flood caused by a lake-enhanced rainband is presented. The flood took place near Erie, Pennsylvania, on 17 September 1996. It was found that the flood resulted from a complex interplay of several scales of forcing that converged over the Erie region. In particular, the flood occurred during a period when 1) a lake-enhanced convective rainband pivoted over the city of Erie with the pivot point remaining quasi-stationary for about 5 h; 2) a deep, surface-based no-shear layer, favorable for the development of strong lake-induced precipitation bands, passed over the eastern portion of Lake Erie; 3) the direction of flow in the no-shear layer shifted from shore parallel to onshore at an angle that maximized frictional convergence; 4) an upper-level short-wave trough contributed to low-level convergence, lifting, and regional destabilization; and 5) a strong land–lake diurnal temperature difference produced a lake-scale disturbance that locally enhanced the low-level convergence....

Numerical simulations on the explosive cyclogenesis over the kuroshio current

In this paper, the Pennsylvania State University-NCAR Mesoscale Model (MM4) is used to investigate the explosive oceanic cyclone of 14-15 March 1988 over the warm Kuroshio Current. A series of numerical simulations on this cyclogenesis indicates that the favorable weather conditions and strong baroclinity in the low- and middle-level are essential to its explosive development. The explosive cyclogenesis occurred over a wide range of sea surface temperatures (SST's), which was then characterized by strong baroclinity, the low-level jet (LLJ) was initially formed under the favorable atmospheric circulation and then this LLJ advected the moisture and heat northward for the explosive development of the cyclone, the LLJ played an important role in the process of cyclogenesis. Sensitivity experiments show that the latent heating was a key factor to explosive cyclogenesis, the latent heating deepened the short-wave trough, which resulted in the rapid intensification of the cyclone; while in the explosive intensification stage and continuous development stage, there was less contribution of local surface processes for the explosion of the cyclone.

Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Abstract On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU—CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; w...

Mesoscale Convective System over the Yellow Sea - A Numerical Case Study

A mesoscale convective system (MCS) case that developed over the Yellow Sea (12–13 July 1993) is studied by using a 23-level, 30 km-mesh Penn State/NCAR mesoscale model MM5. This MCS was generated in northern China, south of the Changma front, in a convectively unstable environment, under the influence of a short-wave trough accompanied by a marked cold vortex aloft.

Mesoscale Model Simulation of the 4–5 January 1995 Lake-Effect Snowstorm

Abstract The Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5), running on a triply nested grid, was used to simulate the intense lake-effect snowstorm of 4–5 January 1995. On the finest grid (5-km resolution) centered over Lake Ontario, MM5 produced a snowband in the correct location having a size and orientation similar to the band observed by the WSR-88D radar at Binghamton, New York. The simulated precipitation distribution agreed well with the observed snowfall during the first 18 h during the time when the snowband was in its midlake position extending into the Tug Hill plateau. During the last 12 h of the simulation, when both the observed and simulated snowbands lay along the south shore of Lake Ontario, the simulated snowfall at inland locations of Oswego County was less than observed. During this period, the simulated precipitation over Lake Ontario appeared to be excessive, although no radar data or ground truth was available to confirm this. Two short-wave troughs interacted w...

Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part II: Microphysics, Vapor and Ice Mass Budgets, and Optical and Radiative Properties

Abstract The 2D/3D cloud model complex with explicit microphysics and radiation described in Part I is used to simulate the development of a midlatitude cirrus cloud, including interaction with radiation. To account for the effects of the interaction of various scales of motion on cloud development, a synoptic-scale vertical velocity field is superimposed on the mesoscale velocity field generated by the model, mimicking the effects of an upper-level shortwave trough. The main results under the conditions simulated here are the following. Cirrus cloud growth is much slower than assumed previously, because the process of vapor deposition to ice crystals is far from instantaneous: the crystal phase relaxation time (i.e., the characteristic time of vapor absorption by crystals) takes 0.5–2.0 h. Even after 1 h of cloud development, supersaturation with respect to ice can remain 5%–10%, while the condensed ice is only 40%–60% of the amount that would be realized assuming that all excess vapor is transformed int...

A numerical investigation of a moderate coastal storm with intense precipitation

In this study, the development of a moderate coastal storm with intense precipitation that occurred during 12–14 February 1993 is examined using a high‐resolution version of the Canadian Regional Finite‐element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis events and also the distribution and intensity of heavy mixed precipitation (rain and snow) associated with the storm. It is found that the cyclogenesis takes place in response to the low‐level inshore advection of high‐θe air from the maritime boundary layer, and the approach of a mid‐level shortwave trough with a warm pool above that is previously associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold fronts near the cyclone centre. Diagnosis of the control and sensitivity simulations reveals that the low‐level inshore warm advection and the propagation of the stratospheric warm pool contribute more significantly to the surface pressure falls during the incipient stage, whereas the mid‐level shortwave trough plays an important role in the cyclogenesis at later stages. Overall, latent heat release accounts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation of intense precipitation. The diabatic and kinematic structures near the rain‐snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE model reproduces well the rain‐snow boundary structures as previously observed. Moreover, a thermally indirect circulation (perturbation) can be seen in the vicinity of the rain‐snow boundary. It is found, however, that melting of snow tends to produce a weak negative or negligible impact on the cyclogenesis, as opposed to previous hypotheses.