Prefrontal Air Research Articles

TAMU TRACER: Targeted Mobile Measurements to Isolate the Impacts of Aerosols and Meteorology on Deep Convection

Abstract Difficulty in using observations to isolate the impacts of aerosols from meteorology on deep convection often stems from the inability to resolve the spatiotemporal variations in the environment serving as the storm’s inflow region. During the U.S. Department of Energy (DOE) Tracking Aerosol Convection interactions Experiment (TRACER) in June–September 2022, a Texas A&M University (TAMU) team conducted a mobile field campaign to characterize the meteorological and aerosol variability in air masses that serve as inflow to convection across the ubiquitous mesoscale boundaries associated with the sea and bay breezes in the Houston, Texas, region. These boundaries propagate inland over the fixed DOE Atmospheric Radiation Measurement (ARM) sites. However, convection occurs on either or both the continental or maritime sides or along the boundary. The maritime and continental air masses serving as convection inflow may be quite distinct, with different meteorological and aerosol characteristics that fixed-site measurements cannot simultaneously sample. Thus, a primary objective of TAMU TRACER was to provide mobile measurements similar to those at the fixed sites, but in the opposite air mass across these moving mesoscale boundaries. TAMU TRACER collected radiosonde, lidar, aerosol, cloud condensation nuclei (CCN), and ice nucleating particle (INP) measurements on 29 enhanced operations days covering a variety of maritime, continental, outflow, and prefrontal air masses. This paper summarizes the TAMU TRACER deployment and measurement strategy, instruments, and available datasets and provides sample cases highlighting differences between these mobile measurements and those made at the ARM sites. We also highlight the exceptional TAMU TRACER undergraduate student participation in high-impact learning activities through forecasting and field deployment opportunities.

Concentration of bacterial aerosols in response to synoptic weather and land‐sea breeze at a seaside site downwind of the Asian continent

AbstractAirborne bacteria are a major group of bioaerosols, but their temporal and spatial variations in the atmosphere are poorly understood due to a lack of accurate information at an appropriate temporal resolution. In this study, we quantified the concentration and viability of airborne bacteria at a seaside site on the southwestern coast of Japan in spring to investigate their association with synoptic weather, which was governed alternately by cyclones and anticyclones moving in the middle‐latitude westerly flow. The concentration of bacteria, on the order of 105–106 cells m−3, showed a small amplitude of variation, while the viability significantly differed in the range of 23–96% under different weather conditions. Cyclones transported bacteria from the Asian continent in their postfrontal air. Compared to bacteria under other weather conditions, the bacteria in postfrontal air had a large concentration, a low viability, and a close correlation with coarse aerosol particles. In contrast, bacterial concentration and viability in prefrontal air varied over small ranges, and no correlation between bacteria and aerosol particle number concentration was observed. During anticyclones, although bacterial concentration was similar to that in prefrontal air, bacterial viability was high, and the concentration had pulse increases of approximately 2 times the usual level when the wind changed to land/sea breezes. The increases were attributed to the accumulation of bacteria in stagnant air due to local sources. These results show the following two distinct patterns of bacterial concentration and viability at the seaside location of this study: (1) during postfrontal long‐distance dust plume transport related to a cyclone approaching from the Asian continent, the bacterial concentration is correlated with the coarse aerosol particle number concentration and the bacterial viability is low, and (2) pulse increases of viable bacteria are observed at anticyclone conditions corresponding to local sources and land‐sea breeze conditions.

Transport of bacterial cells toward the Pacific in Northern Hemisphere westerly winds

Viable and non-viable bacteria in the air were investigated on the southwestern Japan coast to outline the manner by which bacteria were transported and to quantify the bacterial abundance and viability in the Northern Hemisphere westerly winds. The observations were conducted when the weather was governed by cyclones or anticyclones associated with Asian continent outflow between 12 October 2011 and 7 April 2013. Bacterial concentration in thermodynamically different air parcels was in the same order but different ranges: 4.5 × 105–1.3 × 106 cells m−3 in postfrontal air, 2.4 × 105–1.4 × 106 cells m−3 in prefrontal air, 5.4 × 105–9.9 × 105 cells m−3 in the air between cyclones and anticyclones and 2.9 × 105–4.1 × 105 cells m−3 in anticyclone air. In postfrontal air, the concentration correlated closely with coarse aerosol particles (diameter > 1.0 μm). In contrast, bacteria did not show a correlation with coarse particles in prefrontal air and anticyclone air. Bacterial viability was approximately 70% on average of all samples. However, the viability in postfrontal air was smaller than 60% if cases of stationary fronts with stagnant air were excluded. These results indicate that air parcels following fast-moving cold fronts in the westerly wind flow constantly and efficiently conveyed airborne bacteria from the Asian continent toward northwestern Pacific and the bacteria were characterized by coarse particle-correlated high abundance and low viability. The bacteria in slowly moving anticyclone and prefrontal air, characterized by low abundance and high viability, were more likely a mixture of long-range transported ones and regionally or locally originated ones.

Investigations of Transported and Local Emissions on Particle Compositions in Korea

ABSTRACTThe synoptic meteorological patterns, aerosol optical thickness (AOT) and changes in the chemical composition of aerosol particles are indicators that allow the study of the effects of emissions on air quality. These approaches have been applied to two sets of intensive campaign measurement data in Gwangju, Korea, allowing the identification of long-range transported (LRT) and local emission impacts in spring of 2008 and 2010. Prefrontal air masses originating from polluted regions of the Asian continent influenced the particulate properties on March 12, 13, 27, April 12–13, and May 7, 2008, resulting in SO4–2 concentrations of 44.1 µg/m3, 44.6 µg/m3, 26.4 µg/m3, 21.9 µg/m3 and 26.4 µg/m3, respectively, compared to a local baseline concentration of 8.5 µg/m3. Air masses affected by mixed burning (LRT-MB) arrived in Gwangju between April 23 to 27, 2008 and influenced the particulate organic carbon (OC) concentrations by as much as 20.0 µgC/m3. Pollution events in 2010 have been classified into three classes: sulfate-nitrate (S-N) dominant, chloride-enriched (Cl–) and mixed burning influenced (Local MB) emissions. From the diurnal patterns of SO4–2 and NO3–, their concentrations were found to be lowest at midday and highest in the evening, with their chemical transformation reactions consistent with known processes that lead to the formation of NH4NO3. Cl-enriched emissions not related to sea salt and biomass burning emissions were observed on various occasions; a strong correlation (adjusted R2 = 0.96) between excess NH4+ and Cl– exists for the pooled cases. The data suggested contributions from local municipal solid waste combustion and biomass burning. Overall analysis of ionic balance suggests that the LRT particles may provide an excess SO4–2. The neutralizing capacity of NH3 for local emissions was demonstrated from the detection of sufficient amount of NH4+ that has been reacted with Cl–.

Soot particles at an elevated site in eastern China during the passage of a strong cyclone

Atmospheric particles larger than 0.2μm were collected at the top of Mt. Tai (36.25°N, 117.10°E, 1534m a.s.l.) in eastern China in May 2008 during the passage of a strong cyclone. The particles were analyzed with electron microscopes and characterized by morphology, equivalent diameter and elemental composition. Soot particles with coating (coated soot particles) and those without apparent coating (naked soot particles) were predominant in the diameter range smaller than 0.6μm in all samples. The number–size distribution of the relative abundance of naked soot particles in the prefrontal air was similar to that in the postfrontal air and their size modes were around 0.2–0.3μm. However, the distribution of inclusions of coated soot particles showed a mode in the range of 0.1–0.3μm. The coating degree of coated soot particles, which was defined by the ratio of the diameter of inclusion to the diameter of particle body, showed a mode around 0.5 with the range of 0.3–0.6. These results indicate that the status of soot particles in the prefrontal and postfrontal air was similar although air pollution levels were dramatically different. In addition, the relative abundance of accumulation mode particles increased with the decrease of soot particles after the front passage.

Aged status of soot particles during the passage of a weak cyclone in Beijing

Atmospheric particles at an urban site in Beijing from Apr. 18 to 21, 2009, when a weak synoptic cyclone passed and the air quality was relatively fine, were investigated by using electron microscopes. Particles collected from different stages of the cyclone were characterized by their shape, size and elemental composition. Soot particles were predominant and concentrated in the size range of 0.2–1.0 μm, and out of this range were rarely detected. Most of the soot particles exhibited aged shape, i.e. shrunk clusters with liquid coating (the so-called core-shell structure), compared to the open spherule’s chain or agglomerate shape of fresh soot particles. Morphologies of the particles in the prefrontal and postfrontal air were compared. The mean equivalent diameter of the particles in the prefrontal air was 0.36–0.38 μm and in the postfrontal air was 0.35–0.41 μm. More than 50% of the particles had aspect ratios between 1.0 and 1.4, and the remains frequently smaller than 2.0. In terms of circularity factor, 57–64% particles in the prefrontal air and 41–51% in the postfrontal air appeared in the range of 0.6–1.0. These results indicate that the aged status of soot particles in the prefrontal and postfrontal air were not very different, although the quality of the postfrontal air was much better than that of the prefrontal air, suggesting that fresh soot particles in chain or aggregate shape could be modified into core-shell structure rapidly even in less polluted air. In addition, a primary estimate showed that aging had caused the decrease of the cross-section areas of soot parts in the particles. ► Soot particles during a cyclone passage in Beijing were studied. ► Aged status of soot in prefrontal and postfrontal air was not very different. ► Fresh soot particles could be modified rapidly in less polluted postfrontal air. ► Aging caused the decrease of cross-section areas of soot parts in the particles.

An Empirical Perspective on Cold Fronts

Abstract Oklahoma Mesonetwork data are used to illustrate important atmospheric features that are not well shown by the usual synoptic data. For example, some shifts of wind from south to north that are shown as cold fronts on synoptic charts are not cold fronts by any plausible definition. As previously discussed by Fred Sanders and others, such errors in analysis can be reduced by knowledge of the wide variety of weather phenomena that actually exists, and by more attention to temperatures at the earth's surface as revealed by conventional synoptic data. Mesoscale data for four cases reinforce previous discussions of the ephemeral nature of fronts and deficiencies in the usual analyses of cold fronts. One type of misanalyzed case involves post-cold-frontal boundary layer air that is warmer than the prefrontal air. A second type is usually nocturnal, with a rise of local temperature during disruption of an inversion and a wind shift with later cooling that accompanies advection of a climatological gradient of temperature.

Finescale Vertical Structure of a Cold Front as Revealed by an Airborne Doppler Radar

Abstract In the afternoon of 24 May 2002, a well-defined and frontogenetic cold front moved through the Texas panhandle. Detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary. This paper primarily uses reflectivity and Doppler velocity data from an airborne 95-GHz radar, as well as flight-level thermodynamic data, to describe the vertical structure of the cold front as it intersected with the dryline. The prefrontal convective boundary layer was weakly capped, weakly sheared, and about 2.5 times deeper than the cold-frontal density current. The radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail (∼40 m). The echo structure and dual-Doppler-inferred airflow in the vertical plane reveal typical features such as a nose, a head, a rear-inflow current, and a broad current of rising prefrontal air that feeds the accelerating front-to-rear current over the head. The 2D cross-frontal structure, including the frontal slope, is highly variable in time or alongfront distance. Along this slope horizontal vorticity, averaging ∼0.05 s−1, is generated baroclinically, and the associated strong cross-front shear triggers Kelvin–Helmholtz (KH) billows at the density interface. Some KH billows occupy much of the depth of the density current, possibly even temporarily cutting off the head from its trailing body.

A Review of Cold Fronts with Prefrontal Troughs and Wind Shifts

Abstract The conceptual model of a classical surface-based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface. Many cold fronts, however, do not conform to this model—time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front (hereafter called a prefrontal trough and prefrontal wind shift, respectively). Although many authors have recognized these prefrontal features previously, a review of the responsible mechanisms has not been performed to date. This paper presents such a review. Ten disparate mechanisms with different frontal structures have been identified from the previous literature. These mechanisms include those external to the front (i.e., those not directly associated with the cold front itself): synoptic-scale forcing, interaction with lee troughs/drylines, interaction with fronts in the mid- and upper troposphere, and frontogenesis associated with inhomogeneities in the prefrontal air. Mechanisms internal to the front (i.e., those directly associated with the structure and dynamics of the front) include the following: surface friction, frontogenesis acting on alongfront temperature gradients, moist processes, descent of air, ascent of air at the front, and generation of prefrontal bores/gravity waves. Given the gaps in our knowledge of the structure, evolution, and dynamics of surface cold fronts, this paper closes with an admonition for improving the links between theory, observations, and modeling to advance understanding and develop better conceptual models of cold fronts, with the goal of improving both scientific understanding and operational forecasting.

The Cold Front of 15 April 1994 over the Central United States. Part I: Observations

Abstract Detailed observations of the interactions of a cold front and a dryline over the central United States that led to dramatic undulations in the boundary layer, including an undular bore, are investigated using high-resolution water vapor mixing ratio profiles measured by Raman lidars. The lidar-derived water vapor mixing ratio profiles revealed the complex interaction between a dryline and a cold-frontal system. An elevated, well-mixed, and deep midtropospheric layer, as well as a sharp transition (between 5- and 6-km altitude) to a drier region aloft, was observed. The moisture oscillations due to the undular bore and the mixing of the prefrontal air mass with the cold air at the frontal surface are all well depicted. The enhanced precipitable water vapor and roll clouds, the undulations associated with the bore, the strong vertical circulation and mixing that led to the increase in the depth of the low-level moist layer, and the subsequent lifting of this moist layer by the cold-frontal surface, as well as the feeder flow behind the cold front, are clearly indicated. A synthesis of the Raman lidar–measured water vapor mixing ratio profiles, satellite, radiometer, tower, and Oklahoma Mesonet data indicated that the undular bore was triggered by the approaching cold front and propagated south-southeastward. The observed and calculated bore speeds were in reasonable agreement. Wave-ducting analysis showed that favorable wave-trapping mechanisms existed; a low-level stable layer capped by an inversion, a well-mixed midtropospheric layer, and wind curvature from a low-level jet were found.

Separated status of the natural dust plume and polluted air masses in an Asian dust storm event at coastal areas of China

Asian dust particles usually refer to mineral particles which originate from arid and semiarid areas in the Asian continent and disperse eastward in a wide range in the atmosphere. Their appearances in the downstream marine areas are always accompanied by high concentrations of sulfate and nitrate, suggesting their significance as a medium for pollutant conversion and transportation. However, analysis of particles collected at a coastal site in east China before and after the cold front arrival of a heavy dust–loading low‐pressure system in this study revealed that air masses polluted by anthropogenic emissions from local or regional areas and the natural Asian dust plume from northwest China were in different air parcels. Anthropogenic pollutants were in the prefrontal air while the so‐called Asian dust particles were in the postfrontal air. There was a large amount of anthropogenic mineral particles in the polluted air masses, and the particles were abundant in sulfur, sodium, and calcium, while the so‐called Asian dust particles were very “clean” (meaning lack of anthropogenic pollution). Continuous records of PM10, SO2, and O3 concentrations indicated that the cold front was the boundary layer between the polluted air masses and the clean dust plume. Hence, at the coastal areas the Asian dust plume and polluted air masses did not mix with each other and were separated in two consecutive air parcels by the cold front although they were in the same low‐pressure system. In addition, the analysis also revealed that calcium was not a good indicator for Asian dust from natural sources because the anthropogenic mineral particles contained significant calcium.

Marine boundary layer dust and pollutant transport associated with the passage of a frontal system over eastern Asia

Aerosol chemical composition and number size distributions were measured aboard the R/V Ronald H. Brown during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) from 14 March to 20 April 2001. This manuscript focuses on the prefrontal and postfrontal air masses sampled aboard the ship in the Sea of Japan between 6 and 15 April 2001 to illustrate the different chemical sources/mixtures off the coast of Asia resulting from the contrasting meteorological transport patterns. The prefrontal air masses had a dominant accumulation mode composed of pollution and volcanic aerosols. The aerosol was predominately ammonium sulfate and organic carbon. Minor amounts of dust were present in the marine boundary layer (MBL) as a result of subsidence from a pronounced Taklimakan dust aerosol layer aloft. The sea salt in both the submicron and supermicron modes was highly depleted in chloride from reaction with sulfuric and nitric acid vapors. The passage of a large low‐pressure center, surrounded by a widespread distribution of airborne dust, on 10 April brought elevated concentrations of submicron and supermicron Gobi desert dust to the ship. The supermicron dust particles contained high concentrations of sulfate, nitrate, organic, and elemental carbon. The MBL aerosol properties and controlling processes described here provide data to evaluate and refine chemical transport models.

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...

Observations Of A Progressive Warm-Fresh Oceanic Front and A Cool-Moist Atmospheric Front At Duck, North Carolina

This paper presents a unique set of observations of nearlycoincident and progressive oceanic and marine atmospheric boundary-layer (MABL) fronts in a coastal zone. The event was observed during the afternoon of 12 May 1996 at the United States Army Corps of Engineers, Coastal Engineering Research Center, Field Research Facility pier at Duck, North Carolina. The oceanic front was warm and fresh. Current variabilityaccompanied the oceanic front. A marked MABL front preceded the oceanic front by several minutes and had characteristics of a sea-breezefront. This MABL front separated warmer and dryer pre-frontal air from cooler and moister post-frontal air. Wind direction and wind speedvariability accompanied the MABL front. The sea-surface roughness signatures of both fronts were detected by an X-band pulsed Doppler radar. Supporting data are used to identify each front detected by the radar and to calculate each front's velocity. In an attempt to explain the sea-surface roughness variations associated with each front, the radar data are compared to corresponding variations in wind speed, wind direction, and air-sea temperature difference.

Orographic influences during winter precipitation events on the Avalon Peninsula, Newfoundland

AbstractPrecipitation enhancement over a low coastal hill in winter is demonstrated for particular associations of synoptically determined onshore airflow and local geography encountered over southeastern Newfoundland. Four such cases, involving a mixture of pre‐warm‐frontal precipitation types at surface temperatures just below freezing, are analysed using comparative surface gauge records from sites at the coast and hill summit and detailed volume scan data from a Doppler radar. Whilst precipitation at the hill summit and upwind coast was of similar overall duration in each case, the surface rates on the hilltop exceeded those at the coast by 1.0–4.0 mm h−1 during different stages of the events. Analysis of the Doppler reflectivity patterns reveals that intensities are especially enhanced near the windward hill crest, supporting the observed association of greatest enhancement with a strong local upslope wind component exceeding 20 m s−1. In the majority of these cases the enhancement is maintained primarily by the ‘seeder–feeder’ mechanism, which appears to be accelerated during precipitation transitions and with the surface warm front 120–150 km distant; however, a preliminary phase of enhancement due to topographically induced uplift of stable pre‐frontal air is also recognised in one case. Copyright © 2000 Royal Meteorological Society

Influence of springtime weather systems on vertical ozone distributions over three North American sites

The Atmosphere/Ocean Chemistry Experiment (AEROCE) '96 Springtime Intensive was designed to quantify major atmospheric chemical species transported from the United States east coast to the North Atlantic Ocean. During the study ozonesondes were launched almost every day between March 22 and May 3, 1996, from Charlottesville, Virginia; Purdue University, Indiana; and Bermuda. Whenever possible, the Charlottesville sondes were timed to fly behind passing cold fronts into midtropospheric to upper tropospheric bands of dry air. The dry air was hypothesized to contain ozone of stratospheric origin and was detected with color‐enhanced satellite water vapor imagery. Soundings were placed in three categories: sondes launched ahead of an approaching cold front, sondes launched behind a passing cold front, and sondes launched into cutoff lows. This stratification explained much of the vertical ozone variation at each site. Tropospheric mean ozone increased with height at all three sites under postfrontal conditions, with relatively little increase with height under prefrontal conditions. Backward trajectories from Charlottesville indicated that the postfrontal air masses originated at high elevations to the northwest, while prefrontal air masses came from relatively lower elevations to the southwest. Transport also explained the lower tropospheric ozone differences between Charlottesville and Bermuda. The enhanced ozone observed in the upper troposphere over Charlottesville compared with the other sites may be linked to upwind conditions more favorable for tropopause folding. However, we believe that selective launching based on the operational use of satellite water vapor imagery allowed us to sample the full range of tropospheric ozone over Charlottesville.

A squall-like narrow cold-frontal rainband diagnosed by combined thermodynamic and cloud microphysical retrieval

Doppler radar, airborne, and sounding data are used to describe the structure of an exceptionally vigorous narrow cold-frontal rainband (NCFR). A combined thermodynamic and cloud microphysical retrieval technique is used to derive the pressure perturbations and buoyancy in balance with the flow field in the vicinity of the NCFR. The NCFR was triggered by a well-defined cold front that penetrated beneath a deep layer of conditionally unstable prefrontal air. The rainband, which persisted for ∼ 6 h, was located over the Gulf Stream, just off the mid-Atlantic Coast of the U.S.A. It consisted of a series of short-lived cells with updraft maxima between 5 and 15 m s −1 and tops around 12 km. The updraft associated with the NCFR was driven primarily by buoyancy in the middle troposphere, which explains its convective nature. Trailing behind the NCFR was a broad region of stratiform precipitation. Pressure perturbations in the NCFR and the trailing stratiform region were produced mainly by a combination of frontal convergence and buoyancy opposition. The pressure perturbations surrounding the cells within the rainband were due primarily to strong updrafts in the presence of wind shear parallel to the rainband. The same dynamic and cloud microphysical processes that operate in stratiform regions behind some squall lines were present in this NCFR. However, in contrast to a squall line with a trailing stratiform region, the low-level cold pool that penetrated beneath the NCFR was maintained primarily by strong cold advection, rather than by evaporational cooling in a shallow rear-to-front flow. The preponderance of advective over diabatic cooling can be used as a criterion to diagnostically differentiate between NCFRs and squall lines.

Nonhydrostatic Simulation of Frontogenesis in a Moist Atmosphere. Part III: Thermal Wind Imbalance and Rainbands

Abstract The dynamical mechanisms contributing to the cross-front ageostrophic circulation are identified in high-resolution (40 to 5 km) nonhydrostatic simulations of moist frontogenesis. In a first step, the importance of the alongfront ageostrophic circulation is assessed. The structure of the intense thermal wind imbalance (TWI) occurring in the vicinity of the surface cold front is diagnosed and explained using a budget of the alongfront vorticity η. It allows one to propose a new balance in terms of the steadiness of the η field in the system moving framework. The TWI is thus found nearly equal to the total cross-front η transport by resolved and subgrid scales. It is shown that, first, the deviation of the prefrontal air toward the front, enhanced by the surface friction and cloud diabatic processes, allows generation of a layer of positive η near at the top of the PBL. Second, the frontal lifting of this η structure is responsible for the basic structure of the TWI. In a second step, a general for...

Synoptic Analysis of the GUFMEX Return-Flow Event of 10–12 March 1988

Abstract Return flow is the moist southerly wind that develops over the Gulf of Mexico after an outbreak of polar air. Surface, aircraft, and special rawinsonde data collected during the Gulf of Mexico Experiment (GUFMEX) are used to describe the return-flow event of 10–12 March 1988. The return flow at the surface contained both modified polar air and prefrontal air. The surface moist layer was capped by a stable subsident layer except over its western extremity, where an elevated mixed layer was observed. At later stages, a moist layer aloft was present as well. These complex airmass structures arose because both advective and diabatic processes are significant in a return flow. The roles of each are inferred qualitatively by comparing the observed mixing-ratio distribution to the equilibrium conditions expected for the observed sea surface temperatures. The surface moisture distribution can be explained by rapid modification of offshore flow to near equilibrium, followed by onshore (return) flow of the...

Organization and Structure of Clouds and Precipitation on the Mid-Atlantic Coast of the United States. Part IV: Retrieval of the Thermodynamic and Cloud Microphysical Structures of a Frontal Rainband from Doppler Radar Data

Abstract The thermodynamic and cloud structure of a relatively weak wintertime frontal rainband are derived from dual-Doppler radar measurements, soundings and high resolution surface observations, and with a cloud model. These derivations are simplified by the high degree of two-dimensionality and steadiness of the rainband. Water vapor, cloud water, cloud ice, rain, and snow are parameterized in the cloud model, subject to a temperature distribution that is constrained to a dynamical balance. Air temperature is derived from buoyancy, which is retrieved from the airflow assuming momentum and heat balance. The results of the thermodynamic and cloud microphysical retrieval are compared with airborne measurements in the rainband. The analysis indicates that the rainband was driven by a weak cold front aloft (CFA), which made the prefrontal air conditionally symmetrically unstable. The CFA appeared as a midlevel intrusion of cold, dry air on the mesoγ scale. The CFA interacted dynamically with the planetary ...