Air Mass Modification Research Articles

Impact of the Sea Effect on Sudden Fog on the Western Coast of the Bohai Sea: A Case Study

The term “sea effect” generally refers to the process of air mass modification after cold air flows above a warm sea surface. Affected by the sea effect, small-scale and sudden fogs have occasionally been observed on the western coast of the Bohai Sea. A more in-depth study of this type of fog is crucial for ensuring the safety of maritime and aerial traffic routes in this region. This study investigated the formation mechanism of this specific type of fog on the morning of 17 October 2007, utilizing both meteorological stations and 255 m tower observations, combined with the results of the Weather Research and Forecasting model (WRF). It is demonstrated that Bohai Sea evaporation and the associated water vapor advection played crucial roles in the formation of fog along the west coast of the Bohai Sea. The cold return flow became more moist as it passed over the warm Bohai Sea, which was the primary contributor to triggering regional fog on the western coast. A moisture budget analysis revealed that water vapor from the Bohai Sea intruded into its western coast along an eastward trajectory, dominating the oscillations in the net moisture flux. The eastern water vapor flux significantly increased at 17:00 on the 16th (Local time, LST), reaching its peak at 21:00. Correspondingly, the fog water growth rate began to increase at 23:00 on the 16th, reaching its maximum at 03:00 on the 17th. A sensitivity experiment on evaporation further indicated that the Bohai sea effect played a decisive role in fog formation. With a tenfold reduction in evaporation from the Bohai Sea and subsequent significant weakening of water vapor advection, the simulated fog along the western coast of the Bohai Sea completely disappeared. Understanding the formation mechanism of this type of fog is beneficial for refining forecasting focal points, thereby enhancing forecast accuracy in a targeted manner.

A Hybrid Dataset of Historical Cool-Season Lake Effects From the Eastern Great Lakes of North America

The moistening of cold air passing over the Great Lakes of North America has a profound impact on the cool season climate of regions downwind, from relatively benign air mass modification to highly-impactful snowfall events. The importance of lake effects has led to the development of varying techniques for systematically identifying lake-effect days. The results of two such methods are merged here to yield a more thorough record of lake-effect days for the eastern Great Lakes. Comparative analysis of the data sets illustrates the different objectives of the two methodologies, where one identifies days with a synoptic setup conducive to lake-effect snowfall, and the other identifies days with lake-effect modification of the overlying air mass. A smaller population of “absolute” lake-effect days are those identified by both methods, while a larger population of “hybrid” lake-effect days are absolute days plus those identified by one method but not the other. For a 51-year study period ending with the 2014–15 cool season, the absolute data set yields a mean of about 15 lake-effect days per year, or 8% of the November through April season, while the hybrid data set yields a mean of 56 lake-effect days per year, or 31% of the season. The frequencies of absolute, air mass modification-defined, and hybrid lake-effect days decreased through the study period, with days within the hybrid data set declining at a statistically significant rate of 2.8 days per decade, although most obviously from the late 1970s through the early 2000s. The result is a general drying of the cool-season lake-effect hydroclimate. The merged data set offers a more thorough historical record of days available for atmospheric and hydroclimatic study of the lake-effect phenomenon within the eastern Great Lakes region.

Processes contributing to cloud dissipation and formation events on the North Slope of Alaska

Abstract. Clear-sky periods across the high latitudes have profound impacts on the surface energy budget and lower atmospheric stratification; however an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is limited. A method to identify clear periods at Utqiaġvik (formerly Barrow), Alaska, during a 5-year period (2014–2018) is developed. A suite of remote sensing and in situ measurements from the high-latitude observatory are analyzed; we focus on comparing and contrasting atmospheric properties during low-level (below 2 km) cloud dissipation and formation events to understand the processes controlling clear-sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant during the periods bookending clear conditions, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation on the North Slope of Alaska. Further, meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear-sky period: namely, horizontal advection, which was dominant in winter and early spring, and quiescent air mass modification, which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud or fog layer developing near the surface. This low cloud formation is driven largely by air mass modification under relatively quiescent synoptic conditions. Near-surface aerosol particles concentrations changed by a factor of 2 around summer formation events. Thermodynamic adjustment and increased aerosol presence under quiescent atmospheric conditions are hypothesized as important mechanisms for fog formation.

Impact of the Land-Sea Temperature Contrast on the Snowfall Structure over the Western Coastal Region of the Korean Peninsula

In this study, the structure of the typical snowfall which is caused by air-mass modification and is confined over the western coastal region of the Korean Peninsula is investigated using the Weather Research and Forecasting model. In the case of the western coastal snowfall, there are many cases wherein the clouds are confined offshore and do not penetrate inland. Therefore, to clarify the structures related to the snowfall penetration, four experiments are performed with different land-surface air temperature. In the control experiment, the horizontal temperature contrast is observed to establish a density barrier over the coastal region, which induces the upward motion that develops snowfall. However, the density barrier blocks the horizontal inland wind flow, which confines the snowfall over the offshore region. To examine the role of the density barrier, three additional experiments are performed with different 2- m air temperature over land. Intensified horizontal temperature contrast is found to induce a strong density barrier and upward motions over the upstream region of the density barrier, which enhances local snowfall. Additionally, the strong density barrier acts as a blocking wall, which interrupts the inland penetration of the snowfall. When the temperature contrast is weakened, the snowfall intensity is reduced but the snowfall is able to penetrate further inland, meaning that the horizontal land/sea temperature contrast is an important factor for the forecasting of the snowfall over the western coastal region of the Korean Peninsula.

Sea surface temperature influence on a winter cold front position and propagation: air–sea interactions of the ‘Nortes’ winds in the Gulf of Mexico

AbstractA high‐resolution, regional atmospheric model with different sea surface temperature (SST) boundary conditions (BC) is used to examine the air–sea interactions of the winter cold fronts (CF) advancing over the Gulf of Mexico (GoM). Comparison with oceanic‐buoy 10‐m wind, 2 m air temperature (AIR.2 m), sea level pressure (SLP) and SST reveals good agreement with observations. The CF propagation speed was significantly affected by the SST: higher SST produced faster CF traveling speeds. Using a 1‐D ocean mixed layer model as BC reduced the air–sea fluxes and the CF propagated slower but in accordance with the reanalysis data; representing an improvement in the numerical modeling of the CF propagation and airmass modification over the GoM.

Study Case of Air-Mass Modification over Poland and Romania Observed by the Means of Multiwavelength Raman Depolarization Lidars

An air-mass modification, on its way from Poland to Romania, observed between 19-21 July 2014 is discussed. The air-mass was investigated using data of two multi-wavelength lidars capable of performing regular elastic, depolarization and Raman measurements in Warsaw, Poland, and in Magurele, Romania. The analysis was focused on evaluating optical properties of aerosol in order to search for similarities and differences in the vertical profiles describing the atmospheric layers above the two stations within given period.

Impacts of Gulf of Mexico SST Anomalies on Southern Plains Freezing Precipitation: ARW Sensitivity Study of the 28–30 January 2010 Winter Storm

AbstractIce storms are an infrequent but significant hazard in the U.S southern Great Plains. Common synoptic profiles for freezing precipitation reveal advection of low-level warm moist air from the Gulf of Mexico (GOM), above a shallow Arctic air mass ahead of a midlevel trough. Because the GOM is the proximal basin and major moisture source, this study investigates impacts of varying GOM sea surface temperature (SST) on the thermodynamic evolution of a winter storm that occurred during 28–30 January 2010, with particular emphasis on the modulation of freezing precipitation. A high-resolution, nested ARW sensitivity study with a 3.3-km inner domain is performed, using six representations of GOM SST, including control, climatological mean, uniform ±2°C from control, and physically constrained upper- and lower-bound basin-average anomalies from a 30-yr dataset. The simulations reveal discernable impacts of SST on the warm-layer inversion, precipitation intensity, and low-level dynamics. Whereas total precipitation for the storm increased monotonically with SST, the freezing-precipitation response was more varied and nonlinear, with the greatest accumulation decreases occurring for the coolest SST perturbation, particularly at moderate precipitation rates. Enhanced precipitation and warm-layer intensity promoted by warmer SST were offset for the highest perturbations by deepening of the weak 850-hPa low circulation and faster eastward progression associated with enhanced baroclinicity and diabatic generation of potential vorticity. Air-parcel trajectories terminating within the freezing-precipitation region were examined to identify airmass sources and modification. These results suggest that GOM SST can affect the severity of concurrent ice-storm events in the southern Great Plains, with warmer basin SST potentially exacerbating the risk of damaging ice accumulations.

Mesoscale Boundary Layer and Heat Flux Variations over Pack Ice–Covered Lake Erie

Abstract The development of extensive pack ice fields on the Great Lakes significantly influences lake-effect storms and local airmass modification, as well as the regional hydrologic cycle and lake water levels. The evolution of the ice fields and their impacts on the atmospheric boundary layer complicates weather forecasters’ ability to accurately predict late-season lake-effect snows. The Great Lakes Ice Cover–Atmospheric Flux (GLICAF) experiment was conducted over Lake Erie during February 2004 to investigate the surface–atmosphere exchanges that occur over midlatitude ice-covered lakes. GLICAF observations taken by the University of Wyoming King Air on 26 February 2004 show a strong mesoscale thermal link between the lake surface and the overlying atmospheric boundary layer. Mesoscale atmospheric variations that developed over the lake in turn influenced heat exchanges with the surface. Boundary layer sensible and latent heat fluxes exhibited different relationships to variations in surface pack ice concentration. Turbulent sensible heat fluxes decreased nonlinearly with increases in underlying lake-surface ice concentration such that the largest decreases occurred when ice concentrations were greater than 70%. Latent heat fluxes tended to decrease linearly with increasing ice concentration and had a reduced correlation. Most current operational numerical weather prediction models use simple algorithms to represent the influence of heterogeneous ice cover on heat and moisture fluxes. The GLICAF findings from 26 February 2004 suggest that some currently used and planned approaches in numerical weather prediction models may significantly underestimate sensible heat fluxes in regions of high-concentration ice cover, leading to underpredictions of the local modification of air masses and lake-effect snows.

Influences of Large-scale Atmospheric Circulation and Local Sea Surface Temperature on Convective Activity over the Sea of Japan in December

The characteristics of the anomalous winter climate around Japan in December 2005, including abnormally heavy snowfall, were analyzed from the viewpoint of interannual variation using reanalysis data for 50 years. The anomalous snowfall near the Sea of Japan in 2005 is attributed to an anomalous air-mass modification over the Sea of Japan, the large changes of thermodynamic characteristics due to the warmth of the Sea of Japan. Thermodynamic budget analyses showed that the air-mass modification in December 2005 over the Sea of Japan was the strongest in more than 40 years. Air-mass modification over the Sea of Japan was strongly related with a large-scale atmospheric north-south dipole pattern consisting of a northern high covering Siberia and a southern low covering a wide area of Japan in the 500 hPa height field in the positive phase (SJ pattern). In addition, the local SST variability during late autumn was significantly related to air-mass modification. However, the Arctic Oscillation was not significantly correlated with the air-mass modification over the Sea of Japan on an interannual timescale.

Formation, Evolution, and Dissipation of Coastal Sea Fog

Evolution of sea fog has been investigated using three-dimensional Mesoscale Model 5 (MM5) simulations. The study focused on widespread fog-cloud layers advected along the California coastal waters during 14–16 April 1999. According to analysis of the simulated trajectories, the intensity of air mass modification during this advection significantly depended on whether there were clouds along the trajectories and whether the modification took place over the land or ocean. The air mass, with its trajectory endpoint in the area where the fog was observed and simulated, gradually cooled despite the gradual increase in sea-surface temperature along the trajectory. Modelling results identified cloud-top cooling as a major determinant of marine-layer cooling and turbulence generation along the trajectories. Scale analysis showed that the radiative cooling term in the thermodynamic equation overpowered surface sensible and latent heat fluxes, and entrainment terms in cases of the transformation of marine clouds along the trajectories. Transformation of air masses along the trajectories without clouds and associated cloud-top cooling led to fog-free conditions at the endpoints of the trajectories over the ocean. The final impact on cloud-fog transition was determined by the interaction of synoptic and boundary-layer processes. Dissipation of sea fog was a consequence of a complex interplay between advection, synoptic evolution, and development of local circulations. Movement of the high-pressure system over land induced weakening of the along-shore advection and synoptic-pressure gradients, and allowed development of offshore flows that facilitated fog dissipation.

Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth

In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (ΔT) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m−2 within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m−2 on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux \(\left(\overline {w'T \, '_v}\right)\) calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux \((\overline {w'q'})\). The ratio between rising thermals \(\left(\overline {w^+T^+_v}\right) \) and cold downdrafts \(\left(\overline {w^-T^-_v}\right)\) differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by \(\overline {w^+T^+_v} \) in the developing stage of the boundary layer, while \(\overline {w^-T^-_v} \) also became important after the development of the boundary layer.

영동대설 사례와 관련된 동해상의 현열속과 잠열속 분포 특성

To investigate the air mass modification related with Yeongdong Heavy snowfall events, we examined sensible and latent heat fluxes on the East Sea, the energy exchange between atmosphere and ocean in this study. Sensible and latent heats were calculated by a bulk aerodynamic method, in which NCEP/NCAR reanalysis data and NOAA/AVHRR weekly SST data with high resolution were used. Among winter precipitation events in the Yeongdong region, 19 heavy precipitation events were selected and classified into three types (mountain, cold-coastal, and warm types). Mountain-type precipitation shows highly positive anomalies of sensible and latent heats over the southwestern part of the East Set When separating them into the two components due to variability of wind and temperature/ specific Humidity, it is shown that the wind components are dominant. Cold-coastal-type precipitation also shows strong positive anomalies of sensible and latent heats over the northern part and over the central-northern part of the East Sea, respectively. It is shown that the sensible heat anomalies are caused mostly by the decrease of surface air temperature. So it can be explained that cold-coastal-type precipitation is closely related with the air mass modification due to cold air advection over warm ocean surface. But in warm-type precipitation, negative anomalies are found in the sensible and latent heat distributions. From this result, it may be postulated that warm-type precipitation is affected by the internal process of the atmosphere rather than the atmosphere-ocean interaction.

Convective Structures in a Cold Air Outbreak over Lake Michigan during Lake-ICE

Abstract The Lake-Induced Convection Experiment provided special field data during a westerly flow cold air outbreak (CAO) on 13 January 1998, which has afforded the opportunity to examine in detail an evolving convective boundary layer. Vertical cross sections prepared from these data, extending from upstream over Wisconsin out across Lake Michigan, show the modifying effects of land–water contrast on boundary layer mixing, entrainment, heating, and moisture flux. Through this analysis, an interesting case of lake-effect airmass modification was discovered. The data show atypical differing heights in vertical mixing of heat and moisture, as well as offshore downwelling and subsidence effects in the atmosphere. Analysis shows evidence of a new observational feature, the moisture internal boundary layer (MIBL) that accords well with the often recognized thermal internal boundary layer (TIBL). The “interfacial” layer over the lake is also found to be unusually thick and moist, due in part to the upstream conditions over Wisconsin as well as the effectiveness of vertical mixing of moist plumes over the lake (also seen in the aircraft datasets presented). Results show that the atmosphere can be much more effective in the vertical mixing of moisture than heat or momentum (which mixed the same), and thus represents a significant departure from the classical bottom-up and top-down mixing formulation. Four scales of coherent structures (CSs) with differing spatial and temporal dimensions have been identified. The CSs grow in a building block fashion with buoyancy as the dominating physical mechanism for organizing the convection (even in the presence of substantial wind shear). Characteristic turbulence statistics from aircraft measurements show evidence of these multiple scales of CSs, ranging from the smallest (microscale) in the cloud-free path region near the Wisconsin shore, to the largest (mesoscale) in the snow-filled boundary layer near the Michigan shore. A large eddy simulation (LES) model has also been employed to study the effects of buoyancy and shear on the convective structures in lake-effect boundary layers. The model simulation results have been divided into two parts: 1) the general relationship of surface heat flux versus wind shear, which shows the interplay and dominance of these two competing forcing mechanisms for establishing convection patterns and geometry (i.e., rolls versus cells), and 2) a case study simulation of convection analogous to the CSs seen in the CFP region for the 13 January 1998 CAO event. Model simulations also show, under proper conditions of surface heating and wind shear, the simultaneous occurrence of differing scales of CSs and at different heights, including both cells and rolls and their coexisting patterns (based on the interplay between the effects of buoyancy and shear).

Aircraft Observations of Air-Mass Modification Upstream of the Sea of Japan during Cold-Air Outbreaks

Russian research aircraft observed the atmospheric boundary layer in 2001 during weak (29 Jan.), intense (2 Feb.), and very intense (3 Feb.) cold air outbreaks. These observations occurred during a field experiment of Winter Meso-scale convective systems Observations, over the Sea of Japan in 2001 (WMO-01). Conditional sampling techniques were used to elucidate air-mass modification processes upstream of the Sea of Japan during cold-air outbreaks. Buoyancy fluxes in the lower boundary layer near the sea surface differed significantly among the three cases. TKE budgets suggest that the differences were characterized mainly by differences in buoyancy production. However, common properties in heat transfer did exist, namely intensity (3.0) and fractional coverage (28%) of rising thermals (w + T + v ). Scale analysis showed that heating by convective-scale motions dominated in the lower boundary layer. Heating by meso-y scale motions became more important in the upper boundary layer. Near the cloud top, on the other hand, contributions by cold downdrafts (w - T - v ) to the total buoyancy flux (w'T' v ) dominated. The cloud-top w'T' v , was about half of that near the sea surface, suggesting that cooling processes near the cloud top are also important in TKE production. Comparisons of fractional spatial coverage of w + T + v with thermal motions over other warm oceans show good agreement with the AMTEX and GALE cases. However, flux intensities in the present case, and in the GALE case, are stronger than in the AMTEX case. Impacts on ocean convection by the upstream, large surface heat fluxes are discussed.

Air mass modification over Europe: EARLINET aerosol observations from Wales to Belarus

For the first time, the vertically resolved aerosol optical properties of western and central/eastern European haze are investigated as a function of air mass transport. Special emphasis is put on clean maritime air masses that cross the European continent from the west and become increasingly polluted on their way into the continent. The study is based on observations at seven lidar stations (Aberystwyth, Paris, Hamburg, Munich, Leipzig, Belsk, and Minsk) of the European Aerosol Research Lidar Network (EARLINET) and on backward trajectory analysis. For the first time, a lidar network monitored continent‐scale haze air masses for several years (since 2000). Height profiles of the particle backscatter coefficient and the particle optical depth of the planetary boundary layer (PBL) at 355‐nm wavelength are analyzed for the period from May 2000 to November 2002. From the observations at Aberystwyth, Wales, the aerosol reference profile for air entering Europe from pristine environments was determined. A mean 355‐nm optical depth of 0.05 and a mean PBL height of 1.5 km was found for clean maritime summer conditions. The particle optical depth and PBL height increased with increasing distance from the North Atlantic. Mean summer PBL heights were 1.9–2.8 km at the continental sites of Leipzig, Belsk, and Minsk. Winter mean PBL heights were mostly between 0.7 and 1.3 km over the seven EARLINET sites. Summer mean 355‐nm optical depths increased from 0.17 (Hamburg, northwesterly airflow from the North Sea) and 0.21 (Paris, westerly flow from the Atlantic) over 0.33 (Hamburg, westerly flow) and 0.35 (Leipzig, westerly flow) to 0.59 (Belsk, westerly flow), and decreased again to 0.37 (westerly flow) at Minsk. Winter mean optical depths were, on average, 10–30% lower than the respective summer values. PBL‐mean extinction coefficients were of the order of 200 Mm−1 at 355 nm at Hamburg and Leipzig, Germany, and close to 600 Mm−1 at Belsk, Poland, in winter for westerly flows. Whereas the optical depth for westerly flows was typically <0.35 during the summer halfyear, it increased to values of 0.5–0.7 over most of the central European sites during easterly flows. Compared to aerosol sources in Poland and southeastern Europe, the highly industrialized and populated western European region was found to contribute only moderately to the European aerosol burden. Comparably clean conditions (low particle optical depth) prevailed at the Munich site, indicating a sensitive influence of the orography on haze conditions. Estimates of the mean effective (upward minus downward) aerosol mass flux into the atmosphere along the way from the Atlantic Ocean to central Europe (Leipzig), and from Leipzig to Belsk, that are consistent with the optical depth increase, yield values of 0.11–0.17 μg/(m2s) and 0.25 μg/(m2s), respectively. These values correspond to mean effective aerosol mass fluxes for 50 km × 50 km grid cells of the order of 10000 Mg/year and 20000 Mg/year, respectively. The estimates are in reasonable agreement with EMEP emission data.

Role of the Land Plume in the Transport of Ozone Over the Ocean During Indoex (1999)

Observations from aircraft, an island station, and tworesearch vessels are used to investigate the development of an elevated mixed layeror land plume over the Arabian sea during the Indian Ocean Experiment Intensive FieldPhase 1999 (INDOEX) through air mass modification. Much of the transport of aerosolsand gases occurs in this plume located above a well-mixed convective marine boundary layerwith a depth of 800–1000 m. The depth of the land plume isapproximately 2000 m with the peak ozone concentrations occurring near the centre of this land plume.Significant latitudinal variations in the concentration of ozone occur in the marineboundary layer and in the plume. Mean ozone concentrations in the land plume decreasedwith distance from the Indian coastline.

The Atmospheric Boundary Layer In An Arctic Wintertime On-Ice Air Flow

A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Straitwas, for the first time, systematically measured on 12 March 1998 by aircraft in thelowest 3 km over a 300-km long distance. The air mass modification and the processesinvolved are discussed. Over the water, air temperature was lower than water temperature so that a convectiveboundary layer (CBL) was present as initial condition. As soon as the CBL passed theice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s-1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures causedby both different ice thickness and changes in the cloud conditions, and (b) leads in theice deck. At the front side of the on-ice flow, the air mass boundary between the warmair and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted withheight. Observations suggest that the stratified IBL was lifted as a slab on top of thecold air.

ヤマセと海風の地域的な気象特性と中山間地（岩手県久慈市〜山形村）への侵入

The meteorological features of Yamase (cold northeasterly wind) and sea breezes are investigated for the summer of 1997, and their invasion to the hilly mountain area on the Pacific side are discussed.When sea breezes are present, solar radiation is generally large, but the temperature decreases on the coastal region of the Pacific due to the cold air masses brought by the wind from the ocean. During the Yamase period, both solar radiation and temperature decrease, and the lowest solar radiation areas are found on the eastern side of the mountains.The Yamase process has three stages: early, peak, and late. In the peak stage, there was a mixed layer from the surface to about 900hPa (1, 000m) and easterly winds reached above 800hPa (2, 000m). In the early and late stages there were stable layers at lower height with easterly winds. In spite of the different vertical profiles of stratification and easterly wind, clouds were formed at the same height (below 1, 500-2, 000m) through the three stages.Northeastern Japan was covered by a large and homogeneous air mass during the Yamase period, however, below 800hPa relative humidity and potential temperatures differed between the Pacific and Japan Sea sides. These differences were due to modifications of the low level air mass, by processes such as damming up of air masses by mountains and heating from the surface.In the study area (hilly mountain area), during the sea breeze period, the low temperatures were confined to the coastal area and the meteorological conditions differed from inland area. On the other hand, the potential temperatures were almost the same from the coastal to inland areas during the Yamase period. In this case, the air mass with mixed layers invaded from the coast to the inland with very small and/or no air mass modification.

The Use of Pre-Storm Boundary-Layer Baroclinicity in Determining and Operationally Implementing the Atlantic Surface Cyclone Intensification Index

The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low-level baroclinicity within the coastal zone during periods of offshore cold advection. The ralative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period often precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advection period and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24–48 h prior to the initial cyclonic intensification) are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting. The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982–April 1990), 116-storm study conducted previously. During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham (RAH) forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.

Evolution of the cloudy boundary layer during the autumnal freezing of the Beaufort Sea

The Beaufort and Arctic Seas Experiment (BASE) occurred during September and October 1994, off the Canadian Beaufort coast. In this paper, we provide a description of the characteristics of the the cloudy boundary layer that were observed from a research aircraft during BASE and provide an interpretation for the coevolution of the atmospheric boundary layer and sea ice characteristics during the autumnal freezing of the Beaufort Sea. Observations from a total of 12 research flights are presented. During the course of the experiment the underlying surface made a transition from late summer melt conditions to full‐fledged winter conditions. All of the observed boundary layer cloud decks are associated with stable temperature profiles, with temperature inversions occurring either above or within each of the cloud decks. Unlike observations of summertime boundary layer clouds, no multiple‐layered clouds in the boundary layer were observed. Examination of the cloud phase indicates a transition from predominantly liquid phase clouds at the beginning of the experiment to predominantly crystalline clouds at the end of the experiment. Local interactions between the clouds and underlying surface were examined in several case studies, including the effects of leads. Using a time series of soundings over a period of 5 days in conjunction with the the National Center for Environmental Prediction analyses, an air mass modification event was examined that began after the intrusion of warm air from the Pacific Ocean. The evolution of the air mass is apparent in decreasing temperatures in the lower atmosphere and formation of a low‐level mixed phase cloud that evolves with time into a deeper cloud layer that is crystalline.