121611894
Characteristics of Low-level Jets in Shanghai during the 2008-2009 Warm Seasons as Inferred from Wind Profiler Radar Data
In order to understand the characteristics of low-level jets (LLJs) over coastal areas of China Continent and its linkages with rainfall during the warm seasons, half-hourly data from a wind profiler radar (WPR) at the Qingpu site during the Meiyu periods of 2008 and 2009 in addition to data from one month prior to and after (non-Meiyu periods) were used to develop a climatology of the LLJs over Shanghai, China. Two peaks in LLJ incidence were revealed at 500–800 m and 2100–2200 m altitudes. Thus, we classified the observed LLJs into two types: (1) boundary-layer jets (BLJs, below 1 km) and (2) synoptic-system-related LLJs (SLLJs, within 1–3 km). BLJs and SLJs showed different behavior in their temporal variation of the occurrence frequency, wind direction, and relation to rainfall. The BLJs displayed a more evident diurnal cycle than the SLLJs, with maximum incidence occurring in nighttime and early morning. The SLLJs occurred more frequently during Meiyu periods than non-Meiyu periods, whereas the occurrence frequency of BLJs increased throughout the warm seasons with no peak during Meiyu periods. BLJs are mostly southerly winds that might be induced by inertial oscillation with a strong background southerly geostrophic wind due to the west-east land-ocean thermal difference. SLLJs most frequently appeared as southwesterly and westerly winds embedded in the East Asian monsoon circulation. The relation between LLJs and precipitation was examined by comparing the frequency of LLJs occurrence on rainy days (cases), with daily (4-hourly) accumulated rainfall within 1 to 10 mm, and non-rainy days (cases) because the missing rate of WPR data is high during heavy rainfall events. Both BLJs and SLLJs occur more often on rainy days than on non-rainy days. In shorter time scales (4 hours), both BLJs and SLLJs tend to occur frequently during, before and after rainy cases, except SLLJs before rainy cases.
119798162
A Diagnostic Study of the Low-Level Jet during TAMEX IOP 5
Abstract During 31 May–2 June 1987 a low pressure center developed around 28°N, 102°E in the lee of the Tibetan Plateau during the passage of a midlatitude trough. It moved northeastward and intensified over the Yangtze River valley and produced widespread rainfall over southeastern China. A strong southwest flow developed in the lower troposphere over southern China as the cyclone deepened. At 0000 UTC 1 June, a well-defined low-level jet (LLJ) was found between the 850- and 700-hPa levels with wind speed exceeding 15 m s−1 at the 850-hPa level and 20 m s−1 at the 700-hPa level. The LLJ transported warm, moist air from the subtropical ocean, increased the moist static energy, and provided a favorable environment for the development of widespread precipitation. It is shown that the development of the LLJ is closely related to the developing lee cyclone to the east of the Tibetan Plateau. Our analysis reveals that the diagnosed secondary circulation associated with the jet-front system differs from those p...
124385323
Doppler Radar Observations of the Development of a Boundary-Layer Nocturnal Jet
Abstract A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s−1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical vel...
123257437
Observation of the Diurnal Cycle in the Low Troposphere of West Africa
Abstract The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground. Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s−1. This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor fro...
124731451
A Case Study of the Summertime Great Plains Low Level Jet
Abstract A case study of the kinematical and dynamical evolution of the summertime Great Plains low level jet (LLJ) is presented. Airborne radar altimetry was used to discern the x and y components of the geostrophic wind at three levels in the lower atmosphere throughout the LLJ episode. Results appear to confirm previous theoretical and numerical studies regarding the importance of the diurnal cycle of heating over sloping terrain in producing an oscillating horizontal pressure gradient force. Inertial turning of the LLJ as a result of frictional decoupling was also documented. It is concluded that the inertial oscillation resulting from the sudden decrease in friction in the lower atmosphere during the early evening is the dominant mechanism in forcing this example of a summertime Great Plains LLJ.
119642161
A Numerical Simulation of the Mei-Yu Front and the Associated Low Level Jet
Abstract A two-dimensional frontal model was used to study the structure and behavior of the Mei-Yu front over East Asia. The Mei-Yu front is characterized by mixed midlatitude-baroclinic and tropical-convective properties, with frequent occurrence of a low-level jet (LLJ) that is highly correlated with heavy convective rainfall. The quasi-steady state responses to a large-scale stretching deformation forcing were obtained by integrating the perturbation equations from an initial state of seasonal-mean zonal flow. Two major sets of experiments were conducted to simulate different midlatitude and subtropical conditions. The midlatitude front extends deeply into the upper troposphere with a strong poleward tilt, whereas the subtropical front is confined to the lower troposphere with less tilt, in good agreement with observations. Along the sloping front, slantwise updrafts develop with a multiband structure. This updraft is more evident in the subtropical cases and in the more moist midlatitude cases. For t...
9446832
On the Role of Sloping Terrain in the Forcing of the Great Plains Low-Level Jet
Abstract The summertime Great Plains low-level jet (LLJ) has been the subject of numerous investigations during the past several decades. Characteristics of the LLJ include nighttime development of a pronounced wind maximum of typically 15–20 m s−1 at levels 300–800 m above the surface and a clockwise rotation of the wind maximum during the course of the night. Maximum frequency of occurrence of the LLJ is found in the southern Great Plains. Theories proposed to explain the diurnal wind maximum of the Great Plains LLJ include inertial oscillation of the ageostrophic wind, the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain, and the western boundary current interpretations. A simple equation system and output from the 12-km horizontal resolution Weather Research and Forecasting Nonhydrostatic Mesoscale Model (NAM) for July 2008 are used to provide evidence as to the importance of the Great Plains topography in driving the LLJ. Summertime heati...
129978828
A Case Study of the Great Plains Low-Level Jet Using Wind Profiler Network Data and a High-Resolution Mesoscale Model
Abstract A detailed case study of one complete episode of a typical summertime Great Plains low-level jet (LLJ) using data collected by the NOAA wind profiler demonstration network is presented. The high temporal and spatial resolution of the data from the profiler network permits a much more detailed picture of the Great Plains LLJ than is possible from previous studies of this phenomenon. A three-dimensional mesoscale numerical model is also used to simulate the episode and to provide information on the physical mechanisms responsible for the initiation, evolution, maintenance, and decay of the LLJ. The position and width of the jet core, as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model. The analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the nigh...
124779252
Barrier Jets during TAMEX
Abstract A barrier jet is frequently found along the northwestern coast of Taiwan in the prefrontal southwesterly flow regime during the Taiwan Area Mesoscale Experiment (TAMEX). It has a maximum wind speed of 14 m s−1 at approximately 1 km above the surface with a vertical wind shear approximately 10 × 10−3 s−1 below and 4 × 10−3 s−1 above that altitude. During TAMEX, the southwesterly monsoon flow strengthens over Taiwan when the low-level pressure trough/surface front moves toward the southeastern China coast. The barrier jet is a result of the stably stratified airflow past an island obstacle under a small–Froude number [<O(1)] flow regime. During the occurrence of a barrier jet, a windward pressure ridge is observed along the southwestern coast due to island blocking. In low levels, the incoming southwesterly flow decelerates off the southwestern coast and moves around the island. Along the western coast, the deflected airflow accelerates northward with a large cross-contour wind component down the p...
120185303
Numerical experiments investigating the orographic effects on a heavy rainfall event over the northwestern coast of Taiwan during TAMEX IOP 13
An unusual heavy coastal rainfall event (>231 mm day−1) occurred during the period of 24–25 June 1987 over the lowland (elevation less than 200 m) and coastal areas in northwest and central Taiwan. The Weather Research and Forecasting (WRF) model is used to investigate the role of synoptic forcing, orographic effects and the diurnal heating cycle on the generation of a prefrontal localized low-level convergence zone offshore leading to the observed coastal rainfall maximum. This case is well simulated by the control experiment initialized at 0000 UTC (0800 LST) 24 June 1987 using the European Centre for Medium-Range Weather Forecasts data. A model sensitivity test without Taiwan’s terrain fails to reproduce the observed coastal rainfall maximum. It is apparent that for this case, synoptic forcing by the Mei-Yu jet/front system is inadequate to initiate deep convection leading to the development of coastal heavy precipitation. The generation of the localized low-level convergence zone is closely related to the simulated strong winds with a large southwesterly wind component (or the barrier jet) along the northwestern coast as the surface front approaches. The development of the simulated barrier jet is due to a 50–60% increase in the meridional pressure gradient as a result of orographic blocking. The diurnal heating cycle also impacts the strength of the simulated barrier jet over the northwestern Taiwan coast. The simulated barrier jet is stronger (~3 m s−1) in the early morning than in the afternoon as orographic blocking is most significant when the surface air is the coldest. The representation of the terrain in the model impacts the simulated barrier jet and rainfall. With a coarse horizontal resolution (45 km), orographic blocking is less significant than the control run with a much weaker meridional wind component over the northwestern coast of Taiwan. The coarse resolution model fails to reproduce the observed rainband off the northwestern coast. Thus, to successfully simulate this type of event, high-resolution mesoscale models adequately depicting Taiwan’s terrain are required.
123897841
The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms
Abstract Transverse circulations in the exit and entrance regions of jet streaks are investigated through numerical simulation, a case study, and an application of the isallobaric wind equation in isentropic coordinates, to study the interaction between upper and lower tropospheric jets and the development of severe convective storms. A hybrid isentropic-sigma coordinate numerical model is used to simulate the mass and momentum adjustments associated with a jet streak propagating in a zonal channel. The numerical results depict a two-layer mass adjustment in the exit and entrance region of the jet streak. The results also verify that the isallobaric wind on lower isentropic surfaces is a primary component of the return branches of transverse circulations and is foxed by the two-layer mass adjustment accompanying the propagating jet streak. Results from the case study of a severe weather out- break show that 1) a low-level jet (LLJ) beneath the exit region of an upper tropospheric jet streak is embedded in...
120299735
On the Formation of the Low-Level Jet over Texas
Abstract The low-level jet (LLJ) in the winter half-year originates as a south wind over the high plains of Colorado. Kansas, western Oklahoma and West Texas. After formation, the LLJ spreads horizontally and upstream toward the Gulf of Mexico. The air in the LLJ becomes humid when the entrance to the jet moves to the Gulf. The water vapor in the LLJ originates in the nearby waters of the Gulf and enters Texas within the tropical air mass which has a recent (1–3 days) polar origin. A mature LLJ often has multiple maxima of the wind speed.
120634443
Synergistic interactions between an upper-level jet streak and diabatic processes that influence the development of a low-level jet and a secondary coastal cyclone
Abstract A series of numerical simulations is presented for the February 1979 Presidents' Day cyclone in order to understand more fully the roles played by upper-level jet streaks the oceanic planetary boundary layer (PBL), and latent heat release in the development of a low-level jet (LU) and secondary cyclogenesis along the East Coast of the United States. Mesoscale model simulations with and without sensible and latent heating show that the diabatic processes, along with the jet streak circulation patterns, contribute to the enhancement of the low-level winds and the initial development of the coastal cyclone. However. none of the mechanisms acting alone is sufficient to yield a satisfactory simulation of the LIJ and secondary cyclogenesis. Furthermore, the model-based diagnostic analyses indicate that a synergistic interaction must exist among these processes to account for the substantial increase in the magnitude of the low-level winds and the decrease in the sea level pressure that mark the seconda...
121486905
Study of low-level jet and extremely heavy rainfall over northern Taiwan in the Mei-Yu season
Abstract Extremely heavy rainfall (at least 100 mm d−1) is one of the most disastrous weather phenomena in the early-summer rainy season of Fast Asia. This season is called the Mei-Yu season in Taiwan. Thirty-five cases of this rainfall event over northern Taiwan in May-June 1965–84 were selected for studying its relationship between the low-level jet (LLJ) and extremely heavy rainfall. As in Japan and China, extremely heavy rainfall is found to be closely associated with a LLJ. For example, there was an 84% likelihood that a LLJ of at least 12.5 m s−1 would be present at 700 mb 12 h before the start of the rainfall event. Conversely, when a LLJ was present over Taiwan in 1980–84, there was a 91% likelihood that such a rainfall event was in progress or would begin on the next day. Results provide evidence that the LIJ plays an important role in the development of this rainfall event. It appears that cumulus convection in the area of rainfall homogenizes the vertical wind profile so as to dissipate the LLJ...
73677320
Characteristics of Low-Level Jets over Northern Taiwan in Mei-Yu Season and Their Relationship to Heavy Rain Events
Abstract The present study investigates the characteristics of low-level jets (LLJs) (≥12.5 m s−1) below 600 hPa over northern Taiwan in the mei-yu season and their relationship to heavy rainfall events (≥50 mm in 24 h) through the use of 12-h sounding data, weather maps at 850 and 700 hPa, and hourly rainfall data at six surface stations during the period of May–June 1985–94. All LLJs are classified based on their height, appearance (single jet or double jet), and movement (migratory and nonmigratory). The frequency, vertical structure, and spatial and temporal distribution of LLJs relative to the onset of heavy precipitation are discussed. Results on the general characteristics of LLJs suggest that they occurred about 15% of the time in northern Taiwan, with a top speed below 40 m s−1. The level of maximum wind appeared mostly between 850 and 700 hPa, with highest frequency at 825–850 hPa. A single jet was observed more often (76%) than a double jet (24%), while in the latter case a barrier jet usually ...
122333141
Characteristics of Low Level Jets over Okinawa in the Baiu and post-Baiu Seasons Revealed by Wind Profiler Observations
Hourly data from a 400-MHz Wind Profiler Radar (WPR) during the warm season months of three years are used to develop a climatology of the low level jet (LLJ) over Okinawa, a Japan subtropical island. Long-term observations of the wind with high temporal and vertical resolution allowed an investigation of the LLJ statistics. Characteristics of the LLJ are investigated in two subperiods: during the Baiu season over Okinawa Island and the post-Baiu season when the Baiu front has moved further north of Okinawa. The investigation result shows that stronger LLJs occur more frequently during the Baiu season, when heavy precipitation is mostly observed compared to the post-Baiu season. The appearance frequency of the maximum speed height of LLJs has a double peak structure in the vertical; lower and higher peaks appear around heights of 600 m and 1.3 km, respectively. In the rain cases with daily rainfall equal and more than 1 mm, another peak also appears around a height of 3 km.The frequency of LLJ occurrence exhibits a diurnal variation with the maximum in the nighttime and early morning, especially in the non-rain cases. The examination with upper-air sounding observations suggests that the nocturnal preference of LLJ occurrence is brought from the reduction of vertical mixing in the atmospheric boundary layer after sunset. Additional investigations of LLJs, using data from the WPR network of the Japan Meteorological Agency for three other stations surrounding Okinawa Island, provide more evidence for the role of the atmospheric boundary layer in generating the diurnal variation of LLJ occurrence.
17848859
CLIMATOLOGY OF THE LOW LEVEL JET
Abstract Geographical and diurnal variations in the frequency of occurrence of strong low level wind maxima are determined using 2 yr. of wind data from 47 rawinsonde stations in the United States. Maximum frequency of occurrence is found in the Great Plains at approximately 37°N. and 98°W. The vast majority of jets in this region occur with southerly flow. Southerly wind maxima appear on both morning and afternoon soundings but occur with much greater frequency, over a larger area, on the morning observations. Twenty-eight morning jet cases are used to determine average synoptic-scale wind and temperature patterns in the vicinity of the jet. Diurnal wind oscillations are examined by comparisons of jet frequencies, speeds, and altitudes on four-times-daily observations. The oscillation is similar to that described by Blackadar; however, there is no apparent tendency for the latitudinal variation in period of the oscillation which Blackadar's model implies.
15977965
Characteristics of Nocturnal Low-Level Jets Observed in the North Florida Area
The seasonal and interannual variability of the nocturnal low-level jets over the north Florida region are investigated using sodar measurements spanning 540 nights. On average, jets are present in 62% of the nocturnal periods examined. The observed jet speeds range between 3 and 21 m s 21 and heights are between 80 and 700 m. Observations show that the low-level jet occurs more frequently (70% of the nocturnal periods) during the colder months November‐February in contrast with the warmer months June‐August (;47%). The presence of southerly jets dominates the summer months, whereas northerly jets are more frequent during winter. Colder months frequently exhibit jets with speeds exceeding 14 m s 21 , often associated with the passage of frontal systems. The interannual variability observed using the North American Regional Reanalysis (NARR) wind profile data during a 4-yr period shows only minimal differences in jet characteristics.A comparison of jet heights with NARR planetary boundary layer heights suggests that jets at the north Florida location frequently occur within the planetary boundary layer. The occurrence and speed of observed low-level jets are linked to both the land‐ocean temperature contrast and to the strength and orientation of surface pressure gradients over the region. A high occurrence of large-amplitude oscillations with approximately a 24-h period near the jet height is shown using the Hilbert‐Huang transform analysis, suggesting that inertial oscillations are one possible cause of jet formation in north Florida.
119865616
A Climatology of the Warm Season Great Plains Low-Level Jet Using Wind Profiler Observations
Abstract Hourly observations from the Wind Profiler Demonstration Network during the warm season months of 1991 and 1992 were used to develop a climatology of the low-level jet (LLJ) over the Great Plains of the central United States. The maximum overall frequency of LLJ occurrence was in the southern part of the Great Plains, while the maximum frequency of the stronger LLJs extended farther north and east (Kansas through Nebraska). The frequency of occurrence for the weakest LLJs exhibited little diurnal variation. In contrast, the strongest jets were about six times more likely to occur within a few hours of local midnight than during the day. Southerly wind events that did not satisfy the criteria for low-level jets (i.e., those that did not include the low-level local maximum of the wind speed profile) showed comparatively little diurnal variability regardless of speed. Analysis of the synoptic patterns associated with LLJ occurrence showed that the LLJ was promoted by the warm sector of an extratropi...
54768421
Climatology of the Low-Level Jet at the Southern Great Plains Atmospheric Boundary Layer Experiments Site
A unique dataset obtained with combinations of minisodars and 915-MHz wind profilers at the Atmospheric Boundary Layer Experiments (ABLE) facility in Kansas was used to examine the detailed characteristics of the nocturnal low-level jet (LLJ). In contrast to instruments used in earlier studies, the ABLE instruments provide hourly, high-resolution vertical profiles of wind velocity from just above the surface to approximately 2 km above ground level (AGL). Furthermore, the 6-yr span of the dataset allowed the examination of interannual variability in jet properties with improved statistical reliability. It was found that LLJs occurred during 63% of the nighttime periods sampled. Although most of the observed jets were southerly, a substantial fraction (28%) was northerly. Wind maxima occurred most frequently at 200–400 m AGL, though some jets were found as low as 50 m, and the strongest jets tended to occur above 300 m. Comparison of LLJ heights at three locations within the ABLE domain and at one location outside the domain suggests that the jet is equipotential rather than terrain following. The occurrence of southerly LLJ varied annually in a way that suggests a connection between the tendency for jet formation and the large-scale circulation patterns associated with El Nino and La Nina, as well as with the Pacific decadal oscillation. Frequent and strong southerly jets that transport moisture downstream do not necessarily lead to more precipitation locally, however.
8405638
Low-Level Jet Climatology from Enhanced Rawinsonde Observations at a Site in the Southern Great Plains
Abstract A climatology of the Great Plains low-level jet (LLJ) is developed from 2 yr of research rawinsonde data obtained up to eight times per day at a site in north-central Oklahoma. These data have better height and time resolution than earlier studies, and show that jets are stronger than previously reported and that the heights of maximum wind speed are closer to the ground. LLJs are present in 47% of the warm season soundings and 45% of the cold season soundings. More than 50% of the LLJs have wind maxima below 500 m above ground level (AGL). Because the 404-MHz radar profiler network in the central United States has its first data points at 500 m AGL, it is likely to miss some LLJ events and will have inadequate vertical resolution of LLJ wind structure. Previous studies have identified LLJs on the basis of a wind speed profile criterion. This criterion fails to separate the classical southerly LLJs from the less frequent northerly jets, which differ in both structure and evolution. Classical sout...
14835898
Global Distribution and Characteristics of Diurnally Varying Low-Level Jets
Abstract This study documents the global distribution and characteristics of diurnally varying low-level jets (LLJs), including their horizontal, vertical, and temporal structure, with a special emphasis on highlighting the underlying commonalities and unique qualities of the various nocturnal jets. Two tools are developed to accomplish this goal. The first is a 21-yr global reanalysis performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) using a horizontal grid spacing of 40 km. A unique characteristic of the reanalysis is the availability of hourly three-dimensional output, which permits the full diurnal cycle to be analyzed. Furthermore, the horizontal grid spacing of 40 km better resolves many physiographic features that host LLJs than other widely used global reanalyses. This makes possible a detailed examination of the systematic onset and cessation of the jets, including time–height representations of the diurnal cycle. The second tool is an index of nocturnal...
122142424
Detecting the Diurnal Cycle of Rainfall Using Satellite Observations
Abstract The diurnal cycle in rainfall varies considerably from region to region in the tropics. Determining this variability is important both for comparing predictions of atmospheric models to real atmospheric behavior and for making sure that estimates of total rainfall from low-altitude satellites are not biased because of their infrequent observations of a given region of the earth. Although there are no data from the proposed Tropical Rainfall Measuring Mission (TRMM) satellite to work with yet, we can ask how well the diurnal cycle in rainfall will be detected when the satellite is eventually collecting data, given the satellite's proposed sampling characteristics. Data analyses for the diurnal cycle are discussed, taking into account the fact that the satellite visits will be irregularly spaced in time. The amplitudes of the first few harmonics will be determined by least-square fits to the satellite observations, and the tests needed to establish the statistical significance of the fitted amplitu...
123135646
Diurnal Variation and Horizontal Extent of the Low-Level Jet over the Northern Gulf of California
Abstract This note describes aspects of the diurnal variation and geographical location of the summertime low-level jet over the Gulf of California, based on wind soundings made at two sites near the northern end of the gulf during a two-week summer field program in 1995. The low-level wind observations from Puerto Penasco, a site along the northern shore of the gulf, showed a strong diurnal cycle decreasing in amplitude to near 0 by the 1800-m altitude. A weaker, but deeper diurnal cycle was observed at Mexicali, an inland site some 200 km west-northwest of Puerto Penasco. The strongest winds were observed at the coastal site; the observations suggest that the strongest southerly winds in the northern gulf lie closer to the eastern shore of the gulf.
120362891
Development of Low-Level Jets during TAMEX
Abstract Detailed synoptic-scale map analysis and diagnosis for all six low-level jet (LLJ) cases that occurred during the Taiwan Area Mesoscale Experiment (TAMEX) were made to study the structure and the development of the LLJs. It was shown that the development of these LLJs was closely linked to the development of a lee trough to the cast of the Tibetan Plateau. Seasonal change had a profound influence on the secondary circulation across the jet-front system. Before the seasonal transition, the formation of a lee trough with closed isobars in low levels east of the Tibetan Plateau occurred as an upstream short-wave trough moved eastward with strengthening westerlies over the Tibetan Plateau in upper levels. After the development of the lee trough, the southwest flow appeared in the southeast quadrant of the Ice trough. As the upstream trough moved farther eastward, it merged with the lee trough, and the low-level southwesterly flow intensified. For most cases, a midlatitude deep trough was present to t...
123684787
Dynamics and Prediction of a Mesoscale Extreme Rain Event in the Baiu Front over Kyushu, Japan
Abstract A high-resolution version of the limited-area primitive equations model of the Bureau of Meteorology Research Centre is used to investigate the dynamics and prediction of the mesoscale, extreme rain event of 1 August 1993 over Kyushu. The model is multiply nested, uses high-order numerics, includes sophisticated parameterizations of physical processes, and has a capability to use cloud imagery from the Japanese Geostationary Meteorological Satellite (GMS) to initialize the vertical motion field. The short-term forecasts of the timing and distribution of rainfall are very encouraging. However, they are sensitive to many components of the forecast system, particularly the diabatic initialization and boundary layer parameterization. Large-scale flow changes during the event were 1) the maintenance of the subtropical ridge to the south of Japan and the development of a trough over Korea, which combined to maintain a warm and moist southwesterly flow over Kyushu, 2) the movement of a cold low from wes...
53386369
A Numerical Study of a Mesoscale Convective System over the Taiwan Strait
On 7 June 1998, a mesoscale convective system (MCS), associated with a mesoscale cyclone, was initiated on the south side of a mei-yu front near Hong Kong and developed over the Taiwan Strait. In this study, numerical simulations for this event are performed using the fifth-generation Pennsylvania State University‐NCAR Mesoscale Model (MM5). The model captures the evolution of the MCS, including the shapes of clouds and the rainfall rate. In the mature phase of the simulated MCS, the MCS is composed of several meso-b- and meso-g-scale convective clusters possessing commalike shapes similar to that of a midlatitude occluded cyclone. The cluster at the head of the ‘‘comma’’ consists of convective clouds that are decaying, while the tail of the comma is made up of a leading active convective line. A mesoscale cyclone, associated with a mesolow, at the tailing region of the leading convective line is well developed below 500 hPa. At 850 hPa, a mesoscale low-level jet (mLLJ) is located on the south side of the mesolow, and is directed toward the comma-shaped convective clusters. At 300 hPa, a mesohigh develops over the leading cluster. A mesoscale upper-level jet (mULJ) is located on the east side of this mesohigh. Relative streamline and trajectory analyses show that the mLLJ, associated with low ue and sinking air motion, is a rear-inflow jet, while the mULJ is the outflow jet of the MCS. Monsoon air from the boundary layer in front of the MCS feeds deep convection within the MCS. Momentum budget calculations are performed in the regions of the mLLJ and mULJ, at the developing and mature stage of the MCS. The pressure gradient force and the horizontal advection are the main contributors to the development of mLLJ in the developing stage. Although the effects of the pressure gradient force are weakened considerably when the MCS reaches maturity, the horizontal advection continues to accelerate the mLLJ. Vertical advection tends to decelerate the mLLJ both in its developing and mature stages. The pressure gradient force and vertical advection are responsible for generating the mULJ in the early stages, and maintaining the mULJ in its mature stage. Strong convective upward motion, which carries the horizontal momentum upward, from the exit of the mLLJ to the entrance of the mULJ, is crucial in the vertical coupling of the mLLJ and mULJ.
122905763
Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season
The level of neutral buoyancy (LNB) around the Japan Islands during the Baiu season in 2001-2005, estimated by lifting a low-level air with the maximum equivalent potential temperature in a vertical direction, is statistically examined from objective analysis data with the horizontal resolution of 20 km, in order to clarify the surrounding atmospheric conditions under which cumulonimbi can form and develop. Three and two peaks are found in the vertical profiles of the appearance rate of the LNB over the sea and on the land, respectively. The high-level (∼ 150 hPa) peak is very weak in June, and the low-level (∼ 900 hPa) peak does not appear on the land. The other peak is found at the middle level (N 700 hPa).The theoretical study shows that the LNB during the Baiu season strongly depends on the lapse rate between the low and middle levels, when the variation of the low-level temperature is within a few degrees. This is because the LNB during the Baiu season rapidly becomes higher as the lapse rate increases. The statistical results show that the lapse rate around the Japan Islands strongly depends on convective activities over upstream regions, especially the southern part of China. Therefore, strong convective activities over the Baiu frontal zone in June produce the middle-level LNB around the Japan Islands to suppress the development of cumulonimbi. In July, the northward shift of the area with strong convective activities considerably releases such suppression, associated with that of the Baiu frontal zone.The middle-level LNB also frequently appears over the other areas around the Baiu frontal zone, although it scarcely appears over the western part of the North Pacific Ocean. Therefore, the middle-level LNB that suggests the existence of cumulonimbi with a cloud top lower than several kilometers is one of the characteristic features around the Baiu frontal zone.