Pseudo-equivalent Potential Temperature Research Articles

Remote Effects of double Typhoons on Record-breaking Rainfall: A case Study in North China

During 18–22 July 2021, an extreme precipitation event occurred in Henan, China, with maximum daily precipitation reached 657 mm, broking their historical records and resulting in severe disaster. This heavy rainstorm occurred with the background of two typhoon (In-fa and Cempaka) activities, but resent studies have only considered the effects of individual typhoons, ignoring their combined effects. The Dynamic Recycling Model and Weather Research and Forecasting Model were applied to investigate the contributions of two typhoons on water vapor transportation. Two transportation paths were determined: the eastern path for In-fa and the southwestern path for Cempaka. These paths contributed up to 78% of the total water vapor over Henan. Two typhoons jointly led to extreme precipitation, and Cempaka showed greater impact than In-fa on the vapor transportation. The precipitation was 39.0% of that in the control test when only Cempaka existed and 34.4% when only considering In-fa. The sensitivity results also indicate that Cempaka mainly induced the early stage of precipitation, while In-fa controlled the later stage. In addition to the effect of water vapor transportation, In-fa also influenced the local atmospheric stability. The pseudo equivalent potential temperature near the surface was 6 K higher when only In-fa existed relative to the scenario in which no typhoon was considered, indicating that In-fa strengthened the convective instability. Therefore, both Cempaka and In-fa contributed to this extreme precipitation event. Cempaka transferred more water vapor than In-fa, and In-fa acted to increase the instability. Their joint contribution amplified the precipitation intensity beyond the cumulative impact of individual effects.

Multiscale Features and Triggering Mechanisms of the Warm-Sector Heavy Rainfall Accompanied by Warm Shear Along the Yangtze–Huaihe Coastal Regions

Abstract Using high-resolution hourly precipitation data and ERA5 reanalysis data, this study employs the K-means method to categorize 32 cases of warm-sector heavy rainfall events accompanied by a warm-type shear line (WSWR) along the Yangtze–Huaihe coastal region (YHCR) from April to September during 2010–17. Considering the synoptic system features of WSWR by K means, the result reveals 15 southwest type (SW-type) and 17 south-biased type (S-type) WSWR events. Composite analysis illuminates the distinct dynamic and thermodynamic features of each type. For the SW-type WSWR, the maximum value of water vapor is concentrated around 850 hPa in the lower troposphere. The YHCR is located at the intersection of the exit area of the 850-hPa synoptic low-level jet (LLJ) and the entrance area of the 600-hPa jet. The suction effects, combined with the location of YHCR on the left side of the boundary layer jet (BLJ), facilitate the triggering of local convection. Conversely, the S-type WSWR shows peak water vapor in the boundary layer. Before the onset of WSWR events, a warm, humid tongue indicated by pseudoequivalent potential temperature θse is present in the boundary layer, signified by substantial unstable energy. The BLJ aids mesoscale ascent on its terminus, enhancing convergence along the coastline. The BLJ also channels unstable energy and water vapor to the YHCR, causing significant rainfall. Typical case studies of both types show similar environmental backgrounds. The scale analysis shows mesoscales of dynamic field are crucial in shaping both types of WSWR, while the large-scale and meso-α-scale dynamic field facilitate the transportation of moist and warm airflow.

Characteristics of Advection Fog at Qingdao Liuting International Airport

The advection fog characteristics at Qingdao Liuting International Airport during 2000–2022 are studied based on surface observation, sounding and reanalysis data. Surface observation data show that there were two types of fog: evaporation fog (EF) dominated by northwesterly wind in winter and cooling fog (CF) dominated by southeasterly wind in spring and summer. CF is thicker than EF due to different planetary boundary layer (PBL) structures. For EF, the middle and low troposphere are affected by dry and cold air, while CF is affected by warm and moist air below 850 hPa. When EF formed, downdrafts and a positive vertical gradient of the pseudo-equivalent potential temperature indicate stable PBL, surface heat flux is upward from sea to atmosphere and surface wind diverges near the air–sea interface. When CF formed, these characteristics are reversed. Fog is significantly affected by sea–land–atmosphere interactions. The moisture source is mainly from surface fluxes released by the Yellow Sea in the case of EF, while it is from moist air at low latitudes and local land transpiration in the case of CF. The difference in temperature between the sea surface and surface air changes from the range of 0–8 K for EF but from −4–0 K for CF.

The Assimilation Effect of Multi-New Types Observation Data in the Forecasts of Meiyu-Front Rainstorm

Meiyu-front rainstorm is one of the main disastrous weather events in summer in East China. In this study, seven assimilation experiments of multi-type observation data such as wind profile data, microwave radiometer data and radiosonde sounding data are designed to forecast the Meiyu-front rainstorm on 15 June 2020. The results show that the seven experiments can basically simulate the orientation of rain belt. The comprehensive experiment which assimilates all types of observations performs the best in simulating the location of heavy rainstorm and shows good performance in simulating the precipitation above moderate rain. For the comprehensive experiment, the forecast deviation of rainstorm and heavy rainstorm is small, and the equitable threat score has also been greatly improved compared with other experiments. It is found that the convective available potential energy is enhanced after the assimilation of surface observation data. In addition, the wind convergence and water vapor transportation are modified after assimilating wind profile data. Accordingly, the precipitation efficiency is improved in the comprehensive experiment. The profiles of pseudo-equivalent potential temperature, vorticity and divergence show that, the assimilation of new-types observation data from wind profiler radar and microwave radiometer increases the instability of atmospheric stratification and enhances the ascending motion in the heavy precipitation center. The above results show that the introduction of various some new-type data before the numerical simulation can reduce the forecast deviation. In addition, the combined assimilation of microwave radiometer and sounding data presents better performance than single data assimilation, which indicates that data mutual complementation is essential to improving forecast accuracy.

Heavy precipitation events in northwestern China induced by northeastward plateau vortex: Two cases comparison

The plateau vortex is one of the dominant weather systems that modulate summer rainfall in northwestern China, a typical arid/semi-arid area worldwide. Two heavy rainfall cases in the summer of 2012 and 2013, accompanying the northeastward movement of plateau vortexes, are selected here. The role of plateau vortexes in these two cases is explored by diagnosing their moist potential vorticity, helicity, and convective clouds characteristics, using numerous data (ERA-Interim reanalysis, multi-sourced precipitation data, FY-2E satellite images). The plateau vortexes enhanced convective instability in the rainfall area by entraining upper-level dry and cold air, manifested as salient vertical gradients of moist potential vorticity and steep pseudo-equivalent potential temperature isolines. The rainfall occurred eastward relative to the vortex movement track and intensified on the left of cloud black body temperature (TBB) with larger gradients at the middle to late stages of the vortex life span. The generation of moisture helicity is dictated by the water vapor transportation, with the negative value concentrated in the water vapor-rich areas. Notably, the two cases differ in tracks, life spans, water vapor sources, and precipitation amounts, which may be attributed to large-scale circulation background, vertical vortex structure, and moisture condition.

Subseasonal dynamical prediction of South China Sea summer monsoon

South China Sea (SCS) summer monsoon (SCSSM) is an important component of the Asian monsoon system, of which the onset and intensity exerting critical impacts on surrounding areas. With the Japan Meteorological Agency (JMA) model prediction derived from the S2S project, the predictions of SCSSM, specifically, the annual evolution, onset, and corresponding meteorological variables are investigated with lead times of 1–34 days. The prediction skill on the associated dynamical and thermal background such as wind, vertical motion, pseudo-equivalent potential temperature, and specific humidity decreases with the growing lead times. For the SCS surroundings, the upper- and lower-level circulations are reasonably predicted for lead times of shorter than 3 weeks. Predictions with fair skill on humidity and pseudo-equivalent potential temperature over land area around the northern SCS last for around 4 weeks, better than those over the southern SCS. Predictions on vertical motion and precipitation are reliable within 1 week, while the skill decreases rapidly after 2 weeks. In general, the predictions on the spatial patterns of circulation are relatively poor during the transition period before monsoon onset and are then improved after that. The model capability on predicting the date of monsoon onset decreases with the increasing lead times. Besides, skillful predictions on climate systems such as the tropical vortex pairs over Indian Ocean and the western North Pacific subtropical high are critical for the successful predictions of monsoon onset.

Investigation of Atmospheric Dynamic and Thermodynamic Structures of Typhoon Sinlaku (2020) from High-Resolution Dropsonde and Two-Way Rawinsonde Measurements

Profiling the vertical atmospheric structure for typhoons remains challenging. Here, the atmospheric dynamic and thermodynamic structures were investigated during the passage of Typhoon Sinlaku (2020) over Xisha Islands in the South China Sea for the period 28 July to 2 August 2020, mainly based on two-way rawinsonde and dropsonde measurements in combination with surface-based automatic weather station observations, disdrometer measurements, and Himawari-8 geostationary satellite images. The study period was divided to three stages: the formation stage of tropical depression (pre-TD), tropical depression (TD), and tropical storm (TS). The wind speed and local vertical wind shear reached the maximum value at 3 km above mean sea level (AMSL) before the typhoon approached the Xisha islands. Pseudo-equivalent potential temperature (θse) was found to decrease with the altitude below 2 km AMSL; temperature inversions occurred frequently within this altitude range, particularly during the TS stage. This seemed a typical capping inversion that indicated a downward motion above 2 km AMSL. The temperature increased slightly with the development of Typhoon Sinlaku (2020) at altitudes of 8–10 km AMSL. This indicated that our observations presumably captured the air mass warmed by the condensation, which was a good signature of an upper air in the tropical cyclone. In addition, wind speed (particularly in the lower stratosphere), specific humidity, and equivalent potential temperature escalated significantly when the tropical depression strengthened into Typhoon Sinlaku (2020), which indicated that the typhoon constantly obtained energy from the sea surface during its passage over the study region. The thermodynamic and dynamic structures of atmosphere advance our understanding of the inner structure of typhoons during the different evolutionary stages.

An eight-year climatology of the warm-season severe thunderstorm environments over North China

Using proximity soundings taken during the warm seasons (May–September) of 2011–2018 over North China, this work investigated the environments of severe thunderstorms that caused the following four types of convective hazards: only hail (H), only thunderstorm high winds (W), only short-duration heavy rainfall (R), and two or three types of the above three hazards (hybrid). The environments for non-severe thunderstorms and no-rain days were also studied for comparison. Soundings in each weather category were further classified into mountain and plain groups. A set of parameters was tested for skill in discriminating between various categories. Results indicate that the environments over the plains, compared with those over the mountains, are generally characterized by stronger instability, more moisture, and higher vertical wind shear. Over both mountains and plains, the precipitable water (PW) shows the greatest skill overall in discriminating between various categories; the instability parameters show considerable skill in discriminating between the severe thunderstorm, non-severe thunderstorm, and no-rain categories but rather limited skill in discriminating between the four severe thunderstorm categories; the wind shear parameters show very limited skill in discriminating between various categories; the lifting condensation level (LCL) distinguishes well the environments of no-rain and R from those of other categories; and the mid-tropospheric pseudo-equivalent potential temperature is the only parameter investigated here that shows utility in discriminating between H and W environments. Probability distributions in two-parameter spaces reveal that great variations exist regarding the environments under which different weather types are most likely to occur.

Thermodynamic and dynamic behavior in a recent dust precipitation event in north China

The dynamic and thermodynamic characteristics of a dust precipitation event that took place on May 11, 2020 in North China are analyzed. With the invasion of cold air from Lake Baikal (107°E, 53°N), the arid area in the southeast of Mongolia (107°–108°E, 47°–48°N) was in a region with a large potential pseudo-equivalent temperature gradient, where interactions between cold and warm air led to instability of the atmospheric stratification, and became the dust source area. Meanwhile, a low-pressure cyclone formed over the area (122.3°E, 47.5°N) to the east of the dust source area. Under the joint action of the westerlies, the cyclone developed and supplied forces to the vertical ascent and transportation of dust. Strong winds carried the dust along the southeast direction to China. As the cyclone strengthened, the cold and humid air from Lake Baikal arrived and supplied water vapor for precipitation over Beijing. On the whole, the low-pressure cyclone provided momentum and water vapor for this dust–precipitation event. It is found that the horizontal helicity belt can be an indicator for the prediction of the dust transport pathway. The quick increase in surface dust concentration to its peak was closely related to the increase in downward movements in the layers above 700 hPa.

Statistical Characteristics and Environmental Conditions of the Warm-Season Severe Convective Events over North China

Based on severe weather reports, surface precipitation observations, surface routine observations, and the European Center for Medium-Range Weather Forecasts ERA5 reanalysis dataset during the warm seasons (May–September) of 2011–2018 over North China, this paper analyzes the statistical characteristics and environmental conditions of three types of severe convective events. Results are compared between events with different altitudes (i.e., mountains and plains), severities (i.e., ordinary and significant), and months. Hail and thunderstorm high winds (THWs) are more common over the mountains whereas short-duration heavy rainfall (SDHR) is more frequent over the plains. The occurrence frequency of severe convective events exhibits distinct monthly and diurnal variations. Analyses of the environmental parameters provide reference for the potential forecasting of severe convective events over this region. Specifically, the 850–500 hPa temperature lapse rate (LR85), pseudo-equivalent potential temperature at 500 hPa (thetase500), and precipitable water (PW) are skillful in distinguishing hail and THW environments from SDHR environments, and thetase500 is useful in discriminating between hail and THW environments. The convective environments over the plains are characterized by significantly higher (lower) PW (LR85) compared with mountains. The skill of these parameters in forecasting the severity of the convective hazards is limited. Probability distributions in the two parameters space indicate that the occurrence of significant hail requires both higher most unstable convective available potential energy (MUCAPE) and stronger 0–6 km bulk wind shear (SHR6) compared with ordinary hail. Compared with ordinary THWs, the significant THWs over the mountains depend more on the SHR6 whereas those over the plains rely more on the MUCAPE. The significant SDHR events over the plains tend to occur under a variety of instability conditions. The thermodynamic parameters (i.e., MUCAPE, thetase500, and downdraft convective available potential energy), and PW are significantly higher in July–August, whereas the LR85 and vertical wind shear are apparently higher in May, June and September.

Study on the Structure of a Horizontal Shear Line over the Tibetan Plateau Based on CRA-Interim Datasets and Its Comparison with ERA-Interim Datasets

The CRA-Interim trial production of the global atmospheric reanalysis for 10 years from 2007 to 2016 was carried out by the China Meteorological Administration in 2017. The structural characteristics of the horizontal shear line over the Tibetan Plateau (TPHSL) based on the CRA-Interim datasets are examined by objectively identifying the shear line, and are compared with the analysis results of the European Centre for Medium-Range Weather Forecasts reanalysis data (ERA-Interim). The case occurred at 18UTC on July 5, 2016. The results show that both of the ERA-Interim and CRA-Interim datasets can well reveal the circulation background and the dynamic and thermal structure characteristics of TPHSL, and they have shown some similar features. The middle and high latitudes at 500 hPa are characterized by the circulation situation oftwo troughs and two ridges, and at 200 hPa, the TPHSL is located in the northeast quadrant of the South Asian High Pressure (SAHP). The TPHSL locates in the positive vorticity zone and passes through the positive vorticity center corresponding to the ascending motion. Near the TPHSL, the contours of pseudo-equivalent potential temperature (θse) tend to be intensive, with a high-value center on the south side of the TPHSL. The TPHSL can extend to 460 hPa and vertically inclines northward. There is a positive vorticity zone near the TPHSL which is also characterized by the northward inclination with the height, the ascending motion near the TPHSL can extend to 300 hPa, and the atmospheric layer above the TPHSL is stable. However, the intensities of the TPHSL's structure characteristics analyzed with the two datasets are different, revealing the relatively strong intensity of geopotential height field, vertical velocity field, vorticity field and divergence field from the CRA-Interim datasets. In addition, the vertical profiles of the dynamic and water vapor thermal physical quantities of the two datasets are also consistent in the east and west part of the TPHSL. In summary, the reliable and usable CRA-Interim datasets show excellent properties in the analysis on the structural characteristics of a horizontal shear line over the Tibetan Plateau.

Geographical distribution of ozone seasonality over China

Up to now, a nation-wide scale study of surface ozone (O3) concentrations in China was limited due to scarce observation. Thanks to the establishment of national air quality monitoring network in 2013, surface O3 data from 1402 stations during 2014–2017 were collected to investigate O3 seasonality. Our analysis reveals that the variations of monthly O3 averaged from daily mean concentration during a year show different temporal profiles depending on latitude. A unimodal structure (UMS) is generally found for latitudes over 35°N, whereas a bimodal structure (BMS) is in most of the cases identified south of 35°N. The peak of UMS is found in the period of May to July, whereas the first and second peaks of BMS are found from April to June, and from July to October, respectively. In addition, the seasonality of O3 presents a strong dependence on pseudo-equivalent potential temperature and monsoonal clouds. The onset and retreat of warm and wet air are correlated to the summer minimum in BMS cases and to the sharp decrease of UMS in July. As far as the relationships between O3 and carbon monoxide are concerned, the effects of clean maritime air masses on the summer trough of O3 are not significant for inland sites. Overall, summer monsoon bringing warm and moist air and subsequent clouds leads to the suppression of photochemical production, thereby contributing directly to the geographical distribution of O3 seasonality.

NUMERICAL PREDICTION OF AN EXTREME RAINSTORM OVER THE PEARL RIVER DELTA REGION ON 7 MAY 2017 BASED ON WRF-ENKF

An ensemble Kalman filter based on the Weather Research and Forecasting Model (WRF-EnKF) is used to explore the effectiveness of the assimilation of surface observation data in an extreme local rainstorm over the Pearl River Delta region on 7 May 2017. Before the occurrence of rainstorm, the signals of weather forecasts in this case are too weak to be predicted by numerical weather model, but the surface temperature over the urban area are high. The results of this study show that the wind field, temperature, and water vapor are obviously adjusted by assimilating surface data of 10-m wind, 2-m temperature, and 2-m water vapor mixing ratio at 2300 BST 6 May, especially below the height of 2 km. The southerly wind over the Pearl River Delta region is enhanced, and the convergence of wind over the northern Guangzhou city is also enhanced. Additionally, temperature, water vapor mixing ratio and pseudoequivalent potential temperature are obviously increased over the urban region, providing favorable conditions for the occurrence of heavy precipitation. After assimilation, the predictions of 12-h rainfall amount, temperature, and relative humidity are significantly improved, and the rainfall intensity and distribution in this case can be successfully reproduced. Moreover, sensitivity tests suggest that the assimilation of 2-m temperature is the key to predict this extreme rainfall and just assimilating data of surface wind or water vapor is not workable, implying that urban heat island effect may be an important factor in this extreme rainstorm.

In Situ and Radar Observations of the Low Reflectivity Ribbon in Supercells during VORTEX2

Abstract During the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign, mobile radars observed a previously undocumented feature: the low-reflectivity ribbon (LRR). The LRR was characterized by reduced reflectivity ZH and differential reflectivity ZDR through a narrow region extending from the intersection of the hook and forward-flank regions of supercells. This study synthesizes kinematic and polarimetric radar observations with in situ measurements taken by the “StickNet” observing network. StickNet data have been used to establish that the LRR is associated with a localized minimum in pseudoequivalent potential temperature Pronounced drops in are observed by nine separate probes in three different supercell thunderstorms. Both single- and dual-Doppler analyses are used to examine the two- and three-dimensional structures of the winds within the LRR, revealing that the LRR is associated with cyclonic vertical vorticity aloft. Polarimetric radar observations are used to study the hydrometeor characteristics and the processes that cause those hydrometers to be present. Special consideration is given to the analysis of the vertical distribution of traditional and polarimetric variables, as well as the evolution of the kinematic fields retrieved by dual-Doppler analysis. The combination of thermodynamic, kinematic, and inferred microphysical observations supports a hypothesis that the LRR comprises sparse, large hail.

ASYMMETRICAL STRUCTURES OF THE TEMPERATURE AND HUMIDITY OF TROPICAL CYCLONES

Based on NCEP/CFSR 0.5° reanalysis data and the best track data from the Japan Tokyo Typhoon Center, composite and comparative analyses demonstrate the asymmetrical structures of the temperature and humidity in tropical cyclones over the Western North Pacific and the South China Sea from 1979 to 2010. The results are shown as follows. (1) With intensifying tropical cyclones, the flow field tends to become gradually more axisymmetric; however, the asymmetry of the specific humidity in the outer regions is more obvious. (2) In general, tropical cyclones have a non-uniform, vertical, “double warm-core” structure. The “warm-cores” in the lower level of weak tropical cyclones and in the higher level of strong tropical cyclones are the stronger of the two. (3) The distribution area of a “warm-core” is enhanced with cyclone intensification and tends to become more axisymmetric. At 200 hPa, the “warm-core” of a weak cyclone has a weak anticyclone in the center, whereas that of a strong cyclone has a weak cyclone in the center. (4) The “wet-core” of a tropical cyclone is primarily located in the lower level (700–C850 hPa). With the cyclone’s intensification, the intensity of the “wet-core” increases and the scope of the 0.8 g?kg?1 specific humidity anomaly tends to expand to higher levels. (5) With the cyclone’s deepening, the pseudo-equivalent potential temperature at different levels in different regions increases. In addition, the largest warming rates at each intensity level in the different regions occur in the core area, followed in turn by the envelope and outer areas.

A Climatology of the Southwest Vortex during 1979–2008

AbstractUsing a new vortex detection and tracing method, a dataset of the Southwest Vortex (SWV) is established based on Japanese 25-year Reanalysis (JRA-25) reanalysis data during 1979–2008. The spatiotemporal features of the SWV are derived from the dataset. In comparison to other seasons, summer yields the least SWVs, but with the highest probability that they will migrate from their region of origin. SWVs mostly emerge in the southwest of the Sichuan Basin and the southeast of the Tibetan Plateau. Migratory SWVs mainly move along either an eastward or southeastward path. Detailed composite analysis of warm-season SWVs shows that the subtropical high is a key factor in determining the direction of migratory SWVs. Furthermore, the steering wind at 700 hPa dominates the moving direction of migratory SWVs. Potential stability diagnosed by pseudo-equivalent potential temperature θse is of certain significance for the evolution and movement of SWVs. On the other hand, migratory SWVs possess relatively great...

Statistical characteristics of environmental parameters for warm season short-duration heavy rainfall over central and eastern China

Water vapor content, instability, and convergence conditions are the key to short-duration heavy rainfall forecasting. It is necessary to understand the large-scale atmospheric environment characteristics of short-duration heavy rainfall by investigating the distribution of physical parameters for different hourly rainfall intensities. The observed hourly rainfall data in China and the NCEP final analysis (FNL) data during 1 May and 30 September from 2002 to 2009 are used. NCEP FNL data are 6-hourly, resulting in sample sizes of 1573370, 355346, and 11401 for three categories of hourly rainfall (P) of no precipitation (P < 0.1 mm h−1), ordinary precipitation (0.1⩽ P < 20 mm h−1), and short-duration heavy rainfall (P ⩾ 20.0 mm h−1), respectively, by adopting a temporal matching method. The results show that the total precipitable water (PWAT) is the best parameter indicating the hourly rainfall intensity. A PWAT of 28 mm is necessary for any short-duration heavy rainfall. The possibility of short-duration heavy rainfall occurrence increases with PWAT, and a PWAT of 59 mm is nearly sufficient. The specific humidity is a better indicator than relative humidity. Both 700- and 850-hPa relative humidity greater than 80% could be used to determine whether or not it is going to rain, but could not be used to estimate the rainfall intensity. Temperature and potential pseudo-equivalent temperature are also reasonable indicators of short-duration heavy rainfall. Among the atmospheric instability parameters, the best lifted index (BLI) performs best on the short-duration rainfall discrimination; the next best is the K index (KI). The three rainfall categories are not well recognized by total totals (TT) or the temperature difference between 850 and 500 hPa (DT85). Three-quarters of short-duration heavy rainfall occurred with BLI less than -0.9, while no short-duration heavy rainfall occurred when BLI was greater than 2.6. The minimum threshold of KI was 28.1 for short-duration heavy rainfall. The importance of dynamic conditions was well demonstrated by the 925- and 850-hPa divergence. The representativeness of 925-hPa divergence is stronger than that of 850 hPa. Three-quarters of short-duration heavy rainfall occurred under a negative divergence environment. However, both the best convective potential energy (BCAPE) and vertical wind shear were unable to discriminate the hourly rainfall intensities.

A Climatology of the Southwest Vortex during 1979-2008

Using a new vortex detection and tracing method, a dataset of the Southwest Vortex(SWV) is established based on Japanese 25-year Reanalysis(JRA-25) reanalysis data during 1979–2008. The spatiotemporal features of the SWV are derived from the dataset. In comparison to other seasons, summer yields the least SWVs, but with the highest probability that they will migrate from their region of origin. SWVs mostly emerge in the southwest of the Sichuan Basin and the southeast of the Tibetan Plateau. Migratory SWVs mainly move along either an eastward or southeastward path. Detailed composite analysis of warm-season SWVs shows that the subtropical high is a key factor in determining the direction of migratory SWVs. Furthermore, the steering wind at 700 hPa dominates the moving direction of migratory SWVs. Potential stability diagnosed by pseudo-equivalent potential temperature ? se is of certain significance for the evolution and movement of SWVs. On the other hand, migratory SWVs possess relatively greater strength than stationary SWVs, due to a stronger low-level jet with enhanced baroclinicity and moisture transport providing more energy to support the growth of SWVs along their paths of movement.

Influence of Low-Level Thermodynamic Structure on the Downdraft Properties of Simulated Supercells

Abstract Idealized simulations using the Weather Research and Forecasting Model (WRF) were performed to examine the role of capping inversions on the near-surface thermodynamic structure of outflow from simulated supercells. Two simulations were performed: one with the traditional noncapped Weisman and Klemp (WK) analytic sounding and the second with a WK sounding modified to contain a capping inversion. Both sounding environments favor splitting storms and a right-moving supercell by 90 min into the simulation. These two supercell simulations evolve in a qualitatively similar fashion, with both storms exhibiting large, quasi-steady updrafts, hook-shaped appendages in the precipitation mixing ratio field, and prominent localized downdrafts. Results show that the supercell simulated in the capped environment has a surface cold pool with larger values of pseudoequivalent potential temperature (θep) than the cold pool of the supercell produced in the noncapped simulation. Parcels in the surface cold pool of the supercell produced in the capped sounding simulation have a lower origin height than those in the surface cold pool of the supercell produced in the noncapped simulation for all times. Although θep values in the surface cold pool are primarily associated with the origin height of downdraft parcels and the environmental θep at that level, it is shown that nonconservation of θep primarily associated with hydrometeor melting can decrease θep values of downdraft parcels as they descend by several degrees.

The genesis of Typhoon Nuri as observed during the Tropical Cyclone Structure 2008 (TCS08) field experiment – Part 2: Observations of the convective environment

Abstract. Analyses of thermodynamic data gathered from airborne dropwindsondes during the Tropical Cyclone Structure (2008) experiment are presented for the disturbance that became Typhoon Nuri. Although previous work has suggested that Nuri formed within the protective recirculating "pouch" region of a westward propagating wave-like disturbance and implicated rotating deep convective clouds in driving the inflow to spin up the tangential circulation of the system-scale flow, the nature of the thermodynamic environment that supported the genesis remains a topic of debate. During the genesis phase, vertical profiles of virtual potential temperature show little variability between soundings on a particular day and the system-average soundings likewise show a negligible change. There is a tendency also for the lower and middle troposphere to moisten. However, the data show that, on the scale of the recirculating region of the disturbance, there was no noticeable reduction of virtual temperature in the lower troposphere, but a small warming (less than 1 K) in the upper troposphere. Vertical profiles of pseudo-equivalent potential temperature, θe, during the genesis show a modestly decreasing deficit of θe in the vertical between the surface and the height of minimum θe (between 3 and 4 km), from 17.5 K to 15.2 K. The findings reported here are consistent with those found for developing disturbances observed in the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment in 2010. Some implications of the findings are discussed.