High Equivalent Potential Temperature Research Articles

Environment and morphology of mesoscale convective systems associated with the Changma front during 9–10 July 2007

Abstract. To understand the different environment and morphology for heavy rainfall during 9–10 July 2007, over the Korean Peninsula, mesoscale convective systems (MCSs) that accompanied the Changma front in two different regions were investigated. The sub-synoptic conditions were analysed using mesoscale analysis data (MANAL), reanalysis data, weather charts and Multi-functional Transport Satellite (MTSAT-IR) data. Dual-Doppler radar observations were used to analyse the wind fields within the precipitation systems. During both the case periods, the surface low-pressure field intensified and moved northeastward along the Changma front. A low-level warm front gradually formed with an east-west orientation, and the cold front near the low pressure was aligned from northeast to southwest. The northern convective systems (meso-α-scale) were embedded within an area of stratiform cloud north of the warm front. The development of low-level pressure resulted in horizontal and vertical wind shear due to cyclonic circulation. The wind direction was apparently different across the warm front. In addition, the southeasterly flow (below 4 km) played an important role in generating new convective cells behind the prevailing convective cell. Each isolated southern convective cell (meso-β-scale) moved along the line ahead of the cold front within the prefrontal warm sector. These convective cells developed when a strong southwesterly low-level jet (LLJ) intensified and moisture was deeply advected into the sloping frontal zone. A high equivalent potential temperature region transported warm moist air in a strong southwesterly flow, where the convectively unstable air led to updraft and downdraft with a strong reflectivity core.

2010년 여름철 수도권 집중관측기간에 나타난 호우 시스템의 대기연직구조

2010년 8월 13일부터 9월 3일까지 수도권지역에서 집중관측(Predictability and Observation Experiment of Korea-2010, ProbeX-2010)이 수행되었으며, 그 일환으로 동두천, 인천공항, 양평에서 6시간 간격으로 라디오존데 관측이 실행되었다. 관측기간 동안 우리나라의 전형적인 호우 패턴인 스콜선형, 정체전선형, 태풍 전면 수렴형, 열대저압부형, 태풍 직접형 호우가 연속적으로 발생하였다. 8월 15일 03 KST 경에는 스콜선형 구름대가 수도권 지역에서 발달하였다. 따뜻하고 습한 하층 공기 위로 건조한 중층 공기가 유입되어 강한 대류 불안정이 형성되었으며 이로 인해 호우가 발생하였다. 8월 23일부터 26일, 8월 27일부터 29일은 각각 정체전선과 태풍 전면 수렴대의 영향을 받아 강우가 발생하였다. 정체전선형 강우 초기에는 열적 불안정이 우세하게 나타났으나 강우 후기에는 역학적 불안정이 강화되었다. 이 기간 중 특히 강한 강수는 서해상에서 남풍류 하층 제트가 발달한 8월 25일에 발생하였다. 태풍 전면 수렴형 강우기간에는 열적 불안정과 역학적 불안정이 모두 유지되는 특징을 보였다. 이 기간 중 특히 강한 강수는 높은 상당온위(>345 K)를 가진 열대 공기가 대기 중하층에 거쳐 유입되었을 때 발생하였다. 8월 27일과 9월 2일에는 각각 열대저압부와 태풍 곤파스의 영향에 의해 강우가 발생하였다. 이 사례들 동안에는 역학적 불안정이 매우 강하게 발달하였다. The intensive observation (ProbeX-2010) with 6-hour launches of radiosonde was performed over Seoul metropolitan area (Dongducheon, Incheon Airport, and Yangpyeong) from 13 Aug. to 3 Sep. 2010. Five typical heavy rainfall patterns occurred consecutively which are squall line, stationary front, remote tropical cyclone (TC), tropical depression, and typhoon patterns. On 15 Aug. 03 KST, when squall line developed over Seoul metropolitan area, dry mid-level air was drawn over warm and moist low-level air, inducing strong convective instability. From 23 to 26 Aug and from 27 to 29 Aug. Rainfall event occurred influenced by stationary front and remote TC, respectively. During the stationary frontal rainy period, thermal instability was dominant in the beginning stage, but dynamic instability became strong in the latter stage. Especially, heavy rainfall occurred on 25 Aug. when southerly low level jet formed over the Yellow Sea. During the rainy period by the remote TC, thermal and dynamic instability sustained together. Especially, heavy rainfall event occurred on 29 Aug. when the tropical air with high equivalent potential temperature (>345 K) occupied the deep low-middle level. On 27 Aug. and 2 Sep. tropical depression and typhoon Kompasu affected Seoul metropolitan area, respectively. During these events, dynamic instability was very strong.

The Creation of a High Equivalent Potential Temperature Reservoir in Tropical Storm Humberto (2001) and Its Possible Role in Storm Deepening

Thirty global positioning system dropwindsondes (GPS sondes) were used to identify and examine the creation of a reservoir of high equivalent potential temperature (θe) in the nascent eye of Tropical Storm Humberto (2001). The θe did not increase in the high surface wind portion of the storm as it does in mature hurricanes; instead air spiraled into the light-wind center of the developing storm where it was trapped by subsidence under a mesoscale convectively generated vortex (MCV). An energy budget revealed that the inflow column took 7 h to reach the storm center during which a combined average surface enthalpy flux of ~230 W m−2 was diagnosed via the bulk aerodynamic equations. This estimate is close to the 250 W m−2 required for balance based on the energy acquired by the column. The high θe in the lowest kilometer, overlain by a near dry-adiabatic layer under the anvil base, resulted in convective available potential energy (CAPE) exceeding 2500 m2 s−2. This conditionally unstable air later served as fuel for the convection within the nascent eyewall. The authors speculate that CAPE of such a large magnitude could accelerate the updraft and stretch the vorticity field, essentially turning garden-variety cumulonimbi into the vortical hot towers argued by several researchers to play a role in tropical cyclone formation and intensification.

Analysis and simulation of mesoscale convective systems accompanying heavy rainfall: The goyang case

We investigated a torrential rainfall case with a daily rainfall amount of 379 mm and a maximum hourly rain rate of 77.5 mm that took place on 12 July 2006 at Goyang in the middlewestern part of the Korean Peninsula. The heavy rainfall was responsible for flash flooding and was highly localized. High-resolution Doppler radar data from 5 radar sites located over central Korea were analyzed. Numerical simulations using the Weather Research and Forecasting (WRF) model were also performed to complement the high-resolution observations and to further investigate the thermodynamic structure and development of the convective system. The grid nudging method using the Global Final (FNL) Analyses data was applied to the coarse model domain (30 km) in order to provide a more realistic and desirable initial and boundary conditions for the nested model domains (10 km, 3.3 km). The mesoscale convective system (MCS) which caused flash flooding was initiated by the strong low level jet (LLJ) at the frontal region of high equivalent potential temperature (θe) near the west coast over the Yellow Sea. The ascending of the warm and moist air was induced dynamically by the LLJ. The convective cells were triggered by small thermal perturbations and abruptly developed by the warm θe inflow. Within the MCS, several convective cells responsible for the rainfall peak at Goyang simultaneously developed with neighboring cells and interacted with each other. Moist absolutely unstable layers (MAULs) were seen at the lower troposphere with the very moist environment adding the instability for the development of the MCS.

Sensitivity of Numerical Simulation of Early Rapid Intensification of Hurricane Emily (2005) to Cloud Microphysical and Planetary Boundary Layer Parameterizations

Abstract An advanced research version of the Weather Research and Forecasting (ARW) Model is used to simulate the early rapid intensification of Hurricane Emily (2005) using grids nested to high resolution (3 km). A series of numerical simulations is conducted to examine the sensitivity of the simulation to available cloud microphysical (CM) and planetary boundary layer (PBL) parameterization schemes. Results indicate that the numerical simulations of the early rapid intensification of Hurricane Emily are very sensitive to the choice of CM and PBL schemes in the ARW model. Specifically, with different CM schemes, the simulated minimum central sea level pressure (MSLP) varies by up to 29 hPa, and the use of various PBL schemes has resulted in differences in the simulated MSLP of up to 19 hPa during the 30-h forecast period. Physical processes associated with the above sensitivities are investigated. It is found that the magnitude of the environmental vertical wind shear is not well correlated with simulated hurricane intensities. In contrast, the eyewall convective heating distributions and the latent heat flux and high equivalent potential temperature (θe) feeding from the ocean surface are directly associated with the simulated intensities. Consistent with recognized facts, higher latent heat release in stronger eyewall convection, stronger surface energy, and high θe air feeding from the ocean surface into the hurricane eyewall are evident in the more enhanced convection and intense storms. The sensitivity studies in this paper also indicate that the contributions from the CM and PBL processes can only partially explain the slow intensification in the ARW simulations. Simulation at 1-km grid resolution shows a slight improvement in Emily’s intensity forecast, implying that the higher resolution is somewhat helpful, but still not enough to cause the model to reproduce the real intensity of the hurricane.

A Statistical Comparison of the Properties of Flash Flooding and Nonflooding Precipitation Events in Portions of New York and Pennsylvania

Abstract Flash floods reported for the forecast area of the National Weather Service Forecast Office at Binghamton, New York (BGM), are compared with similar significant precipitation and flash flood watch events not corresponding to flash flood reports. These event types are characterized by measures of surface hydrological conditions, surface and upper-air variables, thermodynamic properties, and proxies for synoptic-scale features. Flash flood and nonflood events are compared quantitatively via discriminant analysis and cross validation, and qualitatively via scatterplots and composite soundings. Results are presented in the context of a flash flood checklist used at BGM prior to this study. Flash floods and nonfloods are found to differ most significantly in antecedent soil moisture. The wind direction at 850 hPa shows differences between flood and nonflood events, with flooding more common for an easterly to southeasterly direction and nonflooding more common for a northwesterly direction. Southwesterly wind direction is characteristic of both types. In general, nonflooding significant precipitation events are more commonly associated with a better-defined ridge axis of relatively high 850-hPa equivalent potential temperature and larger convective available potential energy as compared to the flash flood events. Several parameters included on the BGM flash flood checklist, though effective at distinguishing significant precipitation events and flash floods from random events, were found to be unable to separate flash floods from nonflooding significant rain events.

Mesoscale Convective Vortices Observed during BAMEX. Part II: Influences on Secondary Deep Convection

Abstract Observations from the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment are used to examine the role of the five mesoscale convective vortices described in Part I on heavy precipitation during the daytime heating cycle. Persistent widespread stratiform rain without deep convection occurs for two strong MCVs in conditionally stable environments with strong vertical shear. Two other MCVs in moderate-to-strong vertical shear have localized redevelopment of deep convection (termed secondary convection) on their downshear side, where conditional instability exists. The strongest of the five MCVs occurs in weak vertical shear and has widespread secondary convection, which is most intense on its conditionally unstable southeast periphery. The two MCVs with only localized secondary convection have well-defined mesoscale vertical motion couplets with downshear ascent and upshear descent above the planetary boundary layer (PBL). Although the amplitude is significantly greater, the kinematically derived vertical motion dipole resembles that implied by steady, vortex-relative isentropic flow, consistent with previous idealized (dry) simulations and diagnoses based on operational model analyses. In the other three cases with either widespread precipitation or weak environmental vertical shear, the kinematic and isentropic vertical motion patterns are poorly correlated. Vertical motions above the PBL provide a focus for secondary convection through adiabatic cooling downshear and adiabatic warming upshear of the MCV center. The MCVs occur within surface frontal zones with large temperature and moisture gradients across the environmental vertical shear vector (Part I). Thus, the effect of vertical motions on conditional instability is reinforced by horizontal advections of high equivalent potential temperature air downshear, and low equivalent potential temperature air upshear within the PBL. On average, the quadrant immediately right of downshear (typically southeast of the MCV center) best supports deep convection because of the juxtaposition of greatest mesoscale ascent, high equivalent potential temperature PBL air, and MCV-induced enhancement of the vertical shear.

An Assimilation and Forecasting Experiment of the Nerima Heavy Rainfa11 with a Cloud-Resolving Nonhydrostatic 4-Dimensional Variational Data Assimilation System

The Meteorological Research Institute of the Japan Meteorological Agency has developed a cloud-resolving nonhydrostatic 4-dimensional variational assimilation system (NHM-4DVAR), based on the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM), in order to investigate the mechanism of heavy rainfall events induced by mesoscale convective systems (MCSs). A horizontal resolution of the NHM-4DVAR is set to 2 km to resolve MCSs, and the length of the assimilation window is 1-hour. The control variables of the NHM-4DVAR are horizontal wind, vertical wind, nonhydrostatic pressure, potential temperature, surface pressure and pseudo relative humidity. Perturbations to the dynamical processes, and the advection of water vapor are considered, but these to the other physical processes are not taken into account.The NHM-4DVAR is applied to the heavy rainfall event observed at Nerima, central part of Tokyo metropolitan area, on 21 July 1999. Doppler radar's radial wind data, Global Positioning System's precipitable water vapor data, and surface temperature and wind data are assimilated as high temporal and spatial resolution data. The Nerima heavy rainfall is well reproduced in the assimilation and subsequent forecast, with respect to time sequence of 10-minute rainfall amount. The formation mechanism of the Nerima heavy rainfall is clarified from this study. A surface convergence line of horizontal winds was made of a southerly sea breeze and north-easterly winds over the Kanto plain around Nerima. Since the rise of temperature over the northern part of the Kanto plain was suppressed, due to a shield of clouds against sunshine, the difference of temperature between the convergence line and its northern side became large. Consequently, the wind convergence was enhanced around Nerima. An air with high equivalent potential temperature was lifted over this enhanced convergence line to generate cumulonimbi that caused the Nerima heavy rainfall.

Observations of the 11 June Dryline during IHOP_2002—A Null Case for Convection Initiation

Abstract The detailed analysis of the three-dimensional structure of a dryline observed over the Oklahoma panhandle during the International H2O Project (IHOP_2002) on 11 June 2002 is presented. High-resolution observations obtained from the National Center for Atmospheric Research Electra Doppler Radar (ELDORA), S-band dual-polarization Doppler radar (S-Pol), water vapor differential absorption lidar (DIAL) Lidar pour l'Etude des Interactions Aérosols Nuages Dynamique Rayonnement et du Cycle de l'Eau (LEANDRE II; translated as Lidar for the Study of Aerosol–Cloud–Dynamics–Radiation Interactions and of the Water Cycle) as well as Learjet dropsondes are used to reveal the evolution of the dryline structure during late afternoon hours when the dryline was retreating to the northwest. The dryline reflectivity shows significant variability in the along-line direction. Dry air was observed to overrun the moist air in vertical cross sections similar to a density current. The updrafts associated with the dryline were 2–3 m s−1 and were able to initiate boundary-layer-based clouds along the dryline. The formation of this dryline was caused by high equivalent potential temperature air pushing northwestward toward a stationary front in the warm sector. Middle-level clouds with radar reflectivity greater than 18 dBZe near the dryline were detected by ELDORA. A roll boundary, which was associated with larger convergence and moisture content, was evident in the S-Pol data. It is found that the instability parameters most favorable for convection initiation were actually associated with the roll boundary, not the dryline. A storm was initiated near the roll boundary probably as a result of the combination of the favorable instability parameters and stronger upward forcing. It is noted that both the 11 June 2002 dryline and the roll boundary presented in this paper would not be identified if the special datasets from IHOP_2002 were not available. Although all model runs [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), Meso Eta, and Rapid Update Cycle (RUC)] suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms. The reasons why the dryline failed to produce any storm inside the IHOP_2002 intensive observation region are discussed. Both synoptic-scale and mesoscale conditions that were detrimental to convection initiation in this case are investigated in great detail.

Numerical Simulation on the Diurnal Variation of Precipitation over Northeastern Bangladesh: A Case Study of an Active Period 14-21 June 1995

The diurnal variation of precipitation over the Bengal region is numerically investigated using a non-hydrostatic mesoscale model. We focused on northeastern Bangladesh, where the maximum of late night-early morning precipitation is observed. Simulated characteristics of diurnal variation of precipitation agree, to a large extent, with observable evidence. The mechanism of the high late night-early morning precipitation in a case of June 14 to 21 is investigated using the model results.At night, strong southwesterly monsoon winds transport a large amount of moisture over Bangladesh. This high equivalent potential temperature (θe) air mass climbs the slope of the Meghalaya Plateau or climbs over a deep (several hundreds meters to 1 km) pool of cold air, and triggers precipitation over northeastern Bangladesh. The triggered systems develop into mesoscale squall lines and propagate southward or south-southwestward at a speed of about 10 m s-1. It is found that the katabatic mountain wind is not a major player in the diurnal variation of precipitation over the northeastern Bangladesh area.

A Modeling Diagnosis of the Development of Mesoscale Convective Systems over the South China Sea during the Summer Monsoon Onset in 1998

In this study, PSU/NCAR MM5 is used to simulate the evolution of the 1998 East Asian summer monsoon onset over the South China Sea (SCS). Simulation outputs are further utilized to study the development of mesoscale convective systems (MCSs) and their interactions with the synoptic/large-scale environment in the SCS region. During the monsoon onset, over southern China and the SCS, high equivalent potential temperature, convective instability, and the southwesterly confluence in the lower troposphere provided favorable large-scale thermodynamic and dynamic conditions for the MCS development. Model simulated results reveal that, in a warm-moist and unstable atmosphere, the downdraft outflows as a result of the MCS development could become a possible mesoscale triggering mechanism for the formation of a new MCS. Sensitivity tests depict the effect of latent heat release playing a significant role in the establishment of monsoon circulation. The terrain effect is significant for local cloud formation over land. For well organized MCSs occurring over the SCS, the influence of the mesoscale wind field is more important. The latent heat release in MCSs heats the atmosphere, reduces the surface pressure, and therefore enhances the low-level wind convergence and cyclonic circulation. The mesoscale adjustment of wind field and mass field, on one hand, enhances the MCS development or initiates the formation of a new MCS, and the organization of MCSs, on the other hand, integrates the mesoscale disturbances into a synoptic structure and forms the local monsoon trough. Without the latent heat release, monsoon circulation cannot establish itself even if the model provides favorable large-scale boundary forcing.

Numerical Modeling of an Orographically Enhanced Precipitation Event Associated with Tropical Storm Rachel over Taiwan

Abstract An orographic rainfall event that occurred on 6–7 August 1999 during the passage of Tropical Storm (TS) Rachel over Taiwan is investigated by performing triply nested, nonhydrostatic numerical simulations using the Naval Research Laboratory's (NRL) Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. By examining both observational data and numerical model output, it is found that this orographic rainfall event may be separated into three distinct stages. During the first stage (0000–1200 UTC 6 August), TS Rachel was located in the South China Sea and tracked northeastward to Taiwan. Meanwhile, TS Paul was steered by the subtropical high over southwest Japan. During the second stage (1200 UTC 6 August–0000 UTC 7 August), the southwesterly monsoon current as well as the circulation of TS Rachel over southwest Taiwan strengthened and formed a low-level jet (LLJ) with high equivalent potential temperature when TS Rachel moved closer to Taiwan. In comparing the control and sensitivity...

Layer inflow into precipitating convection over the western tropical Pacific

AbstractA conceptual model of tropical convection frequently used in convective parametrization schemes is that of a parcel process in which boundary‐layer air, characterized by high equivalent potential temperature, ascends to great heights in convective updraughts, while air above the planetary boundary layer with lower equivalent potential temperature mixes into convective downdraughts and sinks. However, airborne Doppler‐radar data show that organized deep convective systems over the western tropical Pacific warm pool are often characterized by layers of ascending inflow ∼0.5–4 km in depth. These inflow layers do not consist merely of boundary‐layer air.In this study a high‐resolution numerical cloud model is employed to investigate these inflow layers. Input data are from the Tropical Ocean–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Two time periods are selected in December 1992, which represent the onset and peak of a strong westerly phase of the intraseasonal oscillation. Model simulations for 14 December, representative of westerly onset conditions and the beginning of a convectively active period, and 23 December, representative of strong low‐level westerlies and extremely widespread convection, are conducted. To bridge the coarse resolution of the global‐model analysis fields and the fine resolution of the cloud model, hourly output from a mesoscale model is used to supply initial and lateral boundary conditions for the cloud model.Control simulations of 14 and 23 December reveal distinct convective organizations and, specifically, inflow characteristics for the two systems embedded in different large‐scale environmental conditions. In the case of 23 December, convection simulated under conditions of the strong westerly wind and near‐saturated low‐mid troposphere exhibits deep inflow layers. Trajectories computed from the simulation of 23 December under these conditions show a strong layer‐lifting inflow signal. In contrast, the control simulation of 14 December shows a parcel‐like inflow with only the air in the lower part of the inflow actually rising in the deep convective updraughts. One of the main differences between the two simulations is the lack of a deep environmental moist layer in the 14 December case. The control simulation did not capture well the extent of precipitating mesoscale stratiform clouds that developed from earlier convection in the vicinity of the deep convective cells as indicated in the COARE observations. Previous studies have shown that spatially extensive convection is correlated with enhanced mid‐level humidity. To isolate the effect of the mid‐level moist layer on the characteristics of inflow of convective systems, a numerical experiment based on the control simulation of 14 December was conducted. The relative humidity of environment air in the low‐mid troposphere (1.7–6 km layer) was increased to 95%. Trajectory statistics calculated for this sensitivity experiment show increased layer lifting, with a significant amount of air from the upper part of the inflow layer rising in the updraught along with air from just above the surface. Moistening the inflow layer in this sensitivity experiment allows it to saturate more quickly when it encounters the mesoscale cold pool. Once saturated, the relevant static stability is the moist rather than dry static stability, and the whole layer more easily rises over the cold pool. Moistening the inflow layer also modifies the nature of the simulated cold pool itself, which seems to promote layer lifting in the simulation. Possible mechanisms for moistening the mid‐levels are briefly discussed. Copyright © 2002 Royal Meteorological Society

The Large-Scale Environments of the Global Populations of Mesoscale Convective Complexes

Abstract The mean genesis environment was constructed for each of five mesoscale convective complex (MCC) population centers around the world: Africa, Australia, China, South America, and the United States. It is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the United States. In particular, MCCs initiate within prominent baroclinic zones characterized by locally large values of lower-tropospheric vertical wind shear and convective available potential energy (CAPE). Typically, a low-level jet of air with low static stability, high equivalent potential temperature, oriented nearly perpendicular to the baroclinic zone, intrudes into the genesis region and is forced to ascend over a relatively shallow, surface-based layer of relatively cool air. Pronounced warm advection accompanied by strong lower-tropospheric veering overlays the surface-based cool layer. A local maximum in absolute humidity and a local minimu...

台風9426号(Orchid)接近時に関東地方で見られたほぼ停滞するバンド状降雨帯の解析と数値シミュレーション

A quasi-stationary precipitation band, which persisted for more than 10 hours, was observed along a mesoscale front over the Kanto area, when Typhoon 9426 (Orchid) approached the Japan Islands. The band was analyzed using data of special upper sounding at Tsukuba, two Doppler radars, and operational meteorological observations. The band had the carrot-shaped form and characteristic features of the back-building type. In the precipitation band, which aligned from south to north, a multi-cellular structure was observed. New cells were repeatedly generated at the southern edge of the band, and moved northward, expanding westward. Numerical simulations with the horizontal resolution of 2 km were performed using the Meteorological Research Institute mesoscale non-hydrostatic anelastic model. The characteristic features of a quasi-stationary precipitation band was well simulated, although a mesoscale front was generated about 100 km southeast of the observed position. This lasted for more than 3 hours, and reproduced features of the back-building type. In the simulated band, the carrot-shaped form is produced by the northwestward movements of the cells, and the enhancement of the cells on the eastern flank of the band. By changing environmental fields and cloud microphysical processes, sensitivity experiments were conducted to investigate the formation mechanisms of the precipitation band. It was shown that the supply of high equivalent potential temperature air, from the upwind side of the middle level wind, and the vertical wind shear are essential.

Small-Scale Spiral Bands Observed in Hurricanes Andrew, Hugo, and Erin

Abstract Analysis of radar data taken from the three intense hurricanes that passed close to WSR-57 and WSR-88D radar sites at their point of landfall illustrate small-scale spiral bands that are frequently observed within 100 km or so of the hurricane center. Faintly visible in the radar reflectivity images, these bands have scales of 10 km across the band and can extend for 100 km as they spiral outward in a clockwise fashion. They appear to move around the hurricane with speeds close to the tangential wind at the level of the bands and are characterized by enhanced updrafts with higher equivalent potential temperature in the regions of elevated reflectivity. They induce wind speed variations of at least 8 m s−1 across the bands. The authors suggest that these small-scale hurricane spiral bands are similar to boundary layer rolls although they extend through depths of 5–6 km, which is more than would be expected for rolls in the boundary layer near the sea. The data presented here are not sufficient to ...

Effects of revegetation and new artificial water bodies on the climate of northeast Mexico City

An area of 10 000 ha of the ex-Lake Texcoco bed on the NE fringes of Mexico City has been protected since about 1975. At present, some 5500 ha of previously almost denuded, salty, semi-arid land (precipitation: 527 mm/yr) have been covered (to more than 90%) with native grass. In addition, since 1980 a large (1000 ha) rectangular lake and several (5) smaller ponds (total area 500 ha) have been constructed within this “enclosure” adjacent (and upwind) to the capital city. The additional moisture supplied by the water bodies is of the order of about 40 000 m 3/day plus 90 000 m 3/day from evapotranspiration of new grass-covered surfaces, and is likely to have affected favorably the environment in the vicinity of the airport. The fenced area used to be the most important source for the generation of atmospheric dust which plagued the capital city during the dry season. At present, dust storms coming from this direction have drastically reduced their frequency. According to current thought, the changes in albedo and infrared emission caused by vegetating a previously bare region result in an increase in net radiation absorbed at the surface. This increase in available energy is used for evaporation and, depending on the type and cover of vegetation, to heat the air. The new vegetated cover would result in a moister lower atmosphere with higher equivalent potential temperature. In the 14-year period, the fenced area of ex-Lake Texcoco is likely to have gradually changed its albedo to a lower value, particularly the grass-covered area during the wet season, thus inducing an observed increase in air temperature (up to 1.5°C in mean maximum temperature at the site near the lakes and downwind at the airport station). The construction of lakes and ponds has augmented advection of moisture downwind at the airport, which is observed both as an increase of mean dew-point temperatures (1–2°C), as well as a higher occurrence of ground fog. Moreover, the new mesoscale moisture contrast is also likely to have induced a circulation which, in weak synoptic flow situations, may have been partly responsible for an observed increase (downwind) in precipatation during the dry months.

The Windsor Locks, Connecticut Tornado of 3 October 1979: An Analysis of an Intermittent Severe Weather Event

Abstract On 3 October 1979 a violent F4 tornado struck without warning at Windsor Locks, Connecticut just before 1900 UTC. It was the most destructive storm ever to occur in Connecticut. Our case study of the storm was motivated by the rarity of tornadoes in Connecticut and the unfavorable time of the year for such storms in New England. The history of the storm is traced by means of synoptic and subsynoptic scale analyses. Vigorous midtropospheric ascent occurred just in advance of a cold-core cutoff vortex that was opening up due to synoptic scale deformation while moving northeastward toward southern New England. The probable destabilization mechanism was the intersection of the northeastward moving pocket of cool air aloft with a tongue of high equivalent potential temperature surface air that originated over the relatively warm ocean water and was channeled northward through the Connecticut River Valley. The convective cell that eventually became associated with the Windsor Locks tornado was detected...

Thermodynamic Budget Diagrams for the Hurricane Subcloud Layer

The idealized hurricane boundary layer budget from Malkus and Riehl (1960) are reexamined using vector diagrams for the thermodynamic budgets in the light of recent observational studies. A large air-sea temperature difference can only be maintained with both large fluxes through cloud-base level and a large evaporative cooling of the subcloud layer. The high equivalent potential temperature values observed in hurricane eyewalls can be reached if these cloud-base and evaporative fluxes are reduced and the subcloud layer moves toward the sea surface virtual potential temperature.

A SHORT DRY PERIOD OF MID-SUMMER IN JAPAN

1. The abundant summer rainfall in the eastern part of Asia including Japan has been commonly explained by the southwesterly monsoon winds carrying with much moisture from the Pacific Ocean. However, this problem is not so simple because the rainy season in summer has a threefold structure instead of a single continuous period. Except the northern region, there exists in Japan the double or triple maxima of monthly amount of precipitation. Fig. 1. shows the three regimes of precipitation, A, B and C, classified as follows A: Pacific type with double maxima in summer. A1 Primary maximum in. September and the secondary one in June. A2 Inverted type of A1, primary maximum in June and the secondary one in September. B: Japan Sea type with triple maxima. Of them, the primary maximum appears in winter and the other two in summer (June and September). C: Northern type with single maximum in September. In both regions of type A and B, two maxima of summer appear in September and June separated by the July-August minimum forming a short dry period which is very important not only from the geographical but also from the economic points of view. 2. The dry period above-mentioned is considered to be a result of the special type of pressure pattern developing in the special type of pressure patterns developing in the mid-summer over the north Pacific Ocean. After the Bai-u front along which the cloudy and rainy weather persists for a long spell parse: away to the north, the north Pacific anticyclone extends westward to the Japanese Islands and its western part is called the Ogasawara High. Even in a normal year, the main part of Japan, especially its western half, is located underneath a huge anticyclone (Fig, 2.) and fine weather lasts for many days with scarce rainfall, bright sunshine and strong evaporation. Because of the prevalence and westward expansion of the Ogasawara High, northward movement of cyclones and typhoons from the southwestern seas are checked to turn their paths to the westward direction along the outer periphery of the high pressure area and sc the frequencies of cyclones become very scarce (Fig. 3.). This strengthens further the occurrence of clear sky and scanty rain during this period. In connection with this subject, summer rain in the Far East is very important for the theory of non -soon, and it has been commonly explained by the monsoon winds from the Pacific Ocean laden with abundant moisture. However, this is inconsistent with the threefold structure of summer rainy season in Japan. Recent upper air observations afford another interpretation for this subject. That is, water vapor for summer precipitation in the Far East is originally transfered by the subtropical airmass from the equatorial region inflowing as southwesterly winds along the southern coast of Asiatic continent. This airmass is very moist with high equivalent potential temperature and connective instability. With the westward expansion of the Ogasawara High, tropical easterly with less moisture becomes strong and interrupts northward invasions of the southwesterly winds. As a result, fine weather persists for a long period. In such a way, the monsoon does really exist if it means only the seasonal alternation of winds with opposite directions. Hence; summer monsoon in East Asia is not exactly the same as that with abundant rainfall in India and other regions of southeastern Asia. 3. Furthermore, in summer season, the temperature becomes much higher with less rainfall because of intense solar heating unhindered and unweakened by the thick layer of clouds. A striking negative correlation is found between the amount of monthly rainfall and average monthly temperature in summer. Numerical figures for Fig. 4. give an amount of August rainfall in 1939 expressed as a percentage of normal year and those for Fig. 5, the August temperature deviation in 1939 from the normal.