Dry Air Intrusion Research Articles

FIELD SURVEY ON THE ACTUAL CONDITIONS AND IMPROVEMENT OF INDOOR TEMPERATURE AND HUMIDITY IN KINDERGARTENS IN GIFU AREA DURING WINTER

Kindergarten is one of the main living spaces for infants, and its indoor environment is important from the viewpoint of maintaining and promoting health. In this research, we conducted a survey of indoor temperature and humidity environment in winter in kindergartens located in the Gifu area, clarified its characteristics, and examined the relationship between the kindergarten teacher's awareness of indoor environment adjustment and actions. In addition, we proposed a method to improve the humidity environment based on the results, and aimed to clarify the relation between the influence on the humidity environment and the window opening ventilation. Firstly, in 2006 school year, we selected nine kindergartens and measured indoor temperature and relative humidity with the aim of grasping actual conditions of temperature and humidity environment during winter in the classroom which were usually used by children. Measurement objects were about two classrooms of each kindergarten, totaling 17 rooms. There was no humidifier and mechanical ventilation in these rooms. Next, in 2007 school year, we examined the influence on humidity environments by humidification and window-opening ventilation behavior by kindergarten teachers in seven classrooms in six kindergartens, for the purpose of improving the humidity environment without the building repair. Finally, in 2008 school year, in order to grasp the state of ventilation, we measured the concentration of carbon dioxide and recorded the opening and closing state of doors and windows in three rooms in two kindergartens. The time zone analyzed temperature, humidity and carbon dioxide concentration were from 8: 30 to 14: 30 on weekdays. The results of the survey in 2006 school year, the absolute humidity in the classroom tended to be low in January and February, and it was thought that the influence of the dry outdoor absolute humidity was greatly received. As one of the factors, it was thought that kindergarten teacher gives priority to ventilation. As a main factor, it was thought that kindergarten teacher gave priority to ventilation. Based on the results of the survey in 2007 school year, it was inferred that using humidifier was effective in raising the absolute humidity of the classroom, even if differences in outdoor temperature and humidity conditions and window opening ventilation conditions by teachers were taken into consideration. According to the survey results in 2008 school year, the average carbon dioxide concentration in each classroom was below 1500ppm. The number of air changes was estimated and the dairy average was 1.7 to 5.3 [1/h]. From the records of the opening area of the opening and the number of children in the classroom, it seems that the time when all the children stayed in the room was short and all the opening of the room was not closed for a long time. Consequently, there is a possibility that the amount of ventilation required in a kindergarten may be estimated more if assuming that everyone is always in the room. The absolute humidity of the classroom without the humidifier was less than 5.8 g/kg' which was proposed that infection of influenza spread easily. It was thought that this main factor is caused by intrusion of dry outdoor air by window opening ventilation. Therefore it is desirable to raise the absolute humidity in the classroom of kindergarten on taking outdoor environment and the states of ventilation into consideration.

A comparative study on the genesis of North Indian Ocean tropical cyclone Madi (2013) and Atlantic Ocean tropical cyclone Florence (2006)

AbstractA modeling study has been carried out to understand the similarities and differences in the genesis sequence of a Bay of Bengal tropical cyclone Madi (6–13 December 2013) and the Atlantic Ocean tropical cyclone Florence (3–12 September 2006) on the applicability of hypotheses of the marsupial theory of tropical cyclogenesis. We examined the role of the protective pouch and warm core formation during their genesis and intensification phases. We have chosen tropical cyclone Madi and tropical cyclone Florence for our study specifically due to both of these tropical cyclones originated from westward moving parent disturbance embedded in the intertropical convergence zone. Also, the genesis and intensification of tropical cyclone Florence were accompanied by a series of Saharan dust outbreaks. Our results indicated that the dry air intrusion was not a dominant detrimental factor for the genesis of tropical cyclone Madi and showed rapid intensification within the pouch region. However, in the case of the tropical cyclone Florence, the delay in the intensification as a category 1 tropical cyclone from its tropical depression stage was due to entrainment of the dry air into the core of cyclonic vortex up to 700 hPa from above. The results from this study showed that the wave pouch played a most significant role in the vorticity upscale cascade (First hypothesis) and moisture aggregation (Second hypothesis) in pregenesis period of both the tropical cyclones. It also prevented the lateral dry air intrusion (Second hypothesis) from the Saharan Air Layer during the genesis phase of tropical cyclone Florence.

A diagnostic study on heavy rainfall induced by Typhoon Utor (2013) in South China: 1. Rainfall asymmetry at landfall

AbstractAfter striking the Philippines on 11 August 2013, Typhoon Utor made its second landfall over southern South China on 14 August and brought heavy rainfall, causing widespread damages after its landfall. The heavy rainfall associated with Utor includes three main episodes: rainfall at landfall, postlandfall rainfall in the inner core region, and postlandfall rainfall induced by the intensification of southwesterly monsoon flow. In part 1 the rainfall at landfall is analyzed and diagnosed using the surface mesonet observations and the high‐resolution European Centre for Medium‐Range Weather Forecasting Interim reanalysis (ERA‐Interim) data. Results show that when Utor approached and made landfall over the southern coast of South China, it was embedded in a deep‐layer (200–850 hPa) easterly northeasterly vertical wind shear (VWS) environment with heavy rainfall mostly occurring downshear left in its southwestern quadrant. Under a favorable environment with moderate convective available potential energy and moderate low‐level VWS, a surface cold pool was found in the west‐southwestern quadrants of Utor triggered by the middle‐lower tropospheric dry air intrusion from inland to the north. Both frontogenesis and quasi‐balanced, ageostrophic Q‐vector diagnostics demonstrate that the quasi‐balanced forcing near the surface cold cool contributed to the enhancement of convection in the west‐southwestern quadrants of Utor. It is concluded that the environmental VWS and the consequent surface cold pool dynamics play an important role in rainfall asymmetry in Utor when it made landfall over the southern coast of South China.

Hurricane Irene Sensitivity to Stratified Coastal Ocean Cooling

Cold wakes left behind by tropical cyclones (TCs) have been documented since the 1940s. Many questions remain, however, regarding the details of the processes creating these cold wakes and their in-storm feedbacks onto tropical cyclone intensity. This largely reflects a paucity of measurements within the ocean, especially during storms. Moreover, the bulk of TC research efforts have investigated deep ocean processes—where tropical cyclones spend the vast majority of their lifetimes—and very little attention has been paid to coastal ocean processes despite their critical importance to shoreline populations. Using Hurricane Irene (2011) as a case study, the impact of the cooling of a stratified coastal ocean on storm intensity, size, and structure is quantified. Significant ahead-of-eye-center cooling (at least 6°C) of the Mid-Atlantic Bight occurred as a result of coastal baroclinic processes, and operational satellite SST products and existing coupled ocean–atmosphere hurricane models did not capture this cooling. Irene’s sensitivity to the cooling is tested, and its intensity is found to be most sensitive to the cooling over all other tested WRF parameters. Further, including the cooling in atmospheric modeling mitigated the high storm intensity bias in predictions. Finally, it is shown that this cooling—not track, wind shear, or dry air intrusion—was the key missing contribution in modeling Irene’s rapid decay prior to New Jersey landfall. Rapid and significant intensity changes just before landfall can have substantial implications on storm impacts—wind damage, storm surge, and inland flooding—and thus, coastal ocean processes must be resolved in future hurricane models.

Why did the storm ex-Gaston (2010) fail to redevelop during the PREDICT experiment?

Abstract. An analysis is presented of the failed re-development of ex-Gaston during the 2010 PREDICT field campaign based on the European Centre for Medium Range Weather Forecast (ECMWF) analyses. We analyze the dynamics and kinematics of ex-Gaston to investigate the role of dry, environmental air in the failed redevelopment. The flow topology defined by the calculation of particle trajectories shows that ex-Gaston's pouch was vulnerable to dry, environmental air on all days of observations. As early as 12:00 UTC 2 September 2010, a dry layer at and above 600 hPa results in a decrease in the vertical mass flux and vertical relative vorticity. These findings support the hypothesis that entrained, dry air near 600 hPa thwarted convective updraughts and vertical mass flux, which in turn led to a reduction in vorticity and a compromised pouch at these middle levels. A compromised pouch allows further intrusion of dry air and quenching of subsequent convection, therefore hindering vorticity amplification through vortex tube stretching. This study supports recent work investigating the role of dry air in moist convection during tropical cyclogenesis.

2013년 8월 6일 한반도에서 발달한 다세포(Multicell) 대류계의 특성 분석

Damages caused by torrential rain occur every year in Korea and summer time convection can cause strong thunderstorms to develop which bring dangerous weather such as torrential rain, gusts, and flash flooding. On 6 August 2013 a sudden torrential rain concentrated over the inland of Southern Korean Peninsula occurred. This was an event characterized as a mesoscale multicellular convection. The purpose of this study is to analyze the conditions of the multicellular convection and the synoptic and mesoscale nature of the system development. To this end, dynamical and thermodynamic analyses of surface and upper-level weather charts, satellite images, soundings, reanalysis data and WRF model simulations are performed. At the beginning stage there was a cool, dry air intrusion in the upper-level of the Korean Peninsula, and a warm humid air flow from the southwest in the lower-level creating atmospheric instability. This produced a single cell cumulonimbus cloud in the vicinity of Baengnyeongdo, and due to baroclinic instability, shear and cyclonic vorticity the cloud further developed into a multicellular convection. The cloud system moved southeast towards Seoul metropolitan area accompanied by lightning, heavy precipitation and strong wind gusts. In addition, atmospheric instability due to daytime insolation caused new convective cells to develop in the upstream part of the Sobaek Mountain which merged with existing multicellular convection creating a larger system. This case was unusual because the system was affected little by the upper-level jet stream which is typical in Korea. The development and propagation of the multicellular convection showed strong mesoscale characteristics and was not governed by large synoptic-scale dynamics. In particular, the system moved southeast crossing the Peninsula diagonally from northwest to southeast and did not follow the upper-level westerly pattern. The analysis result shows that the movement of the system can be determined by the vertical wind shear.

Extratropical Impacts on Atlantic Tropical Cyclone Activity

Abstract With warm sea surface temperature (SST) anomalies in the tropical Atlantic and cold SST anomalies in the east Pacific, the unusually quiet hurricane season in 2013 was a surprise to the hurricane community. The authors’ analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August 2013 can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant mid- to upper-tropospheric dryness and strong vertical wind shear hindered TC development. Using the empirical orthogonal function analysis, the active Rossby wave breaking in August 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the main development region (MDR) relative SST and higher than that with the Niño-3.4 index. This study highlights the extratropical impacts on Atlantic TC activity, which may have important implications for the seasonal predictability of Atlantic TCs.

Impact of environmental moisture on tropical cyclone intensification

Abstract. The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts relative to the storm path. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity that deforms the quasi-Lagrangian boundary of the storm and changes the moisture transport into the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. In contrast, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

Abstract. In a joint NRL/Manila Observatory mission, as part of the Seven SouthEast Asian Studies program (7-SEAS), a 2-week, late September 2011 research cruise in the northern Palawan archipelago was undertaken to observe the nature of southwest monsoonal aerosol particles in the South China Sea/East Sea (SCS/ES) and Sulu Sea region. Previous analyses suggested this region as a receptor for biomass burning from Borneo and Sumatra for boundary layer air entering the monsoonal trough. Anthropogenic pollution and biofuel emissions are also ubiquitous, as is heavy shipping traffic. Here, we provide an overview of the regional environment during the cruise, a time series of key aerosol and meteorological parameters, and their interrelationships. Overall, this cruise provides a narrative of the processes that control regional aerosol loadings and their possible feedbacks with clouds and precipitation. While 2011 was a moderate El Niño–Southern Oscillation (ENSO) La Niña year, higher burning activity and lower precipitation was more typical of neutral conditions. The large-scale aerosol environment was modulated by the Madden–Julian Oscillation (MJO) and its associated tropical cyclone (TC) activity in a manner consistent with the conceptual analysis performed by Reid et al. (2012). Advancement of the MJO from phase 3 to 6 with accompanying cyclogenesis during the cruise period strengthened flow patterns in the SCS/ES that modulated aerosol life cycle. TC inflow arms of significant convection sometimes span from Sumatra to Luzon, resulting in very low particle concentrations (minimum condensation nuclei CN < 150 cm−3, non-sea-salt PM2.5 < 1 μg m−3). However, elevated carbon monoxide levels were occasionally observed suggesting passage of polluted air masses whose aerosol particles had been rained out. Conversely, two drier periods occurred with higher aerosol particle concentrations originating from Borneo and Southern Sumatra (CN > 3000 cm−3 and non-sea-salt PM2.5 10–25 μg m−3). These cases corresponded with two different mechanisms of convection suppression: lower free-tropospheric dry-air intrusion from the Indian Ocean, and large-scale TC-induced subsidence. Veering vertical wind shear also resulted in aerosol transport into this region being mainly in the marine boundary layer (MBL), although lower free troposphere transport was possible on the western sides of Sumatra and Borneo. At the hourly time scale, particle concentrations were observed to be modulated by integer factors through convection and associated cold pools. Geostationary satellite observations suggest that convection often takes the form of squall lines, which are bowed up to 500 km across the monsoonal flow and 50 km wide. These squall lines, initiated by cold pools from large thunderstorms and likely sustained by a veering vertical wind shear and aforementioned mid-troposphere dry layers, propagated over 1500 km across the entirety of the SCS/ES, effectively cutting large swaths of MBL aerosol particles out of the region. Our conclusion is that while large-scale flow patterns are very important in modulating convection, and hence in allowing long-range transport of smoke and pollution, more short-lived phenomena can modulate cloud condensation nuclei (CCN) concentrations in the region, resulting in pockets of clean and polluted MBL air. This will no doubt complicate large scale comparisons of aerosol–cloud interaction.

Rapid deepening of tropical cyclones in the northeastern Tropical Pacific: The relationship with oceanic eddies

The National Hurricane Center best track archive data for the northeastern Tropical Pacific includes tropical cyclones (TC) that exhibited rapid and/or explosive deepening during the period 1993-2009. Concurrently, satellite altimeter observations show the northeastern Tropical Pacific populated by cyclonic and anticyclonic oceanic eddies. These two sources of data are used to study the role of oceanic eddies in the spatial distribution of the oceanic heat content and the rapid and/or explosive deepening of TCs in the northeastern Tropical Pacific. Analysis of the results demonstrates that: (1) Interaction between TCs and anticyclonic eddies occurred in~73% of the TCs within the analyzed period; (2) 90% of the TCs that reached major hurricane category had experienced an interaction with anticyclonic eddies; (3)~18% (3%) of the TCs were characterized by rapid (explosive) deepening and 74% of these TCs reached the deepening status after direct and/or indirect interaction with anticyclonic eddies; moreover, 86% of them reached a major hurricane category. Analysis of the atmospheric conditions demonstrates the important role of dry air intrusion along the vertical profile of the lower atmosphere as an inhibitor of tropical cyclone deepening.

Merger of African easterly waves and formation of Cape Verde storms

There are two easterly wave tracks over West Africa, one to the south and the other to the north of the African Easterly Jet (AEJ). A northern wave sometimes merges with a southern wave near the coast of West Africa. The dynamic and thermodynamic evolution of the waves during the merger and the impacts of the merger on subsequent storm development are examined in this study. Three groups of storms are identified: non‐merger developers, merger developers and merger non‐developers. Relative to non‐merger developers, merger developers have a weaker circulation near the surface prior to the merger, but the merger leads to a stronger and deeper wave pouch, which is more conducive to tropical cyclogenesis. The dryness of the northern disturbances does not seem to hinder the storm development. Composite analysis shows that merger developers are associated with a warmer and broader heat low over West Africa and a more extensive AEJ, which helps to maintain the northern waves over the East Atlantic.Merger developers and non‐developers resemble each other prior to the merger, but dry air intrusion west of the wave trough in the middle and upper troposphere contributes to the non‐development of some mergers. The dry air in non‐developers is related to a strong and broad upper‐tropospheric trough and the associated subsidence west of the low‐level wave pouch. The trough advects dry air from the extratropical Atlantic in the middle and upper troposphere and also induces a strong westerly vertical shear. Composites of aerosol optical depth suggest that the Saharan Air Layer has a limited contribution to the dryness in the vicinity of the non‐developers.

The dynamical structure of intense Mediterranean cyclones

This paper presents and analyzes the three-dimensional dynamical structure of intense Mediterranean cyclones. The analysis is based on a composite approach of the 200 most intense cyclones during the period 1989–2008 that have been identified and tracked using the output of a coupled ocean–atmosphere regional simulation with 20 km horizontal grid spacing and 3-hourly output. It is shown that the most intense Mediterranean cyclones have a common baroclinic life cycle with a potential vorticity (PV) streamer associated with an upper-level cyclonic Rossby wave breaking, which precedes cyclogenesis in the region and triggers baroclinic instability. It is argued that this common baroclinic life cycle is due to the strongly horizontally sheared environment in the Mediterranean basin, on the poleward flank of the quasi-persistent subtropical jet. The composite life cycle of the cyclones is further analyzed considering the evolution of key atmospheric elements as potential temperature and PV, as well as the cyclones’ thermodynamic profiles and rainfall. It is shown that most intense Mediterranean cyclones are associated with warm conveyor belts and dry air intrusions, similar to those of other strong extratropical cyclones, but of rather small scale. Before cyclones reach their mature stage, the streamer’s role is crucial to advect moist and warm air towards the cyclones center. These dynamical characteristics, typical for very intense extratropical cyclones in the main storm track regions, are also valid for these Mediterranean cases that have features that are visually similar to tropical cyclones.

Climatological characteristics of fronts in the western North Pacific based on surface weather charts

AbstractComposite front climatology in the western North Pacific is determined using a newly developed 1.0° gridded data set. Here we propose a research strategy for determining the spatiotemporal distribution of fronts using weather chart images published by the Japan Meteorological Agency, one of the major data providers in the region. A preliminarily investigation of the internal data characteristics for the period of 2000–2010 is undertaken, and the final 4 years of data are used for an analysis of front climatology to avoid the effect of any spurious trends. This enables in‐depth analyses to be conducted, which have not previously been possible in the region, including the composites of cross‐sectional patterns for the thermal fields and precipitation near fronts, front length seasonality, and the significance of the thermal gradient near the fronts, in addition to determining the frontal frequency and spatial distribution of frontal precipitation. Pixel‐wise analysis reveals that 56% of the local precipitation maximum is located on the warm side of a cold front caused by less tilted upward motion on the warm side, with the intrusion of the upper level cold dry air into the warm side. This new data set also enables a further analysis of the occluded fronts, which are not correctly distinguished in the existing objective detection method.

Environmental influences on the intensity changes of tropical cyclones over the western North Pacific

Abstract. The influence of environmental conditions on the intensity changes of tropical cyclones (TCs) over the western North Pacific (WNP) is investigated through examination of 37 TCs during 2000–2011 that interacted directly with the western North Pacific subtropical high (WNPSH). Comprehensive composite analysis of the environmental conditions is performed for two stages of storms: one is categorized as intensifying events (maximum wind speed increases by 15 kn over 48 h) and the other is categorized as weakening events (maximum wind speed decreases by 15 kn over 48 h). Comparison of the composite analysis of these two cases show that environmental conditions associated with the WNPSH play important roles in the intensity changes of TCs over the WNP. When a TC moves along the southern periphery of the WNPSH, the relatively weaker easterly environmental vertical wind shear helps bring warm moist air from the south and southeast to its southeast quadrant within 500 km, which is favorable for the TC to intensify. However, when a TC moves along the western edge of the WNPSH, under the combined influences of the WNPSH and an upper-level westerly trough, a strong westerly vertical shear promotes the intrusion of dry environmental air associated with the WNPSH from the north and northwest, which may lead to the inhibition of moisture supply and convection over the western half of the TC and thus its weakening. These composite results are consistent with those with additional geographic restrictions, suggesting that the dry air intrusion and the vertical wind shear (VWS) associated with the WNPSH, indeed affect the intensity changes of TCs over the WNP beyond the difference related solely to variations in geographical locations. The average sea surface temperature (SST) of 27.6 °C for the weakening events is also lower than an average of 28.9 °C for the strengthening events, but remains above the critical value of 27 °C for TC intensification, suggesting that the SST may be regarded as a less positive factor for the weakening events.

ECMWF and GFS model forecast verification during DYNAMO: Multiscale variability in MJO initiation over the equatorial Indian Ocean

AbstractImproving understanding and prediction of the initiation of the Madden‐Julian Oscillation (MJO) over the equatorial Indian Ocean (IO) was a main objective of the Dynamics of the MJO (DYNAMO) field campaign. This study evaluates the European Centre for Medium‐Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction Global Forecast System (GFS) operational global model forecasts of MJO‐2 in DYNAMO (November–December 2011). The model rainfall and 850 hPa zonal winds are compared with the Tropical Rainfall Measurement Mission (TRMM) rainfall and operational analysis in the equatorial IO. The ECMWF forecasts of the total equatorial rainfall were more consistent with TRMM observations. The GFS did not accurately predict the onset of equatorial convection beyond 5 day lead time. Instead, the GFS rainfall was mainly off the Equator in the Intertropical Convergence Zone (ITCZ). The GFS did not capture synoptic scale systems that bring in dry air intrusions from the subtropics and disrupt convection in the ITCZ which is favorable for equatorial convection associated with the MJO initiation. DYNAMO observations suggest that multiscale interactions among convection, synoptic systems, and the MJO large‐scale circulations were important for the initiation of MJO‐2. The model forecasts and analysis of 850 hPa zonal wind are decomposed into mesoscale, synoptic, and large‐scale flows. Results show that (1) both models have good initial conditions and 1–2 day forecasts and (2) the ECMWF has significantly better skill than the GFS in terms of forecasting equatorial convection, synoptic systems, and the large‐scale MJO circulation at 5–15 day lead times.

Potential vorticity diagnosis of tropical cyclone Usagi (2001) genesis induced by a mid-level vortex over the South China Sea

In this study, we first show that tropical cyclone (TC) Usagi evolved from a mid-level vortex over the South China Sea (SCS) in August 2001. The initial disturbance of TC Usagi had a maximum potential vorticity (PV) near 500 hPa, and an anticyclonic circulation with a cold core near the surface. The cyclonic circulation and its warm core of the mid-level vortex developed gradually downward toward the surface when environmental easterly and dry air intruded from the upper troposphere; finally, the mid-level vortex evolved into TC Usagi under favorable environment conditions such as weak vertical wind shear, deep moist layer, etc. To investigate the dynamic and thermodynamic processes during TC Usagi genesis, the technique of piecewise PV inversion is employed. The results show that the actions of upper-layer PV and potential temperature anomalies were not important in TC Usagi genesis. Surface-layer thermal anomalies mainly produced negative disturbances of temperature at the vortex center below 800 hPa, which was unfavorable to the genesis of a cyclonic circulation near the surface. Middle-to-lower-layer latent heat played a key role in TC Usagi genesis and downward development of dynamic and thermodynamic processes. The actions of dry air intrusion from the upper troposphere, environmental westerly changing into easterly in the middle and lower troposphere, and baroclinic structure of the vortex were also important. The cyclonic circulation of the mid-level vortex could develop downward quickly from the middle troposphere toward the surface. However, whether the warm core of the vortex developed near the surface depended on the combined actions of surface-layer thermal anomaly and middle-to-lower-layer latent heat. Finally, we present a conceptual model of TC Usagi genesis induced by a mid-level vortex over the SCS.

Equatorial Dry Air Intrusion and Related Synoptic Variability in MJO Initiation during DYNAMO

Abstract Dynamics of the Madden–Julian oscillation (DYNAMO) was conducted over the equatorial Indian Ocean (IO) from October 2011 to March 2012. During mid- to late November, a strong Madden–Julian oscillation (MJO) event, denoted MJO-2, initiated in the western IO and passed through the DYNAMO observation array. Dry air intrusions associated with synoptic variability in the equatorial region played a key role in the evolution of MJO-2. First, a sharp dry air intrusion surging from the subtropics into the equatorial region suppresses convection in the ITCZ south of the equator. This diminishes subsidence on the equator associated with the ITCZ convection, which leads to an equatorward shift of convection. It is viewed as a contributing factor for the onset of equatorial convection in MJO-2. Once the MJO convection is established, a second type of dry air intrusion is related to synoptic gyres within the MJO convective envelope. The westward-propagating gyres draw drier air from the subtropics into the equatorial region on the west side of the MJO-2. This dry air intrusion contributes to a 1–2-day break in the rainfall during the active phase of MJO-2. Furthermore, the dry air intrusion suppresses convection in the westerlies of the MJO in the IO. This favors the abrupt shutdown of MJO convection during transition to the suppressed phase in DYNAMO. The two types of dry air intrusions can redistribute convection from the ITCZ to the equator and favor the eastward propagation of the MJO convection. Further study of multiple MJO events is necessary to determine the generality of these findings.

Evidence of a LNOX influence on middle/upper troposphere ozone‐mixing ratios at Dakar, Senegal during Northern Hemisphere summer season

AbstractWe present results for the vertical distribution of tropospheric ozone from 40 measurements at Dakar, Senegal (14.59°N, 17.5°W) from June to September of 2008, 2010, 2011 and 2012. Under pre‐monsoon conditions (June), many dry air intrusions are found in the lower troposphere transitioning to humid conditions during July, August and September. Measurements show the highest ozone‐mixing ratios (50–100 ppb) in the 400–200 hPa layer and easterly wind flow during each month. Lightning‐NOX (LNOX) is the only viable source to explain the vertical distribution of ozone, with a large amount of upstream lightning over West Africa during summer.

Predictability and Dynamics of a Nonintensifying Tropical Storm: Erika (2009)

Abstract In this study, the predictability of Tropical Storm Erika (2009) is evaluated by analyzing a 60-member convection-permitting ensemble initialized with perturbations from a real-time ensemble Kalman filter (EnKF) system. Erika was forecast to intensify into a hurricane by most operational numerical models, but in reality it never exceeded 50 kt (1 kt = 0.51 m s−1). There is a fairly large spread in the final intensities of the 60 ensemble members indicating large uncertainty in the deterministic prediction of Erika's intensity at 36–48-h lead times. An investigation into which factors prevented intensification of the weaker ensemble members provides insight that may aid in the forecasting of the intensity of future tropical cyclones under similar conditions. A variety of environmental and storm-related factors are examined, and the parameters that have the greatest relation to future intensity are determined based on ensemble sensitivity and correlation analysis. It appears that midlevel relative humidity, absolute vorticity, and the distribution of convection relative to the storm center all play a role in determining whether a given ensemble member intensifies or not. In addition, although differences in deep-layer shear among ensemble members are difficult to discern, many of the ensemble members that do not intensify fail to do so because of apparent dry air intrusions that wrap around the centers of the storms, particularly in the 700–500-hPa layer. In the presence of moderate shear, this dry air is able to penetrate the cores of the cyclones, thereby preventing further development.

A Numerical Study of the Impacts of Dry Air on Tropical Cyclone Formation: A Development Case and a Nondevelopment Case

Abstract The impacts of dry air on tropical cyclone formation are examined in the numerical model simulations of ex-Gaston (2010) and pre-Fay (2008). The former, a remnant low downgraded from a short-lived tropical cyclone, can be regarded as a nondeveloping system because it failed to redevelop, and the latter developed into a tropical cyclone despite lateral dry air entrainment and a transient upper-level dry air intrusion. Water vapor budget analysis suggests that the mean vertical moisture transport plays the dominant role in moistening the free atmosphere. Backward trajectory analysis and water budget analysis show that vertical transport of dry air from the middle and upper troposphere, where a well-defined wave pouch is absent, contributes to the midlevel drying near the pouch center in ex-Gaston. The midlevel drying suppresses deep convection, reduces moisture supply from the boundary layer, and contributes to the nondevelopment of ex-Gaston. Three-dimensional trajectory analysis based on the numerical model simulation of Fay suggests that dry air entrained at the pouch periphery tends to stay off the pouch center because of the weak midlevel inflow or gets moistened along its path even if it is being wrapped into the wave pouch. Lateral entrainment in the middle troposphere thus does not suppress convection near the pouch center or prevent the development of Tropical Storm Fay. This study suggests that the upper troposphere is a weak spot of the wave pouch at the early formation stage and that the vertical transport is likely a more direct pathway for dry air to influence moist convection near the pouch center.