Warm Core Research Articles

Formation and Development of a Mountain-induced Secondary Center inside Typhoon Morakot (2009)

The structural evolution of Typhoon Morakot (2009) during its passage across Taiwan was investigated with the WRF model. When Morakot approached eastern Taiwan, the low-level center was gradually filled by the Central Mountain Range (CMR), while the outer wind had flowed around the northern tip of the CMR and met the southwesterly monsoon to result in a strong confluent flow over the southern Taiwan Strait. When the confluent flow was blocked by the southern CMR, a secondary center (SC) without a warm core formed over southwestern Taiwan. During the northward movement of the SC along the west slope of the CMR, the warm air produced within the wake flow over the northwestern CMR was continuously advected into the SC, contributing to the generation of a warm core inside the SC. Consequently, a well-defined SC with a warm core, closed circulation and almost symmetric structure was produced over central western Taiwan, and then it coupled with Morakot’s mid-level center after crossing the CMR to reestablish a new and vertically stacked typhoon. Therefore, the SC inside Morakot was initially generated by a dynamic interaction among the TC’s cyclonic wind, southwesterly wind and orographic effects of the CMR, while the thermodynamic process associated with the downslope adiabatic warming effect documented by previous studies supported its development to be a well-defined SC. In summary, the evolution of the SC in this study is not in contradiction with previous studies, but just a complement, especially in the initial formation stage.

Evaluation of Warm‐Core Structure in Reanalysis and Satellite Data Sets Using HS3 Dropsonde Observations: A Case Study of Hurricane Edouard (2014)

AbstractUsing National Aeronautics and Space Administration Hurricane and Severe Storm Sentinel (HS3) dropsonde observations in the eye of Hurricane Edouard (2014), its warm‐core structure in reanalysis/analysis and satellite data sets are evaluated. All of the data sets generally underestimate the warm‐core strength of Edouard. Advanced Microwave Sounding Unit‐A shows the smallest error in the upper troposphere but the largest error in the lower troposphere. Its successor, Advanced Technology Microwave Sounder, significantly improves the representation of low‐level warming while maintaining its performance in the middle and upper troposphere. Two versions of the Global Forecast System analysis at different resolutions and Climate Forecast System Reanalysis version 2 show better performance than the other reanalyses. Modern Era Retrospective Analysis for Research and Application (MERRA)‐2, Japanese 55‐year Reanalysis, and European Centre for Medium‐Range Weather Forecasts Interim Reanalysis could represent the vertical warm‐core structure, but they markedly underestimate the warm‐core strength. MERRA and North American Regional Reanalysis (especially North American Regional Reanalysis) have the worst skills when reproducing warm cores. MERRA‐2 considerably outperforms its predecessor, MERRA. The warm‐core evolution of Edouard in six reanalysis/analysis data sets is diagnosed and compared. Different physical processes such as diabatic heating and vertical and horizontal advection may be responsible for warm‐core generation at different stages during its lifetime. The inaccurate representation of these physical processes in the reanalyses/analyses results in errors in the warm‐core strength and height. It is suggested that vortex relocation/bogusing, assimilation of tropical cyclone (TC) observation data, and model parameterizations are important to adequately represent warm cores in terms of strength and three‐dimensional structure. This study can shed some light on the selection of suitable data sets for estimating the real‐time warm‐core strength or for TC‐related climate studies. The need for better observations of TCs and model improvements is also highlighted.

The role of the Indian Summer Monsoon variability on Arabian Peninsula summer climate

This study investigates the influence of the Indian Summer Monsoon (ISM) on the atmospheric circulation over the Arabian Peninsula (AP) using the European Centre for Medium Range Weather Forecasts’ twentieth century reanalysis (ERA-20C) for the period 1901–2010. After describing the summer climate of the AP using various dynamic and thermodynamic parameters, we investigate the link between extreme ISMs and atmospheric circulation over the AP on inter-annual time scale. Analysis of composites of different parameters during extreme monsoon (strong and weak) years reveals that the ISM plays an important role in the summer circulation over the AP and adjoining regions. The major noticeable changes in modulating circulation during extreme monsoons are: (1) a strengthening of lower tropospheric northerly winds, westerly winds passing through the Tokar Gap, Shamal winds, and the upper tropospheric easterly jet stream during strong ISM; (2) a northward (southward) shift of the subtropical westerly jet stream during strong (weak) monsoon years; (3) the development of strong upper level ridge above the surface thermal low during strong ISM years, which result in a baroclinic structure over the AP and adjoining regions; (4) an increase in adiabatic warming, and hence aridity, over the AP during strong monsoon years, caused by intense subsidence of the middle to upper troposphere due to zonal overturning circulation; and (5) convective instability during strong monsoon years caused by an intensification of the upward motion over the southern AP. Furthermore, during strong monsoons, the availability of excess moisture leads to atmospheric instability, which in turn triggers the formation of clouds that lead to more rainfall over the southwestern AP. Finally, the westward propagation of a Gill-type Rossby waves induced by the ISM play an important role in the variations of the AP summer climate by enhancing the warm core structure over the AP and through their interaction with the midlatitude westerlies during strong monsoons.

Impact of 2015–2016 ENSO on the winter bloom and associated phytoplankton community shift in the northeastern Arabian Sea

Abstract The present study explores the impact of 2015–16 El-Nino Southern Oscillation (ENSO) on the winter phytoplankton bloom in the northeastern Arabian Sea (NEAS). In situ Chlorophyll a (Chla) from the bio-Argo float (2013–2016) and phytoplankton pigment data collected onboard R.V. Sindhu Sankalp (February 2013 and 2016) have been used to decipher the 2015–2016 ENSO modulated variability. During a normal winter, convective mixing deepens the mixed layer (ML) and increases the availability of nutrients in the euphotic zone. During the 2015–16 ENSO, the intensity of the convection (evaporation) was weaker than normal and this, in turn, weakened the convective mixing in the following winter. However, during the 2015–16 ENSO event, a warm core eddy deepened the ML further by pushing the nutracline downwards, thereby reducing the availability of nutrients. Changes in the physical condition during the 2015–16 ENSO altered the nutrient availability which in turn affected the Chla production and associated phytoplankton community structure. The Bio-Argo observations show 50% reduction in Chla during February 2016 compared to a normal year, which we attribute to the ENSO event of 2015–2016. Additionally, pigment data shows a reduction in the abundance of microplankton (diatoms and dinoflagellates) and an increase in abundance of picoplankton in the NEAS. Our results clearly indicate that 2015 ENSO played a dominant role in altering the phytoplankton community composition and in turn the productivity of the Arabian Sea.

Atmospheric Signature of the Agulhas Current

AbstractWestern boundary currents play an important role in the climate system by transporting heat poleward and releasing it to the atmosphere. While their influence on extratropical storms and oceanic rainfall is becoming appreciated, their coastal influence is less known. Using satellite and climate reanalysis data sets and a regional atmospheric model, we show that the Agulhas Current is a driver of the observed band of rainfall along the southeastern African coast and above the Agulhas Current. The Agulhas current's warm core is associated with sharp gradients in sea surface temperature and sea level pressure, a convergence of low‐level winds, and a co‐located band of precipitation. Correlations among wind convergence, sea level pressure, and sea surface temperature indicate that these features show high degree of similarity to those in the Gulf Stream region. Model experiments further indicate that the Agulhas Current mostly impacts convective rainfall.

Estimation of Hurricane Maximum Wind Speed Using Temperature Anomaly Derived From Advanced Technology Microwave Sounder

Estimation of hurricane intensity is typically relying on aircraft observations and remote sensing data. This letter develops a satellite algorithm to estimate the hurricane surface maximum wind speed. The Advanced Technology Microwave Sounder (ATMS) observations are used to retrieve the atmospheric temperatures. The temperature anomaly field at each pressure level within the hurricane is derived with respect to its environment. It is shown that for a tropical cyclone entering the hurricane stage, a warm core anomaly occurs in upper troposphere. From the GPS dropsonde data of three hurricane cases (Harvey, Irma, and Maria) in 2017, a linear relationship is derived between the maximum warm core value and maximum surface wind speed. Thus, ATMS-derived warm core anomaly is a very good indicator of storm intensity and can be used in the operational weather forecasts.

Medicanes as subtropical cyclones: the December 2005 case from the perspective of surface pressure tendency diagnostics and atmospheric water budget

Mediterranean cyclones with tropical‐like characteristics such as spiral cloud coverage and a central cloud‐free “eye” are referred to as medicanes. These systems have been analyzed due to their relation with high‐impact weather. In previous studies, the identification of medicanes has been typically performed subjectively, using satellite pictures, but also objectively through three‐dimensional diagnosis of a warm core and an axisymmetric structure. Despite the presence of these characteristics, it is still unclear if medicanes show dynamical similarities with tropical cyclones.We analyse the (thermo‐)dynamics of a recognized medicane that occurred in December 2005 by applying different diagnostics to a high‐resolution simulation. These diagnostics are focused on the intensification, dynamical structure and water budget of this representative case, aiming to highlight extratropical and tropical cyclone characteristics.Three stages in the medicane life cycle are identified. In stage I, a potential vorticity (PV) streamer reaches the Mediterranean, triggering deep convection and deepening the medicane's central surface pressure due to diabatic heating. When the lowest central pressure is reached (stage II), the medicane presents a warm core and an axisymmetric structure. However, convection is rather weak and the PV streamer evolves into a cut‐off system which contributes to the deepening of the medicane's surface pressure. Finally, stage III corresponds to the decay phase where the medicane tends to weaken and lose its axisymmetric structure.Our results highlight the detrimental role of deep convection prior to the mature stage of the medicane, as well as the possibility of positive or negative feedback of upper‐tropospheric dynamics on the central surface pressure. In addition, we show that the medicane warm core might be achieved due to front seclusion, while the “eye” formation is associated with dry air intrusions. Our analysis suggests that medicanes are hybrid systems combining characteristics of both tropical and extratropical cyclones and thus they plausibly correspond to subtropical cyclones.

Phytoplankton response to the contrasting physical regimes in the eastern Arabian Sea during north east monsoon

Abstract Phytoplankton abundance and composition in two contrasting physical regimes - convective mixing in the northeastern Arabian Sea (NEAS) and Arabian Sea mini warm pool (ASMWP) in the southeastern Arabian Sea (SEAS) - were investigated during the northeast monsoon (NEM) of 2015 and 2017. Observations in 2015 were carried out late during the season, and only one station in the north (at 21°N latitude) fell within the zone of convective mixing where microplankton was dominated by diatoms. In 2017, convective mixing occurred even at 16°N latitude, but the microplankton contribution was low, presumably due to low Si/N ratios. Within the convective mixing regime of the NEAS, chlorophyll (Chl) a concentrations were higher in 2015 (maximum 1080 ng L−1; average 493 ng L−1) than in 2017 (maximum 673 ng L−1; average 263 ng L−1). In contrast, picophytoplankton were dominant in the ASMWP of the SEAS with peak abundance associated with the subsurface chlorophyll maximum. A warm core eddy was present in 2015 in the SEAS where four times higher Prochlorococcus counts were found within the core of the eddy than at its periphery. This study provides the first description of the phytoplankton community in the ASMWP. Our results clearly demonstrate phytoplankton response to the contrasting physical conditions, highlighting the role of bio-physical coupling in the productivity of the Arabian Sea.

Predictability of the 2012 Great Arctic Cyclone on medium-range timescales

Arctic Cyclones (ACs) can have a significant impact on the Arctic region. Therefore, the accurate prediction of ACs is important in anticipating their associated environmental and societal costs. This study investigates the predictability of the 2012 Great Arctic Cyclone (AC12) that exhibited a minimum central pressure of 964 hPa on 6 August 2012, using five medium-range ensemble forecasts. We show that the development and position of AC12 were better predicted in forecasts initialized on and after 4 August 2012. In addition, the position of AC12 was more predictable than its development. A comparison of ensemble members, classified by the error in predictability of the development and position of AC12, revealed that an accurate prediction of upper-level fields, particularly temperature, was important for the prediction of this event. The predicted position of AC12 was influenced mainly by the prediction of the polar vortex, whereas the predicted development of AC12 was dependent primarily on the prediction of the merging of upper-level warm cores. Consequently, an accurate prediction of the polar vortex position and the development of the warm core through merging resulted in better prediction of AC12.

Mesoscale Eddy Activity and Transport in the Atlantic Water Inflow Region North of Svalbard

AbstractMesoscale eddies are known to transport heat and biogeochemical properties from Arctic Ocean boundary currents to basin interiors. Previous hydrographic surveys and model results suggest that eddy formation may be common in the Atlantic Water (AW) inflow area north of Svalbard, but no quantitative eddy survey has yet been done for the region. Here vorticity and water property signatures are used to identify and track AW eddies in an eddy‐resolving sea ice‐ocean model. The boundary current sheds AW eddies along most of the length of the continental slope considered, from the western Yermak Plateau to 40°E, though eddies forming east of 20°E are likely more important for slope‐to‐basin transport. Eddy formation seasonality reflects seasonal stability properties of the boundary current in the eastern portion of the study domain, but on and immediately east of the Yermak Plateau enhanced eddy formation during summer merits further investigation. AW eddies tend to be anticyclonic, have radii close to the local deformation radius, and be centered in the halocline. They transport roughly 0.16 Sv of AW and, due to their warm cores, 1.0 TW away from the boundary current. These findings suggest eddies may be important for halocline ventilation in the Eurasian Basin, as has been shown for Pacific Water eddies in the Canadian Basin.

The Inner-Core Temperature Structure of Hurricane Edouard (2014): Observations and Ensemble Variability

Abstract The inner-core thermodynamic structure of Hurricane Edouard (2014) is explored, primarily through an examination of both high-altitude dropsondes deployed during NASA’s Hurricane and Severe Storm Sentinel (HS3) and a 60-member convection-permitting ensemble initialized with an ensemble Kalman filter. The 7-day forecasts are initialized coincident with Edouard’s tropical depression designation and include Edouard’s significant intensification to a major hurricane. Ten-member ensemble groups are created based on timing of near–rapid intensification (RI) onset, and the associated composite inner-core temperature structures are analyzed. It is found that at Edouard’s peak intensity, in both the observations and the simulations, the maximum inner-core perturbation temperature (~10–12 K) occurs in the midlevels (~4–8 km). In addition, in all composite groups that significantly intensify, the evolution of the area-averaged inner-core perturbation temperatures indicate that weak to moderate warming (at most 4 K) begins to occur in the low to midlevels (~2–6 km) ~24–48 h prior to RI, and this warming significantly strengthens and deepens (up to ~8 km) ~24 h after RI has begun. Despite broad similarities in the evolution of Edouard’s warm core in these composites, variability in the height and strength of the maximum perturbation temperature and in the overall development of the inner-core temperature structure are present among the members of the composite groups (despite similar intensity time series). This result and concomitant correlation analyses suggest that the strength and height of the maximum perturbation temperature is not a significant causal factor for RI onset in this ensemble. Fluctuations in inner-core temperature structure occur either in tandem with or after significant intensity changes.

Chapter 18 - Efferent thermoregulatory pathways regulating cutaneous blood flow and sweating

Cutaneous vasoconstrictor nerves regulate heat retention, and are activated by falls in skin or core temperature. The efferent pathways controlling this process originate within the preoptic area. A descending GABAergic pathway, activated by warm skin or core, indirectly inhibits sympathetic premotor neurons in the medullary raphé. Those premotor neurons drive cutaneous vasoconstriction via excitatory glutamatergic and serotonergic connections to spinal preganglionic neurons. Cold skin and/or cold core temperatures activate a direct preoptic-to-raphé excitatory pathway. The balance of inhibitory and excitatory influences reaching the medullary raphé determines cutaneous blood flow. During fever, prostaglandin E2 inhibits preoptic GABAergic neurons, resulting in disinhibition of the excitatory preoptic-to-raphé pathway, and hence, cutaneous vasoconstriction. A weaker, parallel source of descending excitatory drive reaches cutaneous preganglionic neurons from the rostral ventrolateral medulla. Sweating follows local heating of the preoptic area in cats and monkeys, and heated humans show sweating-related activation of this same region in functional magnetic resonance imaging (fMRI) studies. A descending pathway that drives sweating has been traced in cats from the hypothalamus to putative premotor neurons in the parafacial region at the pontomedullary junction. The homologous parafacial region in humans also shows sweating-related activation in fMRI studies. The central pathways that drive active vasodilatation in human nonacral skin remain unknown.

Dramatic Inner-Core Tropopause Variability during the Rapid Intensification of Hurricane Patricia (2015)

Abstract Dropsondes with horizontal spacing as small as 4 km were released from the stratosphere in rapidly intensifying Hurricane Patricia (2015) during the Office of Naval Research Tropical Cyclone Intensity experiment. These observations provide cross sections of unprecedented resolution through the inner core of a hurricane. On 21 October, Patricia exhibited a strong tropopause inversion layer (TIL) across its entire circulation, with a maximum magnitude of 5.1 K (100 m)−1. This inversion weakened between 21 and 22 October as potential temperature θ increased by up to 16 K just below the tropopause and decreased by up to 14 K in the lower stratosphere. Between 22 and 23 October, the TIL over the eye weakened further, allowing the tropopause to rise by 1 km. Meanwhile over Patricia’s secondary eyewall, the TIL restrengthened and bulged upward by about 700 m into what was previously the lower stratosphere. These observations support many aspects of recent modeling studies, including eyewall penetration into the stratosphere during rapid intensification (RI), the existence of a narrow inflow layer near the tropopause, and the role of subsidence from the stratosphere in developing an upper-level warm core. Three mechanisms of inner-core tropopause variability are hypothesized: destabilization of the TIL through turbulent mixing, weakening of the TIL over the eye through upper-tropospheric subsidence warming, and increasing tropopause height forced by overshooting updrafts in the eyewall. None of these processes are seen as the direct cause of RI, but rather part of the RI process that includes strong increases in boundary layer moist entropy.

INFLUENCE OF MODEL HORIZONTAL RESOLUTION ON THE INTENSITY AND STRUCTURE OF RAMMASUN

We use the WRF (V3.4) model as the experimental model and select three horizontal resolutions of 15, 9, and 3km to research the influence of the model’s horizontal resolution on the intensity and structure of the super-strong typhoon Rammasun (1409) in 2014. The results indicate that the horizontal resolution has a very large impact on the intensity and structure of Rammasun. The Rammasun intensity increases as the horizontal resolution increases. When the horizontal resolution increases from 9km to 3km, the enhancement of intensity is more obvious, but the strongest intensity simulated by 3km horizontal resolution is still weaker than the observed strongest intensity. Along with the increase of horizontal resolution, the horizontal scale of the Rammasun vortex decreases, and the vortex gradually contracts toward its center. The vortex structure changes from loose to compact and deep. The maximum wind radius, thickness of the eye wall, and outward inclination of the eye wall with height decrease, and the wind in the inner core region, updraft motion along the eye wall, and strength of the warm core become stronger. Additionally, the pressure gradient and temperature gradient of the eye wall region increase, and the vortex intensity becomes stronger. When the horizontal resolution increases from 9km to 3km, the change in the Rammasun structure is much larger than the change when the horizontal resolution increases from 15km to 9km. When the model does not employ the method of convection parameterization, the Rammasun intensity simulated with 3km horizontal resolution is slightly weaker than the intensity simulated with 3km horizontal resolution when the Kain–CFritsch (KF) convection parameterization scheme is adopted, while the intensity simulated with 9km horizontal resolution is much weaker than the intensity simulated with 9km horizontal resolution when the KF scheme is adopted. The influence of the horizontal resolution on the intensity and structure of Rammasun is larger than the influence when the KF scheme is adopted.

Eddy-induced cross-shelf export of high Chl-a coastal waters in the SE Bay of Biscay

Different remote sensing data were combined to characterise a winter anticyclonic eddy in the southeastern Bay of Biscay and to infer its effects on cross-shelf exchanges, in a period when typical along shelf-slope currents depict a cyclonic pattern. While the joint analysis of available satellite data (infrared, visible and altimetry) permitted the characterisation and tracking of the anticyclone properties and path, data from a coastal high-frequency radar system enabled a quantitative analysis of the surface cross-shelf transports associated with this anticyclone. The warm core anticyclone had a diameter of around 50km, maximum azimuthal velocities near 50cms−1 and a relative vorticity of up to −0.45f. The eddy generation occurred after the relaxation of a cyclonic wind-driven current regime over the shelf-slope; then, the eddy remained stationary for several weeks until it started to drift northwards along the shelf break. The surface signature of this eddy was observed by means of high-frequency radar data for 20 consecutive days, providing a unique opportunity to characterise and quantify, from a Lagrangian perspective, the associated transport and its effect on the Chl-a surface distribution. We observed the presence of mesoscale structures with similar characteristics in the area during different winters within the period 2011–2014. Our results suggest that the eddy-induced recurrent cross-shelf export is an effective mechanism for the expansion of coastal productive waters into the adjacent oligotrophic ocean basin.

Isentropic Analysis on the Intensification of Hurricane Edouard (2014)

Abstract An isentropic analysis technique is adopted in this study to investigate the intensification of Hurricane Edouard (2014) predicted by an experimental real-time convection-permitting hurricane analysis and forecast system. This technique separates the vertical mass transport in terms of equivalent potential temperature θe for the rising air parcels at high entropy from the subsiding air at low entropy. It is found that as Edouard intensifies the vertical circulation becomes wider via the expansion of upward (downward) mass flux to higher (lower) θe. In the early developing stages, the asymmetric convection dominates the vertical circulation and leads to a remarkable upward mass flux maximum center in the upper troposphere. When Edouard becomes intense, the axisymmetric convection becomes important to the upper-level vertical mass transport while the asymmetric convection still dominates the low-level vertical mass transport. Development of the warm core in the eye leads to double maxima along the θe axis for both the isentropic-mean relative humidity and tangential velocity. The isentropic-mean properties such as the mid- to upper-level relative humidity, vertical velocity, and radial outflow decrease considerably while the mid- to upper-level vorticity enhances on the high-θe side before the onset of rapid intensification. The isentropic analysis also reveals that as Edouard intensifies the eye characterized by warm and dry core first forms in the low to middle troposphere and then gradually expands upward. The abovementioned results indicate that the isentropic framework may have the advantages of binning common variables with θe that could reflect the changes of the tropical cyclone structure in the inner-core region without a prior specification of the location of the storm center.

Computing Ocean Surface Currents From GOCI Ocean Color Satellite Imagery

One of the significant challenges in physical oceanography is getting an adequate space/time description of the ocean surface currents. One possible solution is the maximum cross-correlation (MCC) method that we apply to hourly ocean color images from the Geostationary Ocean Color Imager (GOCI) over five years. Since GOCI provided a large number of image pairs, we introduce a new MCC search strategy to improve the computational efficiency of the MCC method saving 95% of the processing time. We also use an MCC current merging method to increase the total spatial coverage of the currents, proving a 25% increase. Five-year mean and seasonal time-average flows are computed to capture the major currents in the area of interest. The mean flows investigate the Kuroshio path, support the triple-branch pattern of the Tsushima Warm Current (TC), and reveal the origin of the TC. The evolution of a warm core ring shed by the Kuroshio near the northeast coast of Honshu, Japan, is clearly depicted by a sequence of three monthly MCC composites. We capture the evolution of the Kuroshio meander over seasonal, monthly, and weekly time scales. Three successive weekly MCC composite maps demonstrate how a large anticyclonic eddy, to the south of the Kuroshio meander, influences its formation and evolution in time and space. The unique ability to view short space/time scale changes in these strong current systems is a major benefit of the application of the MCC method to the high spatial resolution and rapid refresh GOCI data.

Structure analyses of the explosive extratropical cyclone: A case study over the Northwestern Pacific in March 2007

The synoptic situation and mesoscale structure of an explosive extratropical cyclone over the Northwestern Pacific in March 2007 are investigated through weather station observations and data reanalysis. The cyclone is located beneath the poleward side of the exit of a 200 hPa jet, which is a strong divergent region aloft. At mid-level, the cyclone lies on the downstream side of a well-developed trough, where a strong ascending motion frequently occurs. Cross-section analyses with weather station data show that the cyclone has a warm and moist core. A ‘nose’ of the cold front, which is characterized by a low-level protruding structure in the equivalent potential temperature field, forms when the cyclone moves offshore. This ‘nose’ structure is hypothesized to have been caused by the heating effect of the Kuroshio Current. Two low-level jet streams are also identified on the western and eastern sides of the cold front. The western jet conveys cold and dry air at 800–900 hPa. The wind in the northern part is northeasterly, and the wind in the southern part is northwesterly. By contrast, the eastern jet carries warm and moist air into the cyclone system, ascending northward from 900 hPa to 600–700 hPa. The southern part is dominated by the southerly wind, and the wind in the northern part is southwesterly. The eastern and western jets significantly increase the air temperature and moisture contrast in the vicinity of the cold front. This increase could play an important role in improving the rapid cyclogenesis process.

Numerical study on the spray and thermal characteristics of R404A flashing spray using OpenFOAM

Flashing spray is of great importance in a variety of engineering fields, which occurs as the pressurized liquid is released into a low-pressure environment below its saturation pressure. The initial stage close to the nozzle exit is furthest from thermodynamic equilibrium and the information of droplets is difficult to obtain by experimental ways due to the high density of droplets. This paper conducts a systematically numerical study on the spray and thermal characteristics of R404A flashing spray with high volatility and low saturation temperature, based on the open source tool OpenFOAM-2.4.0. Key physics involved in the complex process, including droplet flash boiling, evaporation, droplet transport, breakup, heat transfer and interaction between droplet and gas phases are taken into consideration, through Reynolds Averaged Navier Stokes simulations (RANS) with Eulerian Lagrangian framework for the continuous phase and discrete phase respectively. The spray morphology and droplet diameter coincide reasonably with those in the experiment. It presents explosive atomization with large cone angle and quick expansion towards the radial direction near the nozzle exit. Droplet diameter and temperature undergoes a rapid reduction, while velocity accelerates in this region. A warm core with high droplet temperature exists in the spray central region, and droplet velocity is also larger than that at spray periphery. The ambient pressure significantly affects spray profile, penetration distance and velocity. The whole spray profile transforms to the expansion shape as the pressure increases above 0.5 MPa whereby it presents contractive one in low ambient pressure. Higher ambient pressure leads to lower droplet velocity but with longer penetration distance. These numerical results provide further insight into the mechanism of the flashing spray with volatile cryogens.

A Methodology to Adjust ATMS Observations for Limb Effect and Its Applications

AbstractThe Advanced Technology Microwave Sounders (ATMS), carried on the Suomi National Polar‐orbiting Partnership (SNPP) satellite, was launched on 28 October 2011. The ATMS is a follow‐on instrument to advanced microwave sounding unit (AMSU), currently flying on National Oceanic and Atmospheric Administration (NOAA) satellites. The primary new ATMS features are a reduced hardware package and improved gap coverage. One thing in common about cross‐track sounders is a scan perpendicular to the motion of the satellite, allowing a broad swath of measurements to be taken. But an undesirable feature is that the measurements vary with scan angle because of changes in the optical pathlength through the Earth's atmosphere between the Earth and the satellite. One approach to this problem is to limb adjust the measurements to a fixed view angle. The limb correction algorithm applied to ATMS is based on the heritage methodology originally applied to MSU and later to AMSU. The limb correction method is applied to each of the 96 ATMS field of view (FOV) per scan line, adjusting the off‐nadir FOV to the nadir view with fitting error generally within the instrumental noise. The limb‐adjusted brightness temperature were used in the original, legacy TIROS Operational Vertical Sounder, and Advanced TIROS Operational Vertical Sounder NOAA sounding product algorithms and more recently to derive the total precipitation water (TPW) retrieval over ocean, with a bias of 0.046 mm and a standard deviation of 3.43 mm, when compared with European Centre for Medium‐Range Weather Forecasts TPW data. The limb‐corrected brightness temperature can be used to detect the atmospheric weather features, such as the warm cores for tropical cyclones, and the imagery presents snapshots for quick weather signal diagnosis.