Thermal Ridge Research Articles

A CloudSat–CALIPSO view of cloud and precipitation in the occluded quadrants of extratropical cyclones

AbstractUsing 10 years of satellite‐borne radar and lidar observations coupled with a novel method for automated occlusion identification, composite transects of cloud and precipitation across occluded thermal ridges of extratropical cyclones are, for the first time, constructed. These composites confirm that occluded sectors are characterized by the most extensive cloud cover and heaviest precipitation in any of the frontal regions of the cyclone. Hydrometeor frequency in occluded sectors is sensitive to the cyclone's ascent strength but not to the mean precipitable water in the cyclone's environment. This result is in contrast to the strong relationships between hydrometeor frequency and both precipitable water and ascent strength as previously reported in warm frontal regions. In both hemispheres, cloud and precipitation increase with the maximum value of the equivalent potential temperature at 700 hPa within the occluded thermal ridge, until a threshold is reached. For very large values of maximum equivalent potential temperature, hydrometeors become less frequent while precipitation rates increase. It is suggested that this conjunction is a by‐product of an increase in the frequency of convection in those instances. While in the Northern Hemisphere occluded sectors exhibit deeper and wider cloud structures than their Southern Hemisphere counterparts, their hydrometeor occurrence frequencies are less. The differences in maximum equivalent potential temperature of the thermal ridges in both hemispheres does not appear to explain the more frequent hydrometeors in the Southern Hemisphere. These relationships offer new perspectives on the interplay between cloud processes and cyclone evolution, as well as new observational constraints for process evaluation of Earth system models.

GEFSv12 High- and Low-Skill Day-10 Tornado Forecasts

Abstract On average, modern numerical weather prediction forecasts for daily tornado frequency exhibit no skill beyond day 10. However, in this extended-range lead window, there are particular model cycles that have exceptionally high forecast skill for tornadoes because of their ability to correctly simulate the future synoptic pattern. Here, model initial conditions that produced a more skillful forecast for tornadoes over the United States were exploited while also highlighting potential causes for low-skill cycles within the Global Ensemble Forecasting System, version 12 (GEFSv12). There were 88 high-skill and 91 low-skill forecasts in which the verifying day-10 synoptic pattern for tornado conditions revealed a western U.S. thermal trough and an eastern U.S. thermal ridge, a favorable configuration for tornadic storm occurrence. Initial conditions for high skill forecasts tended to exhibit warmer sea surface temperatures throughout the tropical Pacific Ocean and Gulf of Mexico, an active Madden–Julian oscillation, and significant modulation of Earth-relative atmospheric angular momentum. Low-skill forecasts were often initialized during La Niña and negative Pacific decadal oscillation conditions. Significant atmospheric blocking over eastern Russia—in which the GEFSv12 overforecast the duration and characteristics of the downstream flow—was a common physical process associated with low-skill forecasts. This work helps to increase our understanding of the common causes of high- or low-skill extended-range tornado forecasts and could serve as a helpful tool for operational forecasters. Significance Statement This research provides a framework for the anticipation of a more (or less) skillful 10-day tornado forecast in an operational numerical weather prediction system. High-skill forecasts were associated with substantial tropical convection and warm sea surface temperature throughout the Pacific Ocean and Gulf of Mexico, whereas the underlying cause of low-skill forecasts were typically associated with a blocking anticyclone over eastern Russia. These findings are important because they permit increased or decreased confidence in a long-range forecast of tornado occurrence based on a dynamical prediction system.

A 10-Year Climatology of Midlevel Mesoscale Vortices in China

Abstract The mesoscale vortex (MV) is an important rain-producing system. In this study, the reanalysis data and satellite precipitation products are used to classify MVs into three categories: mesoscale convective vortex (MCV), mesoscale stratiform vortex (MSV), and mesoscale dry vortex (MDV). Then, these three categories of midlevel MVs in China from 2007 to 2016 are investigated. A total of 21 053 MVs are obtained. Most MVs form in the northwest of parent convection, and 45% of MVs generate secondary convection. The Tibetan Plateau is the main MV source region. Steered by the westerlies, MVs mainly move eastward. MCV is active in summer, MDV in winter, and MSV in spring and autumn. MCV diurnal variations are closely related to local topography, and MDVs mainly form around midnight. Composite analyses show that MCVs form near the high-value center of convective available potential energy at the development stage of parent convection. The composite MCV forms near the low pressure trough and the thermal ridge at 500 hPa, and a low-level jet exists to the south of the MCV center. At the initiation and maturity stages of MCV, strong convergence and divergence respectively exist at low levels and 400 hPa. The vortex circulation mainly locates near 500 hPa. Above the vortex is a warm core associated with the latent heat release, and below is a cold anomaly related to the cold pool. In the downshear region, there is strong low-level convergence and ascending motion, higher humidity, and greater latent heat release, which favor the formation of secondary convection.

Spring Saharan Cyclones over Saudi Arabia: Preliminary Study of the Impacts on Climate

Saharan cyclones are an important feature of the seasonal spring climate of Saudi Arabia. It was found in an objectively tracking scheme that of 688 Sahara tracks, 53 of them reached Saudi Arabia during the period 1958–2018. Two distinct regions of cyclogenesis are identified: the first is Area A (Algeria, in particular, south of the Atlas Mountains) and the second is Area B (Libya and Egypt). The 53 tracks that pass or abate over Saudi Arabia are ultimately categorized into five key routes. The Saharan cyclones intensify near or over the Mediterranean, developing a relatively strong pressure gradient during their lifetimes which weakens as they pass over the Arabian Peninsula. Cyclones develop usually under a tropical trough over North Africa, which is accompanied by a thermal ridge with a very strong temperature gradient. This pattern is also found at upper levels of the atmosphere. During the cyclones’ movement over Saudi Arabia, they extend upwards, and at higher levels, pressure levels of up to 500 hPa can be observed. The study also revealed that more tracks occur during the negative phase of the North Atlantic Oscillation, reflecting the role played by subtropical high pressure in controlling the movement of the Saharan cyclones. Cyclones originating over Area B are more frequent in recent decades than in the earlier decades. Cyclones originating in Area B are found to have more influence on the climate of Saudi Arabia than those occurring in Area A. Saharan cyclones that reach Saudi Arabia substantially increase the frequency of dust storms, raise the minimum temperature and maximum wind speed, and cause a reduction in minimum visibility. These results suggest how they may contribute to weather forecasts for the region.

Composite Analysis of Polar Mesocyclones over the Western Part of the Sea of Japan

Polar mesocyclones occur frequently over the Sea of Japan during winter in association with cold air outbreaks from the Eurasian continent. In this study, the general characteristics of polar mesocyclones over the western part of the Sea of Japan are examined using composite analysis. The synoptic-scale environment associated with these mesocyclones is characterized by a negative sea level pressure anomaly to the east that causes a cold air outbreak at low levels. There is also a geopotential height trough moving eastward at upper levels. In the cold air outbreak, a convergence zone known as the Japan Sea polar airmass convergence zone (JPCZ), collocated with a thermal ridge, develops on the lee side of the mountains at the root of the Korean Peninsula. These polar mesocyclones are generated when the upper-level trough approaches the JPCZ from the west. However, the behavior of the JPCZ and the movement of the polar mesocyclones differ depending on the location of the upper-level trough. A piecewise potential vorticity inversion analysis revealed that the circulation associated with the upper-level trough modifies the low-level winds, which affects the direction of extension of the JPCZ as well as the genesis location and the movement of the polar mesocyclones.

The Origin of Western European Warm-Season Prefrontal Convergence Lines

Abstract The authors investigate the origin of prefrontal, warm-season convergence lines over western Europe using the Weather Research and Forecasting Model. These lines form east of the cold front in the warm sector of an extratropical cyclone, and they are frequently the focus for convective development. It is shown that these lines are related to a low-level thermal ridge that accompanies the base of an elevated mixed layer (EML) plume generated over the Iberian Peninsula and northern Africa. Using Q-vector diagnostics, including the components that describe scalar and rotational quasigeostrophic frontogenesis, it is shown that the convergence line is associated with the rearrangement of the isentropes especially at the western periphery of the EML plume. The ascending branch of the resulting ageostrophic circulation coincides with the surface velocity convergence. The modeling results are supported by a 3-yr composite analysis of cold fronts with and without preceding convergence lines using NCEP–NCAR Reanalysis-1 data.

Convective Storm Structures and Ambient Conditions Associated with Severe Weather over the Northeast United States

Abstract This study documents the convective storm structures and ambient conditions associated with severe storms (wind, hail, and tornado) over the northeastern United States for two warm seasons (May–August), including 2007 and a warm season comprising randomly selected days from 2002 to 2006. The storms were classified into three main convective organizational structures (cellular, linear, and nonlinear) as well as several subcategories. The same procedure was applied to the highly populated coastal zone of the northeastern United States, including New Jersey, Connecticut, Rhode Island, and New York. The coastal analysis included six warm seasons from 2002 to 2007. Over the Northeast, severe wind events are evenly distributed among the cellular, linear, and nonlinear structures. Cellular structures are the primary hail producers, while tornadoes develop mainly from cellular and linear structures. Over the coastal zone, primarily cellular and linear systems produce severe wind and hail, while tornadoes are equally likely from all three convective structures. Composites were generated for severe weather days over the coastal region for the three main convective structures. On average, severe cellular events develop during moderate instability [most unstable CAPE (MUCAPE) ~1200 J kg−1], with low-level warm-air advection and frontogenesis at the leading edge of a thermal ridge collocated with an Appalachian lee trough. Severe linear events develop in a similar mean environment as the cellular events, except that most linear events occur with a surface trough upstream over the Ohio River valley and half of the linear events develop just ahead of progressive midlevel troughs. Nonlinear severe events develop with relatively weak mean convective instability (MUCAPE ~460 J kg−1), but they are supported by midlevel quasigeostrophic (QG) forcing for ascent.

A Synoptic Climatology and Composite Analysis of the Alberta Clipper

Abstract Surface and upper-air analyses from the ECMWF Tropical Ocean Global Atmosphere (TOGA) dataset are used to construct a climatology of 177 Alberta clippers over 15 boreal cold seasons (October–March) from 1986/87 to 2000/01. The Alberta clipper (hereafter simply clipper) occurs most frequently during December and January and substantially less frequently during October and March. These cyclones generally move southeastward from the lee of the Canadian Rockies toward or just north of Lake Superior before progressing eastward into southeastern Canada or the northeastern United States, with less than 10% of the cases in the climatology tracking south of the Great Lakes. Characteristics of the structure and evolution of clippers during a 36-h period leading up to departure of the cyclone from the lee of the Canadian Rockies and a 60-h period after departure as the cyclone traverses central and eastern North America are examined through composite analyses. Over the course of the predeparture period, a cyclone over the Gulf of Alaska approaches the west coast of North America, and through its interaction with the mountainous terrain of western North America spawns a surface lee trough, characterized by a thermal ridge at 850 hPa, to the east of the Canadian Rockies. This thermal ridge dampens considerably as the composite clipper moves into central North America away from the immediate lee of the Canadian Rockies. The composite clipper system evolves from a lee cyclone with its nonclassical thermal structure to a more classically structured midlatitude cyclone as it moves through central and eastern North America largely as a result of rotation of the low-level thermal gradient and the increasing westward tilt with height of the composite clipper over the last 36 h of the postdeparture period. The thermal gradient rotation is dynamically linked to convergence of the along-isentrope component of the Q vector and thus to the ascent that sustains the clipper and creates some of its characteristic sensible weather elements. Such dynamical forcing is a direct consequence of the persistent westward displacement of the 500-hPa vorticity maximum with respect to the composite clipper sea level pressure minimum that characterizes the postdeparture period.

The Effect of Latent Heat Release on the Evolution of a Warm Occluded Thermal Structure

The effect of latent heat release on the development of the occluded thermal structure in a major winter storm is examined through comparison of full physics (FP) and no-latent-heat-release (NLHR) simulations of the event performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Though both simulations possess a well-developed occluded thermal ridge near the surface, the 3D structure of their respective occluded quadrants is quite different. In particular, the FP simulation depicts the canonical, troposphere-deep warm occluded thermal structure, whereas the NLHR simulation produces only a shallow, poorly developed one. Consistent with these differences in tropospheric thermal structure, the FP cyclone displays a robust treble clef potential vorticity (PV) distribution in the upper troposphere in its postmature phase, while a considerably less robust version characterizes the NLHR simulation. The PV minimum of the treble clef overlies a poleward sloping column of warm, weakly stratified air that extends through the depth of the troposphere and is a signature of the trowal, the essential structural feature of warm occluded cyclones. Consequently, examination of the role played by latent heat release in production of the occluded thermal structure in this case is made through consideration of its influence on the evolution of the upper-tropospheric PV morphology. It is found that direct dilution of upper-tropospheric PV by midtropospheric latent heat release initiates formation of a local, upper-tropospheric PV minimum, or low PV tongue, to the northwest of the surface cyclone center. The production of this PV minimum initiates a cutting off of the upper-tropospheric PV anomaly associated with the surface development. The upper-tropospheric circulation associated with this cutoff anomaly, in turn, forces the advection of low (<1 PVU) values of PV into the developing PV trough. This combination of kinematic and diabatic processes acts to produce both the tropopause PV treble clef as well as the underlying warm occluded thermal structure in the FP simulation. In contrast, though an adiabatic kinematic tendency for production of a treble clef PV morphology operates in the NLHR simulation, the resulting PV and thermal structures are weaker and slower to evolve than those produced in the FP simulation. Thus, it is suggested that latent heat release plays an indispensable role in the production of the characteristic occluded thermal structures observed in nature.

A Potential Vorticity and Wave Activity Diagnosis of Optimal Perturbation Evolution

A diagnosis of singular vector (SV) evolution (computed for the L2 streamfunction norm) in the Eady model using potential vorticity (PV) and Eliassen–Palm (E–P) flux diagnostics is performed. In addition, a partitioning of the vertical component of the E–P flux vector based on the results of the piecewise PV inversion is introduced to better elucidate the fundamental mechanisms for SV amplification. The initial PV structures of the Eady model SVs on both the long- and shortwave sides of the Eady model shortwave cutoff are characterized by initially upshear tilted interior PV anomalies. The results of the PV and the E–P flux diagnostics for optimal perturbations reveal a three-stage process for the SV evolution: 1) a superposition of interior PV anomalies (diagnosed by a positive vertical component of the E–P flux), 2) a subsequent intensification (characterized by maxima in the E–P flux near the boundaries) of the SV boundary potential temperature anomalies (BTAs) by winds attributed to interior PV, and 3) finally a transient or sustained mutual interaction between the BTAs (associated with a nearly nondivergent interior E–P flux). The PV inversion demonstrates that a significant fraction of the observed SV amplification may be attributed to the initially upshear tilted PV anomalies, and that the initial BTAs, while important in describing the initial SV structure, play a minimal role in the subsequent evolution. The results of the experiments in which the Eady model SV structures were altered by removing either the BTAs or the interior PV suggest that data assimilation schemes that more heavily weight targeted surface observations to targeted tropospheric wind and temperature observations may result in a forecast correction that is either excessive (for short optimization times) or insufficient for optimization times comparable to or larger than the time required for the PV to be rendered vertical by the shear flow. An analysis of SVs calculated about the time-varying basic state of an NWP model reveals that the SV PV initially consists of upshear tilted structures located beneath a depressed tropopause in a region of strong low-level thermal gradient (and concomitant vertical shear). The subsequent nonlinear evolution of the SV PV in the NWP model is congruous with that observed in the Eady model: The vertical superposition of SV PV, followed by amplification of a surface thermal ridge by winds attributed to the SV PV, characterize the SV amplification. These results suggest the relevance of the SV amplification mechanisms identified in the Eady model in more realistic flows, and indicate the potential utility of applying the aforementioned diagnostic techniques to understanding SV development in observed flows.

Using Piecewise Potential Vorticity Inversion to Diagnose Frontogenesis. Part I: A Partitioning of theQVector Applied to Diagnosing Surface Frontogenesis and Vertical Motion

The technique of piecewise potential vorticity (PV) inversion is used to identify the nondivergent wind fields attributed to upper-, middle-, and lower-tropospheric PV anomalies in addition to the irrotational wind with the goal of diagnosing the respective wind fields’ frontogenetic potentialities. Frontogenesis is diagnosed using a piecewise separation of the Q vector into parts associated with the partitioned wind field. Partitioned geostrophic Q vectors are used to diagnose the vertical motion attributed to the upper-, middle-, and lower-tropospheric PV anomalies. Insight gained from this new diagnostic technique is demonstrated by examining a particular case of extratropical marine cyclogenesis resulting from the interaction of an upper-tropospheric short-wave trough with a surface thermal wave. In the early stages of development, the largest contributor to surface frontogenesis was associated with winds attributed to the lower-tropospheric thermal wave. As the cyclone matured, the contributions of the upper-tropospheric PV and near-surface potential temperature anomalies to surface frontogenesis increased. Winds attributed to the upper-tropospheric PV were frontogenetical north of the thermal ridge axis and frontolytical south of the thermal ridge in the warm sector. The upper-tropospheric PV acted to amplify the thermal ridge while simultaneously narrowing the warm sector. The patterns of geostrophic Q vectors associated with the upper-tropospheric PV suggest that ascent should be favored in the narrowing surface thermal ridge. The contribution to surface frontogenesis due to lower- and middle-tropospheric PV, whose increase is imputed to latent heat release, was variable during the evolution of the cyclone—suggesting that the location of diabatically generated PV anomalies relative to frontal zones can have a significant impact on frontogenesis and associated frontal precipitation distribution. Throughout the evolution of the cyclone, the irrotational wind was frontogenetical along the warm and cold fronts with the magnitude of the irrotational frontogenesis increasing as the surface cyclone amplified. For the case considered, partitioned geostrophic Q vector “forcing” for vertical motion revealed approximately equal contributions from the upper-tropospheric PV anomalies and the near-surface thermal perturbations. Characteristic patterns of Q vectors and Q-vector divergence are identified and presented for cases including an upper trough interacting with a surface baroclinic zone and a propagating surface edge wave.

The Separate Roles of Geostrophic Vorticity and Deformation in the Midlatitude Occlusion Process

Abstract Separate vector expressions for the rate of change of direction of the potential temperature gradient vector resulting from the geostrophic vorticity and geostrophic deformation, referred to as QVR and QDR, respectively, are derived. The evolution of the thermal structure and forcing for quasigeostrophic vertical motion in an occluded cyclone are investigated by examining the distributions of QVR and QDR and their respective convergences. The dynamics of two common structural transformations observed in the evolution of occluded cyclones are revealed by consideration of these separate forcings. First, the tendency for the sea level pressure minimum to deepen northward and/or westward into the cold air west of the triple point is shown to be controlled by the convergence of QVR, which is mathematically equivalent to thermal wind advection of geostrophic vorticity, a well-accepted mechanism for forcing of synoptic-scale vertical motion. Second, the lengthening of the occluded thermal ridge and surf...

Quasigeostrophic Forcing of Ascent in the Occluded Sector of Cyclones and the Trowal Airstream

A numerical model-based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the Q vector into its along- and acrossisentrope components, Qs and Qn, respectively. The Qn component describes the geostrophic contribution to the rate of change of the magnitude of =pu (traditional frontogenesis), whereas the Qs component describes the geostrophic contribution to the rate of change of direction of =pu (rotational frontogenesis). It is shown that convergence of Qs simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant. The absence of significant Qn convergence there suggests that quasigeostrophic (Q-G) frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three-dimensional structural changes in the occluded sector of post-mature phase midlatitude cyclones. A cyclonically ascending, cloud- and precipitation-producing airstream that originates in the warm-sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified Q-G forcing. This airstream is referred to as the ‘‘trowal airstream’’ and it is shown to be responsible for the production of the ‘‘wrap around’’ cloud and precipitation commonly associated with occluded systems. The relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed.

The Effect of Large-Scale Flow on Low-Level Frontal Structure and Evolution in Midlatitude Cyclones

Abstract Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach. Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving i...

Investigation of the planetary boundary layer height variations over complex terrain

The effects of sea-breeze interactions with synoptic forcing on the PBL height over complex terrain are investigated through the use of a 3-D mesoscale numerical model. Two of the results are as follows. First, steep PBL height gradients — order of 1500 m over a grid interval of 10 km — are associated with the sea-breeze front and are enhanced by the topography. Second, a significant horizontal shift in the maximum PBL height relative to the mountains, is induced by a corresponding displacement of the thermal ridge due to the mountains, in the presence of large scale flow.

Basalt evolution at low pressure: implications from an experimental study in the system CaO-FeO-MgO-Al2O3-SiO2

New experiments have been performed in the system CaO+MgO+Al2O3+SiO2 (CMAS)+FeO at atmospheric pressure. Most of the experiments were conducted on Fe-rich compositions, in the low-temperature field of the assemblage liq(liquid)+an(anorthite) +aug(augite)+ol(olivine), and mostly along five isotherms. Others were located on, or nearby the assemblage boundaries. These experiments, together with the previously reported high temperature experiments (Shi and Libourel 1991; Libourel et al. 1989), permit contouring the complete liq+an+aug+ol divariant field, and tracing out some of its boundaries. The boundary of the assemplage liq+an+aug+ol consists of six segments, with the appearance of one of the following phases, orthopyroxene, pigeonite, tridymite, bustamite, kirschsteinite, and spinel, as an additional phase. Within the stability field of the assemblage liq+an+aug+ol, the compositions of all the coexisting phases have been described as functions of temperature and silica content in the melt by applying a multiple linear regression method. This allows a quantitative characterization of the divariant assemblage liq+an+aug+ol in the system CMAS+FeO, from 1273°C to 1055°C, with olivine compositions ranging from Mg*[Mg/(Mg+Fe)]=1 to 0.08. Knowing the composition-temperature relationships, the basic T-X configuration of the assemblage liq+an +aug+ol has been analysed, and mass-balance calculations have been performed to examine the FeO effect on different crystallization processes. Addition of FeO to the system CMAS transforms the thermal divide in the assemblage liq+an+di(diopside)+fo(forsterite) into a thermal ridge. With decreasing temperature, the spine of the thermal ridge moves towards Si-poor compositions at Mg-rich end but towards Si-rich compositions at the Fe-rich end. This indicates that late-stage tholeiitic liquids can follow a trend of silica enrichment without the crystallization of an oxide phase. Crystallization paths of the assemblage liq+an+aug+ol are determined by the detailed T-X relations of the thermal ridge with the melt evolving away from the spine. The boundary reactions with decreasing temperature have also been characterized numerically.

Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome

The Guinea Dome is a permanent, quasi‐stationary feature on the eastern side of the thermal ridge extending zonally across the tropical North Atlantic. The dome is a part of the large‐scale near‐surface flow fields associated with the North Equatorial Current, the North Equatorial Countercurrent and the North Equatorial Undercurrent. In the present study, historical and recently obtained hydrographic data are combined to investigate the thermohaline structure and geostrophic flow field in the vicinity of the dome. It is shown that the Guinea Dome exists throughout the year both in subthermocline and thermocline layers, that it has a corresponding cyclonic geostrophic flow, and that seasonal changes occur with respect to its vertical structure, horizontal extent, and position. The observational results are then compared with simulations from a general circulation model of the tropical Atlantic. A seven‐year simulation forced by observed monthly winds is run to compute a monthly climatology. The model adequately simulates the Guinea Dome with respect to its structure, flow field, and seasonal variability.

The effects of FeO on the system CMAS at low pressure and implications for basalt crystallization processes

The common occurrence of olivine, clinopyroxene and plagioclase in natural basalts may suggest that crystallization of these phases exerts important controls on the evolution of natural basalts at low pressure. In order to understand the effects of such an assemblage on the evolution of basalt, an experimental study of the divariant assemblage 1+an+cpx+ol in the system CMAS + FeO was undertaken at 1 bar and under very reducing conditions. Experiments have been performed along three isothernal sections. Detailed electron microprobe data have been collected on all the coexisting phases. Combining these data with available data in the system CMAS, the compositions of all the coexisting phases have been described as functions of temperature and CaO content in the melt by applying a multiple linear regression method. This allows a quantitative characterization of the divariant assemblage 1+an+epx+ol in the system CMAS + FeO, from 1275°C to 1160°C, with liquid compositions ranging from mg#=1 to mg#=0.28. Knowing the composition-temperature relationships, the basic T-X configuration of the assemblage 1+an+cpx+ol has been analysed, and mass balance calculations have been performed to examine the FeO effect on different crystallization processes. Addition of FeO to the system CMAS transforms the thermal divide in the assemblage 1+an+cpx+ol to a thermal ridge, and shifts the spine of the thermal ridge towards SiO2-poor compositions with decreasing temperature. This indicates that tholeiitic basalts can be relatively silica-poor, and evolve towards slightly more silica-poor compositions. With increasing FeO content, pigeonite replaces ortheonstatite as a crystallizing phase along the silica-rich boundary of the assemblage 1+an+cpx+ol. In some iron, and silica rich composition, olivine possibly terminates its crystallization or even starts resorption prior to pigeonite precipitation; this suppresses silica enrichment in the melt. Crystallization paths of the assemblage 1+an+cpx+ol are determined by the detailed T-X relations of the thermal ridge with the melt evolving away from the spine. The liquid evolution trend depends strongly on the initial compositions. A tholeiitic liquid can follow a moderate silica enrichment path at the same very reducing condition of there is a high modal proportion of olivine. ‘A Fenner mechanism therefore is at work at the same place as a Bowen mechanism’.

Dynamical aspects of the october storm, 1987: A study of a successful fine‐mesh simulation

AbstractA diagnostic study of a successful numerical simulation of the October storm 1987 is described. This good, non‐operational forecast was obtained using the Meteorological Office's fine‐mesh model run from a revised analysis at 00 GMT on 15 October 1987. the Meteorological Office's mesoscale model was also run using initial conditions at 12 GMT 15 October obtained by interpolating the above fine‐mesh forecast fields to examine the benefits of very high horizontal resolution (15km grid length).Diagnostic maps and cross‐sections derived are used to address two scientific issues: the role of weak moist slantwise stability (or, alternatively, positive slantwise convective potential energy) the dynamical significance of the striking cloud structure (the cloud head) seen in the satellite imagery early in the storm's life. It appears that the baroclinic zone, in which the storm develops, is highly receptive to forcing by an upper trough and the warm boundary‐layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy. the cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone. Its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent. the low‐level temperature field exhibits, throughout the development period, a remarkable shape‐preserving thermal ridge of considerable intensity. the sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately. It is found that on the timescale of the simulation, surface energy fluxes have negligible impact on the explosive development. On the other hand, the contribution of latent‐heat release to the development appears to dominate the dry baroclinic instability process.

A Case Study of GWE Satellite Data Impact on GLA Assimilation Analyses of Two Ocean Cyclones

The impact of GWF satellite data on GLA analyses of the thermal structure of two oceanic extratropical cyclones are presented. The two storms developed nearly simultaneously in the western Pacific and Atlantic oceans during January 1979. In addition to describing the satellite data impact on temperature and static-stability distributions within each storm, implications of the impact on available potential energy and on the baroclinic growth of the cyclone are discussed. In the comparison of CLA analyses with (SAT) and without (NOSAT) the satellite temperature soundings produced by 1,be NOAA/NESS retrieval method, the results show sizable temperature differences between the two analyses for both storms. Characteristic features include the following: 1) warmer SAT temperature within NOSAT thermal through and colder SAT temperatures within NOSAT thermal ridges 2) a difference in sign or magnitude of the temperature impact between the upper troposphere and the lower troposphere, and 3) an evolution of a significant, satellite-data cold bias located within and paralleling the evolution of the low-tropospheric thermal ridge close to the cyclone center. The satellite temperature impact in the low troposphere increases tropospheric static in the central storm area and decreases troposhperic stability in the cold troughs both to the rear and ahead of the cyclone. Although the net effect of area-averaged satellite temperature impact for the storms is small, the association of warmer temperature with NOSAT thermal trough and colder temperatures with thermal ridges reduces the areal temperature stability variance within the Pacific storm area. The reduction for the Atlantic storm region is less. For the Pacific storm the satellite temperature impact led to a 10% reduction of available potential energy, a significant reduction in vertical phase tilt of the tropospheric temperature structure, and a potentially greater than 10–20% reduction of the baroclinic rate of cyclone-scale waves. The decreased baroclinic amplification is inferred from stabilization of the highest lapse rates and a reduction of the largest vertical thermal-wind shears in the low troposphere of the storm area. Overall the results stress the need to identify characteristics of satellite temperature-sounding impacts on the baroclinic structure of cyclone waves, which potentially degrade numerical weather prediction of cyclogenesis.