Equivalent Potential Temperature Research Articles

Atmospheric Overturning across Multiple Scales of an MJO Event during the CINDY/DYNAMO Campaign

Abstract This study investigates the atmospheric overturning of the October 2011 MJO event observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY)/DYNAMO field experiment using a cloud-permitting numerical model. The isentropic analysis is used to sort the vertical mass transport in terms of the equivalent potential temperature of the air parcels, which naturally decomposes the atmospheric overturning between ascending air with high entropy and subsiding air with low entropy. The circulation is further decomposed into contributions of four main scales: basinwide ascent, meridional overturning, regional overturning, and convection. Results show that the convective scale dominates the upward mass transport while larger scales play an important role both by allowing a deeper overturning and by modulating convective activity. There are substantial changes in the atmospheric overturning during different phases of this MJO event. Increased convective activity at low levels precedes the onset of the MJO by several days. The initiation of the MJO itself is associated with a substantial increase in the atmospheric overturning over the Indian Ocean. The subsequent eastward propagation of the MJO event can be clearly captured by the evolutions of convective-scale vertical mass fluxes at different altitudes. The equivalent potential temperatures of the rising and subsiding air parcels in the convective-scale overturning are also increased in the troposphere during the active phase of the MJO.

Thermodynamic structure of the convective boundary layer (CBL) over the Indian monsoon region during CAIPEEX campaigns

Abstract. Spatial and temporal variability in the convective boundary layer (CBL) height for the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) study period are examined using the data collected from high-resolution radiosondes during May–September 2009 over the Indian monsoon region. In total, 57 radiosonde launchings were carried out at ∼ 11:00–17:00 IST over six different stations covering a large geographical region, ranging from latitude ∼ 13 to 32° N and longitude 73 to 92° E. Of the total 57 launchings, 17 were made during cloudy conditions during which relative humidity (RH) was found to be greater than 83 % for an ∼ 1.0 km layer at various altitudes below 6 km. Within the layer the difference between saturated equivalent potential temperature and equivalent potential temperature is small, and it satisfies the condition that RH > 83 % for about 1 km is considered as the cloudy layer. There are eight cases when the cloud-topped boundary layer (CTBL) and 19 cases when fair-weather boundary layer (FWBL) is observed. The CBL heights are obtained using thermodynamic profiles, which vary from ∼ 0.4 to 2.5 km a. g. l. The formation of the cloud layers above the boundary layer generally lowers the CBL height and is responsible for its day-to-day variability. The development of the cloud beneath the boundary layer generally elevates the CBL, which is also responsible for the large day-to-day variability in the CBL. The FWBL identified using relative invariance of the thermodynamic profiles varies from ∼ 2.0 to 5.5 km, which is clearly marked by a local minimum in the refractivity gradient. During cloudy days, the CBL is found to be shallow and the surface temperature lower when compared to clear-sky days. The CBL and the lifting condensation level (LCL) heights are randomly related and are found to be at a lower height during cloudy days when compared to clear-sky days. Finally, the typical comparison between the CBL height obtained using thermodynamic profiles and backscattering profiles using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) is examined.

Formation of a coastal barrier jet in the Gulf of Mexico due to the interaction of cold fronts with theSierraMadreOriental mountain range

The passage of three cold fronts (CFs) over the Gulf of Mexico was simulated with the Weather Research and Forecasting model to study the mechanisms associated with the formation of a jet parallel to the Sierra Madre Oriental (SMOr) mountain range near the port of Veracruz. Dew‐point temperature (Td= 20 °C) and available convective potential energy best simulated the horizontal propagation of the intenseCFsover the Gulf, and equivalent potential temperature helped identify the vertical position of the fronts. Additional simulations with reduced topography showed that mountain heights exceeding 2.5 km, where theSMOris closest to the coast, were crucial for the formation of a coastal barrier jet. The Froude number (Fr < 1) and the ageostrophic component of the wind at low levels were used to identify the formation of the jet; the Rossby radius of deformation determined its horizontal extent which was 81 km into the Gulf of Mexico in agreement with scatterometer winds during one of theCFs. The interaction of the northeasterly winds (known as Nortes) with the topography produced a blocking effect resulting in a stable region of cold air, a rain‐shadow and a coastal barrier jet of northwesterly direction, which generated strong cold air advection and subsidence in comparison with adjacent zones. The jet region appears as a minimum in the winter precipitation climatology indicating that the barrier jet is an important intraseasonal feature. In two of the case‐studies, maximum jet velocities reached those of a tropical storm (>18 m s−1) and occurred near the surface (<1 km above mean sea level), consistent with observations and similar to other barrier jets in the world. For the first time the jet near the port of Veracruz is documented as a coastal barrier jet. TheCFsalso produced strong Tehuano gap winds.

Multi-scale temporospatial variability of the East Asian Meiyu-Baiu fronts: characterization with a suite of new objective indices

The Meiyu-Baiu front (MBF) is the primary weather system modulating the monsoonal rainfall in East Asia, and it exhibits pronounced variability across multiple temporal and spatial scales. A suite of new objective indices based upon daily reanalysis data are developed to characterize such variability, which include an “intensity” index effectively measuring the meridional gradient of equivalent potential temperature across the frontal zone, a “location” index documenting the central latitude and longitude of the front, and a “stationarity” index corresponding to the standard deviation of the daily central latitude. All these indices are characterized by strong interannual to interdecadal variations. Specifically, the MBF has shown a tendency of southwestward shift since the mid-1990s and become more stationary over southern China with reduced intraseasonal meandering. The western North Pacific subtropical high (WNPSH) is the main circulation feature affecting the location of the MBF on interannual timescales. The MBF moves northward (southward) when the WNPSH is anomalously strong (weak) and the surface high over the region of 35°–50°N/100°–120°E is anomalously weak (strong). When the WNPSH is strong, the MBF is often situated northwest to its climatological position and becomes more stationary. On interdecadal timescales, enhanced activity of high latitude blocking over East Asia is often accompanied with a stronger but less stationary MBF while a negative phase of the Arctic Oscillation favors a southward-shifted and more stationary MBF.

Modeling the contributions of global air temperature, synoptic-scale phenomena and soil moisture to near-surface static energy variability using artificial neural networks

Abstract. The static energy content of the atmosphere is increasing on a global scale, but exhibits important subglobal and subregional scales of variability and is a useful parameter for integrating the net effect of changes in the partitioning of energy at the surface and for improving understanding of the causes of so-called warming holes (i.e., locations with decreasing daily maximum air temperatures (T) or increasing trends of lower magnitude than the global mean). Further, measures of the static energy content (herein the equivalent potential temperature, θe) are more strongly linked to excess human mortality and morbidity than air temperature alone, and have great relevance in understanding causes of past heat-related excess mortality and making projections of possible future events that are likely to be associated with negative human health and economic consequences. New nonlinear statistical models for summertime daily maximum and minimum θe are developed and used to advance understanding of drivers of historical change and variability over the eastern USA. The predictor variables are an index of the daily global mean temperature, daily indices of the synoptic-scale meteorology derived from T and specific humidity (Q) at 850 and 500 hPa geopotential heights (Z), and spatiotemporally averaged soil moisture (SM). SM is particularly important in determining the magnitude of θe over regions that have previously been identified as exhibiting warming holes, confirming the key importance of SM in dictating the partitioning of net radiation into sensible and latent heat and dictating trends in near-surface T and θe. Consistent with our a priori expectations, models built using artificial neural networks (ANNs) out-perform linear models that do not permit interaction of the predictor variables (global T, synoptic-scale meteorological conditions and SM). This is particularly marked in regions with high variability in minimum and maximum θe, where more complex models built using ANN with multiple hidden layers are better able to capture the day-to-day variability in θe and the occurrence of extreme maximum θe. Over the entire domain, the ANN with three hidden layers exhibits high accuracy in predicting maximum θe > 347 K. The median hit rate for maximum θe > 347 K is > 0.60, while the median false alarm rate is ≈ 0.08.

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.

On the movement of tropical cyclone LEHAR

In this paper, an attempt has been made to delineate the physical processes which lead to the westward movement of the North Indian Ocean tropical cyclone LEHAR. The Advanced Weather Research and Forecasting (ARW) model is used to simulate LEHAR with 27 and 9 km resolutions. In addition to that, all terms of the complete vorticity equation are computed to obtain the contribution of each term for the vorticity tendency. The vorticity tendency is calculated in four sectors, namely northwest, northeast, southwest, and southeast and assumed that the cyclone moves from its existing location to the nearest point where the vortices tendency is maximum. The results indicate that the model performed well in simulating the characteristics of cyclone compared with the Satellite and other observations. It is noticed that the vorticity stretching term contributes most to the positive vorticity tendency. The second highest contribution is from the horizontal advection thus indicating the secondary importance of steering. The distribution of lightning flash rates are higher in the SW and followed by NW sectors of the cyclone indicate more strong convective clouds are in SW sector. The equivalent potential temperatures (θ e) at different stages of before, during and after the mature stage of the cyclone reveals that the wind-induced surface heat (WISH) exchange process is a plausible mechanism for the intensification of LEHAR.

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.

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

Pools of air cooled by partial rain evaporation span up to several hundreds of kilometers in nature and typically last less than 1 day, ultimately losing their identity to the large-scale flow. These fundamentally differ in character from the radiatively-driven dry pools defining convective aggregation. Advancement in remote sensing and in computer capabilities has promoted exploration of how precipitation-induced cold pool processes modify the convective spectrum and life cycle. This contribution surveys current understanding of such cold pools over the tropical and subtropical oceans. In shallow convection with low rain rates, the cold pools moisten, preserving the near-surface equivalent potential temperature or increasing it if the surface moisture fluxes cannot ventilate beyond the new surface layer; both conditions indicate downdraft origin air from within the boundary layer. When rain rates exceed sim 2 mm h^{-1}, convective-scale downdrafts can bring down drier air of lower equivalent potential temperature from above the boundary layer. The resulting density currents facilitate the lifting of locally thermodynamically favorable air and can impose an arc-shaped mesoscale cloud organization. This organization allows clouds capable of reaching 4–5 km within otherwise dry environments. These are more commonly observed in the northern hemisphere trade wind regime, where the flow to the intertropical convergence zone is unimpeded by the equator. Their near-surface air properties share much with those shown from cold pools sampled in the equatorial Indian Ocean. Cold pools are most effective at influencing the mesoscale organization when the atmosphere is moist in the lower free troposphere and dry above, suggesting an optimal range of water vapor paths. Outstanding questions on the relationship between cold pools, their accompanying moisture distribution and cloud cover are detailed further. Near-surface water vapor rings are documented in one model inside but near the cold pool edge; these are not consistent with observations, but do improve with smaller horizontal grid spacings.

Structure and Maintenance Mechanism of Long-Lived Concentric Eyewalls Associated with Simulated Typhoon Bolaven (2012)

Abstract Typhoons with long-lived concentric eyewalls (CEs) are more intense than those with short-lived CEs. It is important for more accurate prediction of typhoon intensity to understand the maintenance mechanism of the long-lived CEs. To study the mechanism of the long-term maintenance of CEs, a numerical experiment of Typhoon Bolaven (2012) is performed using a nonhydrostatic model with full physics. Two aspects of the maintenance of simulated CEs are investigated: the maintenance of the inner eyewall and the contraction of the outer eyewall. To examine the maintenance of the inner eyewall, the equivalent potential temperature budget and air parcel trajectories of the simulated inner eyewall are calculated. The results show that the entropy supply to the inner eyewall is sufficient to maintain the inner eyewall after secondary eyewall formation (SEF). During the early period after SEF, entropy is supplied by an axisymmetric inflow, and later it is supplied by nonaxisymmetric flows of the outer eyewall. To examine the contraction of the outer eyewall, the potential vorticity (PV) budget of the outer eyewall is diagnosed. The result reveals that the negative contribution to the contraction of the outer PV peak (i.e., the outer eyewall) in the early period is the negative PV generation due to axisymmetric advection and diabatic heating just inside of the outer PV peak. In the later period, the negative PV generation due to nonaxisymmetric structure is important for the prevention of contraction. The present study reveals that the structure of the outer eyewall plays important roles in the maintenance of long-lived CEs.

A Third-Law Isentropic Analysis of a Simulated Hurricane

Abstract The moist-air entropy can be used to analyze and better understand the general circulation of the atmosphere or convective motions. Isentropic analyses are commonly based on studies of different equivalent potential temperatures, all of which are assumed to fully represent the entropy of moist air. It is, however, possible to rely either on statistical physics or the third law of thermodynamics when defining and computing the absolute entropy of moist air and to study the corresponding third-law potential temperature, which is different from the previous ones. The third law assumes that the entropy for the most stable crystalline state of all substances is zero when approaching absolute zero temperature. This paper shows that the way all these moist-air potential temperatures are defined has a large impact on (i) the plotting of the isentropes for a simulation of Hurricane Dumile, (ii) the changes in moist-air entropy computed for a steam cycle defined for this hurricane, (iii) the analyses of isentropic streamfunctions computed for this hurricane, and (iv) the computations of the heat input, the work function, and the efficiency defined for this steam cycle. The moist-air entropy is a state function and the isentropic analyses must be completely determined by the local moist-air conditions. The large differences observed between the different formulations of moist-air entropy are interpreted as proof that the isentropic analyses of moist-air atmospheric motions must be achieved by using the third-law potential temperature defined from general thermodynamics.

Precipitating Quasigeostrophic Equations and Potential Vorticity Inversion with Phase Changes

Abstract Precipitating versions of the quasigeostrophic (QG) equations are derived systematically, starting from the equations of a cloud-resolving model. The presence of phase changes of water from vapor to liquid and vice versa leads to important differences from the dry QG case. The precipitating QG (PQG) equations, in their simplest form, have two variables to describe the full system: a potential vorticity (PV) variable and a variable M including moisture effects. A PV-and-M inversion allows the determination of all other variables, and it involves an elliptic partial differential equation (PDE) that is nonlinear because of phase changes between saturated and unsaturated regions. An example PV-and-M inversion is provided for an idealized cold-core cyclone with two vertical levels. A key point illustrated by this example is that the phase interface location is unknown a priori from PV and M, and it is discovered as part of the inversion process. Several choices of a moist PV variable are discussed, including subtleties that arise because of phase changes. Boussinesq and anelastic versions of the PQG equations are described, as well as moderate and asymptotically large rainfall speeds. An energy conservation principle suggests that the model has firm physical and mathematical underpinnings. Finally, an asymptotic analysis provides a systematic derivation of the PQG equations, which arise as the limiting dynamics of a moist atmosphere with phase changes, in the limit of rapid rotation and strong stratification in terms of both potential temperature and equivalent potential temperature.

Sensitivity Analysis of Hurricane Arthur (2014) Storm Surge Forecasts to WRF Physics Parameterizations and Model Configurations

Abstract Through a case study of Hurricane Arthur (2014), the Weather Research and Forecasting (WRF) Model and the Finite Volume Coastal Ocean Model (FVCOM) are used to investigate the sensitivity of storm surge forecasts to physics parameterizations and configurations of the initial and boundary conditions in WRF. The turbulence closure scheme in the planetary boundary layer affects the prediction of the storm intensity: the local closure scheme produces lower equivalent potential temperature than the nonlocal closure schemes, leading to significant reductions in the maximum surface wind speed and surge heights. On the other hand, higher-class cloud microphysics schemes overpredict the wind speed, resulting in large overpredictions of storm surge at some coastal locations. Without cumulus parameterization in the outermost domain, both the wind speed and storm surge are grossly underpredicted as a result of large precipitation decreases in the storm center. None of the choices for the WRF physics parameterization schemes significantly affect the prediction of Arthur’s track. Sea surface temperature affects the latent heat release from the ocean surface and thus storm intensity and storm surge predictions. The large-scale atmospheric circulation models provide the initial and boundary conditions for WRF, and influence both the track and intensity predictions, thereby changing the spatial distribution of storm surge along the coastline. These sensitivity analyses underline the need to use an ensemble modeling approach to improve the storm surge forecasts.

Extratropical Factors Affecting the Variability in Summer Precipitation over the Yangtze River Basin, China

Abstract The Yangtze River basin (YRB), a typical East Asian monsoon region, experiences a large year-to-year variability in summer precipitation and is subject to both floods and droughts. There is a well-known seesaw relationship in precipitation between the tropical western North Pacific and the YRB, but more than half of the variance in precipitation in the YRB cannot be explained by this seesaw pattern. The authors therefore investigated other physical factors that might affect precipitation in the YRB. The results indicate that the northeasterly anomaly in the lower troposphere to the north of the YRB plays an important role in the variability in precipitation. This northeasterly anomaly is paired with the southwesterly anomaly to the south of the YRB. They both play an important role in water vapor accumulation over the YRB and intensify the meridional gradient of the equivalent potential temperature θe over the YRB by bringing dry and cool air from the north and wet air from the south. This intensified θe gradient favors convective instability and heavier rainfall in the YRB, as previous studies on mei-yu weather have indicated. Furthermore, it is found that the zonally oriented teleconnection along the Asian westerly jet and the meridional displacement of the jet can affect circulation in the lower troposphere and precipitation in the YRB. These results highlight the role of extratropical circulation anomalies and thus contribute to a more comprehensive understanding of the variability of precipitation in the YRB.

MJO Propagation Shaped by Zonal Asymmetric Structures: Results from 24 GCM Simulations

Abstract Eastward propagation is an essential characteristic of the Madden–Julian oscillation (MJO). Yet, simulation of MJO propagation in general circulation models (GCMs) remains a major challenge and understanding the causes of propagation remains controversial. The present study explores why the GCMs have diverse performances in MJO simulation by diagnosis of 24 GCM simulations. An intrinsic linkage is found between MJO propagation and the zonal structural asymmetry with respect to the MJO convective center. The observed and realistically simulated MJO eastward propagations are characterized by stronger Kelvin easterly waves than Rossby westerly waves in the lower troposphere, which is opposite to the Gill pattern where Rossby westerly waves are 2 times stronger than Kelvin easterly waves. The GCMs simulating stronger Rossby westerly waves tend to show a stationary MJO. MJO propagation performances are robustly correlated with the quality of simulated zonal asymmetries in the 850-hPa equatorial zonal winds, 700-hPa diabatic heating, 1000–700-hPa equivalent potential temperature, and convective instability. The models simulating realistic MJO propagation are exemplified by an eastward propagation of boundary layer moisture convergence (BLMC) that leads precipitation propagation by about 5 days. The BLMC stimulates MJO eastward propagation by preconditioning and predestabilizing the atmosphere, and by generating lower-tropospheric heating and available potential energy to the east of precipitation center. The MJO structural asymmetry is generated by the three-way interaction among convective heating, moisture, and equatorial wave and boundary layer dynamics. In GCMs, differing convective heating representation could produce different MJO structural asymmetry, and thus different propagations. Diagnosis of structural asymmetry may help revealing the models’ deficiency in representing the complex three-way interaction processes, which involves various parameterized processes.

Radar Kinematic Information as a Surrogate for Isentropes in Stratiform Precipitation Systems

AbstractThis study illustrates that dual-Doppler-derived wind shear (vertical gradient of the horizontal wind) in stratiform, nonturbulent flow is structured in long, thin striations. The reason this has not been documented before is that scanning ground-based radars have inadequate vertical resolution, deteriorating with range. Here data from an airborne radar with a fine, range-independent vertical resolution are used. A comparison of the radar-derived wind shear with model output of isentropes in vertical transects in the comma head of two frontal disturbances suggests that the wind shear layers describe material surfaces. Model output itself further confirms the alignment of isentropes with wind shear in vertical transects. Thus, Doppler-radar-derived wind shear (a kinematic conserved variable) may serve as a suitable proxy for thermodynamic conserved variables such as equivalent potential temperature in stratiform precipitation. Furthermore, the presence of shear striations in vertical transects can be used as a marker for nonturbulent flow, and their persistence as an indicator of limited dispersion in such flow.

A self‐weakening mechanism for tropical cyclones

A mechanism leading to the self‐weakening of tropical cyclones is proposed using the Weather and Research and Forecasting model. A comparison between an experiment with variable Coriolis parameter f and one on an f‐plane shows that, after the initial intensification, the former is characterized by a smaller intensity. As opposed to the tropical cyclone on the f‐plane, the one with variable f weakens significantly after reaching maturity. Analyses of the 3D circulation show that the main reason for the weakening is dry intrusion in the mid‐upper troposphere from the west. Once the dry intrusion reaches the inner vortex, strong downdraughts reduce the high equivalent potential temperature in the boundary‐layer inflow. The subsequent updraughts in the eyewall, characterized by lower equivalent potential temperature, are considerably reduced and, consequently, the secondary circulation weakens.Back‐trajectories are used to determine the origin of the dry intrusion. It is found that the air parcels expelled from the storm deep convection into the outflow layer recirculate anticyclonically back into the vortex, causing a self‐weakening of the tropical cyclone. A time span for the recirculation of at least 48 h allows the air parcels to sink substantially before reaching the vortex circulation. Some implications of the intrinsic nature of this process are briefly discussed.

Spatial and Temporal Variability of the Three-Dimensional Flow around African Easterly Waves

This study presents a large-scale trajectory analysis of African easterly waves (AEWs) across West Africa and the eastern Atlantic. Back trajectories were initialized at multiple pressure levels from in and around the vortex center of the AEW troughs to reveal the source regions of environmental inflow. The trajectory analysis highlights a changing influence of environmental air on AEW troughs. Over West Africa, monsoonal flow dominates with source regions of air from the southwest and east to northeast influencing the trough. As the AEW troughs leave West Africa, flow from the northwest becomes increasingly important. Cluster analysis highlighted that the contribution of trajectories from the northwest increased as the AEW troughs move westward and that this cluster also had high variability in environmental characteristics. Correlation analysis of outgoing longwave radiation around the troughs with environmental characteristics 72 h earlier was conducted on 443 AEWs. This analysis reveals that the impact of the various source regions on convective activity within the AEW troughs is consistent with the cluster trajectory analysis. While the AEW troughs were over West Africa, convection was sensitive to midlevel equivalent potential temperature [Formula: see text] around the trough and to the northeast of the trough axis. Over the West African coast and Cape Verde basin, the correlation analysis captures the changing flow regime with sensitivity to [Formula: see text] west of the trough axis at midlevels and northwest at low levels. These results highlight that the cool and dry low-level northerly trade winds over the Canary and Cape Verde basin can be a crucial influence on AEWs as they leave West Africa.

Sensitivity of Northern Great Plains Convection Forecasts to Upstream and Downstream Forecast Errors

Abstract The role of earlier forecast errors on subsequent convection forecasts is evaluated for a northern Great Plains severe convective event on 11–12 June 2013 during the Mesoscale Predictability Experiment (MPEX) by applying the ensemble-based sensitivity technique to Weather Research and Forecasting (WRF) Model ensemble forecasts with explicit convection. This case was characterized by two distinct modes of convection located 150 km apart in western Nebraska and South Dakota, which formed on either side of an axis of high, lower-tropospheric equivalent potential temperature . Convection forecasts over both regions are found to be sensitive to the position of this axis. The convection in Nebraska is sensitive to the position of the western edge of the axis near an upstream dryline, which modulates the preconvective prior to the diurnal maximum. In contrast, the convection in South Dakota is sensitive to the position of the eastern edge of the axis near a cold front, which also modulates the preconvective in that location. The position of the axis is modulated by the positions of both upstream and downstream mid- to upper-tropospheric potential vorticity anomalies, and can be traced backward in time to the initial conditions. Dropsondes sampling the region prior to convective initiation indicate that ensemble members with better representations of upstream conditions in sensitive regions are associated with better convective forecasts over Nebraska.

Tropical Cyclone Outflow and Warm Core Structure as Revealed by HS3 Dropsonde Data

Abstract Dropsonde data collected during the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign from 16 research missions spanning 6 tropical cyclones (TCs) are investigated, with an emphasis on TC outflow and the warm core. The Global Hawk (GH) AV-6 aircraft provided a unique opportunity to investigate the outflow characteristics due to a combination of 18+-h flight durations and the ability to release dropsondes from high altitudes above 100 hPa. Intensifying TCs are found to be associated with stronger upper-level divergence and radial outflow relative to nonintensifying TCs in the sample, regardless of current intensity. A layer of 2–4 m s−1 inflow 20–50 hPa deep is also observed 50–100 hPa above the maximum outflow layer, which appears to be associated with lower-stratospheric descent above the eye. The potential temperature of the outflow is found to be more strongly correlated with the equivalent potential temperature of the boundary layer inflow than to the present storm intensity, consistent with the outflow temperature having a stronger relationship with potential intensity than actual intensity. Finally, the outflow originates from a region of low inertial stability that extends above the cyclone from 300 to 150 hPa and from 50- to 200-km radius. The unique nature of this dataset allows the height and structure of the warm core also to be investigated. The magnitude of the warm core was found to be positively correlated with TC intensity, while the height of the warm core was weakly positively correlated with intensity. Finally, neither the height nor magnitude of the warm core exhibits any meaningful relationship with intensity change.