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J. Halverson, M. Garstang, J. Scala, and W-K. Tao

Abstract

Mesoscale water and energy budgets are diagnosed for a squall line during the Convection and Precipitation Electrification Experiment and combined with the results of the two-dimensional Goddard Cumulus Ensemble Model. The fine temporal and spatial resolution of cloud-scale processes contained in the model is used to reduce uncertainty in the diagnosed water budget residual and, thus, to arrive at a good estimate of storm-total rainfall. Profiles of cumulus heating (Q 1) and drying (Q 2) inferred from the sounding observations are in turn compared with the cloud-scale energy budget terms calculated from the model. This comparison reveals near-agreement in the magnitude and vertical distribution of the peak Q 1 and Q 2, and also the relative size of the heating and drying at different levels in the column.

When the size of the mesoscale convective disturbance is approximately the same as the sounding observation network, it may be wrong to assume that the diagnosed vertical eddy heat transport accounts for most of the total eddy transport of moist static energy, F. The cloud model is used to resolve the relative contribution of the horizontal and vertical eddy flux convergence of heat and moisture, and thus it serves as a guide to interpreting the sounding-diagnosed total flux. The model results suggest that although the mean column vertical flux convergence is significantly larger than the column-mean horizontal flux convergence, the horizontal flux convergence does play a significant role in midlevels of the convective region. This flux convergence may be associated with a strong front-to-rear inflow that develops during the mature stage of the squall line.

This study suggests that when combined with the independent results of a mesoscale cloud model, the sounding diagnostics can provide a sensitivity test for the Tropical Rainfall Measuring Mission measurements of rainfall and diabatic heating over the life cycle of an entire mesoscale convective system.

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G. M. Heymsfield, J. B. Halverson, and I. J. Caylor

Abstract

An extensive wintertime squall line on 13 January 1995 occurring along the U.S. Gulf of Mexico coastline is examined using airborne radar observations combined with conventional data analysis. Flight tracks with the ER-2 Doppler radar (EDOP) mounted on the high-altitude (20 km) ER-2 aircraft provided a unique view of the vertical structure of this line. In this paper, the authors document the squall line structure, and compare and contrast this structure with other published cases.

The squall line had several prominent features that differ from previous studies: 1) the stratiform region was wide in comparison to more typical systems that are 50–100 km wide; 2) the trailing stratiform region consisted of two to three separate embedded trailing bands rather than one continuous band; 3) vertical motions in the trailing stratiform region were nearly twice as strong as previously reported values, with mean values approaching 1 m s−1 between 7- and 9-km altitude, and larger values (1.5 m s−1) in the embedded bands; 4) reflectivities were large with mean stratiform values of about 38 dBZ, and maximum convective values of about 55 dBZ; 5) the squall line rear inflow descended to the surface well behind the leading edge (∼200 km); 6) the convective and squall line inflow region exhibited unique microphysics with small graupel or hail falling out of the tilted squall line updraft, and a wavy, elevated melting region associated with the inflow; and 7) the squall-scale transverse circulation was directly coupled with a jet streak thermally direct circulation, and the ascending branch of this direct circulation may have enhanced production of widespread stratiform rainfall. A conceptual model is presented highlighting the features of this squall line and the coupling of the squall line to the larger-scale flow.

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Liguang Wu, Scott A. Braun, J. Halverson, and G. Heymsfield

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Hurricane Erin (2001) at high resolution (4-km spacing) from its early development as a tropical depression on 7 September 2001, through a period of rapid intensification into a strong hurricane (8–9 September), and finally into a stage during which it maintains its intensity on 10 September. These three stages of formation, intensification, and maintenance in the simulation are in good agreement with the observed evolution of Erin. The simulation shows that during the formation and early portions of the intensification stages, intensification is favored because the environmental wind shear is weak and the system moves over a warm tongue of water. As Erin intensifies, the wind shear gradually increases with the approach of an upper-level trough and strengthening of a low-level high pressure system. By 10 September, the wind shear peaks and begins to decrease, the storm moves over slightly cooler waters, and the intensification ends. Important structural changes occur at this time as the outer precipitation shifts from the northeastern and eastern sides to the western side of the eye. A secondary wind maximum and an outer eyewall begin to develop as precipitation begins to surround the entire eye.

The simulation is used to investigate the role of vertical wind shear in the changes of the precipitation structure that took place between 9 and 10 September by examining the effects of changes in storm-relative flow and changes in the shear-induced tilt. Qualitative agreement is found between the divergence pattern and advection of vorticity by the relative flow with convergence (divergence) generally associated with asymmetric inflow (outflow) in the eyewall region. The shift in the outer precipitation is consistent with a shift in the low-level relative inflow from the northeastern to the northwestern side of the storm. The changes in the relative flow are associated with changes in the environmental winds as the hurricane moves relative to the upper trough and the low-level high pressure system. Examination of the shear-induced tilt of the vortex shows that the change in the tilt direction is greater than that of the shear direction as the tilt shifts from a northerly orientation to northwesterly. Consistent with theory for adiabatic vortices, the maximum low-level convergence and upper-level divergence (and the maximum upward motion) occurs in the direction of tilt. Consequently, both mechanisms may play roles in the changes in the precipitation pattern.

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Haiyan Jiang, Jeffrey B. Halverson, Joanne Simpson, and Edward J. Zipser

Abstract

Part I of this two-part paper examined the satellite-derived rainfall accumulation and rain potential history of Hurricanes Isidore and Lili (2002). This paper (Part II) uses analyses from the Navy Operational Global Atmospheric Prediction System (NOGAPS) to examine the water budget and environmental parameters and their relationship to the precipitation for these two storms. Factors other than storm size are found to account for large volumetric differences in storm total rainfall between Lili and Isidore. It is found that the horizontal moisture convergence was crucial to the initiation and maintenance of Isidore’s intense rainfall before and during its landfall. When the storm was over the ocean, the ocean moisture flux (evaporation) was the second dominant term among the moisture sources that contribute to precipitation. During Isidore’s life history, the strong horizontal moisture flux convergence corresponded to the large storm total precipitable water. The large difference in budget-derived stored cloud ice and liquid water between Isidore and Lili is corroborated from Tropical Rainfall Measuring Mission (TRMM) measurements. During Isidore’s landfall, the decrease in environmental water vapor contributed to rainfall in a very small amount. These results indicate the importance of the environmental precipitable water and moisture convergence and ocean surface moisture flux in generating Isidore’s large rainfall volume and inland flooding as compared with Lili’s water budget history. Both the moisture convergence and ocean flux were small for Lili.

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Haiyan Jiang, Jeffrey B. Halverson, Joanne Simpson, and Edward J. Zipser

Abstract

The Tropical Rainfall Measuring Mission–based National Aeronautics and Space Administration Goddard Multisatellite Precipitation Analysis (MPA) product is used to quantify the rainfall distribution in tropical cyclones that made landfall in the United States during 1998–2004. A total of 37 tropical cyclones (TC) are examined, including 2680 three-hourly MPA precipitation observations. Rainfall distributions for overland and overocean observations are compared. It is found that the TC rainfall over ocean bears a strong relationship with the TC maximum wind, whereas the relationship for overland conditions is much weaker. The rainfall potential is defined by using the satellite-derived rain rate, the satellite-derived storm size, and the storm translation speed. This study examines the capability of the overocean rainfall potential to predict a storm’s likelihood of producing heavy rain over land. High correlations between rain potentials before landfall and the maximum storm total rain over land are found using the dataset of the 37 landfalling TCs. Correlations are higher with the average rain potential on the day prior to landfall than with averages over any other time period. A TC overland rainfall index is introduced based on the rainfall potential study. This index can be used to predict the storm peak rainfall accumulation over land. Six landfalling storms during the 2005 Atlantic Ocean hurricane season are examined to verify the capability of using this index to forecast the maximum storm total rain over land in the United States. The range of the maximum storm overland rain forecast error for these six storms is between 2.5% and 24.8%.

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J. Simpson, E. Ritchie, G. J. Holland, J. Halverson, and S. Stewart

Abstract

With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.

A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.

The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during . In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.

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G. M. Heymsfield, Joanne Simpson, J. Halverson, L. Tian, E. Ritchie, and J. Molinari

Abstract

Tropical Storm Chantal during August 2001 was a storm that failed to intensify over the few days prior to making landfall on the Yucatan Peninsula. An observational study of Tropical Storm Chantal is presented using a diverse dataset including remote and in situ measurements from the NASA ER-2 and DC-8 and the NOAA WP-3D N42RF aircraft and satellite. The authors discuss the storm structure from the larger-scale environment down to the convective scale. Large vertical shear (850–200-hPa shear magnitude range 8–15 m s−1) plays a very important role in preventing Chantal from intensifying. The storm had a poorly defined vortex that only extended up to 5–6-km altitude, and an adjacent intense convective region that comprised a mesoscale convective system (MCS). The entire low-level circulation center was in the rain-free western side of the storm, about 80 km to the west-southwest of the MCS. The MCS appears to have been primarily the result of intense convergence between large-scale, low-level easterly flow with embedded downdrafts, and the cyclonic vortex flow. The individual cells in the MCS such as cell 2 during the period of the observations were extremely intense, with reflectivity core diameters of 10 km and peak updrafts exceeding 20 m s−1. Associated with this MCS were two broad subsidence (warm) regions, both of which had portions over the vortex. The first layer near 700 hPa was directly above the vortex and covered most of it. The second layer near 500 hPa was along the forward and right flanks of cell 2 and undercut the anvil divergence region above. There was not much resemblance of these subsidence layers to typical upper-level warm cores in hurricanes that are necessary to support strong surface winds and a low central pressure. The observations are compared to previous studies of weakly sheared storms and modeling studies of shear effects and intensification.

The configuration of the convective updrafts, low-level circulation, and lack of vertical coherence between the upper- and lower-level warming regions likely inhibited intensification of Chantal. This configuration is consistent with modeled vortices in sheared environments, which suggest the strongest convection and rain in the downshear left quadrant of the storm, and subsidence in the upshear right quadrant. The vertical shear profile is, however, different from what was assumed in previous modeling in that the winds are strongest in the lowest levels and the deep tropospheric vertical shear is on the order of 10–12 m s−1.

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J. B. Halverson, J. Simpson, G. Heymsfield, H. Pierce, T. Hock, and L. Ritchie

Abstract

A combination of multiaircraft and several satellite sensors were used to examine the core of Hurricane Erin on 10 September 2001, as part of the Fourth Convection and Moisture Experiment (CAMEX-4) program. During the first set of aircraft passes, around 1700 UTC, Erin was still at its maximum intensity with a central pressure of 969 hPa and wind speed of 105 kt (54 m s−1).

The storm was moving slowly northwestward at 4 m s−1, over an increasingly colder sea surface. Three instrumented aircraft, the National Oceanic and Atmospheric Administration (NOAA) P3 with radar, the National Aeronautics and Space Administration (NASA) ER-2 at 19 km, newly equipped with GPS dropwindsondes, and the NASA DC-8 with dropwindsondes flew in formation across the eye at about 1700 UTC and again 2.5 h later around 1930 UTC. The storm had weakened by 13 m s−1 between the first and second eye penetrations. The warm core had a maximum temperature anomaly of only 11°C, located at 500 hPa, much weaker and lower than active hurricanes. The core appeared to slant rearward above 400 hPa. Even on the first penetration, airborne radar showed that the eyewall cloud towers were dying. The tops fell short of reaching 15 km and a melting band was found throughout. The tropopause had a bulge to 15.8-km elevation (environment ∼14.4 km) above the dying convection.

The paper presents a consistent picture of the vortex in shear interaction from a primarily thermodynamic perspective. A feature of Erin at this time was a pronounced wavenumber-1 convective asymmetry with all convective activity being confined to the forward quadrants on the left side of the shear vector as calculated from analyses. This is similar to that predicted by the mesoscale numerical models, which also predict that such small amounts of shear would not affect the storm intensity. In Erin, it is remarkable that relatively small shear produced such a pronounced asymmetry in the convection. From the three-dimensional analysis of dropsonde data, horizontal asymmetries in lower and middle tropospheric warming were identified. The warm anomalies are consistent with the pattern of mesoscale vertical motions inferred from the shear-induced wavenumber-1 asymmetry, dipole in rain intensity, and surface convergence.

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S. Lang, W-K. Tao, J. Simpson, R. Cifelli, S. Rutledge, W. Olson, and J. Halverson

Abstract

The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

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W-K. Tao, S. Lang, W. S. Olson, R. Meneghini, S. Yang, J. Simpson, C. Kummerow, E. Smith, and J. Halverson

Abstract

This paper represents the first attempt to use Tropical Rainfall Measuring Mission (TRMM) rainfall information to estimate the four-dimensional latent heating structure over the global Tropics for one month (February 1998). The mean latent heating profiles over six oceanic regions [Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Flux Array (IFA), central Pacific, South Pacific Convergence Zone (SPCZ), east Pacific, Indian Ocean, and Atlantic Ocean] and three continental regions (South America, central Africa, and Australia) are estimated and studied. The heating profiles obtained from the results of diagnostic budget studies over a broad range of geographic locations are used to provide comparisons and indirect validation for the heating algorithm–estimated heating profiles. Three different latent heating algorithms, the Goddard Space Flight Center convective–stratiform heating (CSH), the Goddard profiling (GPROF) heating, and the hydrometeor heating (HH) algorithms are used and their results are intercompared. The horizontal distribution or patterns of latent heat release from the three different heating retrieval methods are very similar. They all can identify the areas of major convective activity [i.e., a well-defined Intertropical Convergence Zone (ITCZ) in the Pacific, a distinct SPCZ] in the global Tropics. The magnitudes of their estimated latent heating release are also in good agreement with each other and with those determined from diagnostic budget studies. However, the major difference among these three heating retrieval algorithms is the altitude of the maximum heating level. The CSH algorithm–estimated heating profiles only show one maximum heating level, and the level varies among convective activity from various geographic locations. These features are in good agreement with diagnostic budget studies. A broader maximum of heating, often with two embedded peaks, is generally derived from applications of the GPROF heating and HH algorithms, and the response of the heating profiles to convective activity is less pronounced. Also, GPROF and HH generally yield heating profiles with a maximum at somewhat lower altitudes than CSH. The impact of different TRMM Microwave Imager (TMI) and precipitation radar (PR) rainfall information on latent heating structures was also examined. The rainfall estimated from the PR is smaller than that estimated from the TMI in the Pacific (TOGA COARE IFA, central Pacific, SPCZ, and east Pacific) and Indian Oceans, causing weaker latent heat release in the CSH algorithm–estimated heating. In addition, the larger stratiform amounts derived from the PR over South America and Australia consequently lead to higher maximum heating levels. Sensitivity tests addressing the appropriate selection of latent heating profiles from the CSH lookup table were performed.

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