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V. Mohan Karyampudi and Toby N. Carlson

Abstract

A Conceptual model of the Saharan Air Layer (SAL) and easterly wave disturbance is presented in light of diagnostic analyses of dust outbreaks.

Numerical simulations of the SAL were carried out to 5 days for two case studies using the Penn State/NCAR limited-area tropical model. The region of simulations encompasses North Africa and the eastern tropical Atlantic Ocean. One set of simulations used a horizontal resolution of 220 km. Analysis of the simulations emphasize the structure of the SAL and easterly wave disturbance and evaluation is made with reference to available observations and a conceptual model. Because both cases are similar, emphasis of the sensitivity tests is placed on the August 1974 case only. These tests include the effect of enhancing the SAL in the initial conditions, the role of surface sensible heating, the role of latent heating in the atmosphere, and the effect of heating due to radiative warming of the aerosol. A fine-mesh simulation of 110 km was also made to resolve the mesoscale features of the SAL.

Topics treated in the discussion include 1) the interaction of the SAL with attendant easterly wave disturbances, 2) the frontal structure of the SAL along the leading and southern boundary of the SAL, 3) forcing of vertical motions and the transverse/vertical circulations in the SAL front, 4) the nature of the anticyclonic curvature of the SAL plume along the coast of Africa and 5) the role of aerosol radiative heating in preserving the characteristics of the SAL as it moves toward the west. A significant conclusion is that the SAL contributes to forcing of vertical motions and cumulus convection and is therefore important (if not necessary) in the initial development of some easterly wave disturbances. Without surface heating over the Sahara or a proper initialization of the desert mixing layer, atmospheric forcing tends to be very much weaker than for the cam where a deep SAL is present.

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V. Mohan Karyampudi and Harold F. Pierce

Abstract

The formations of Hurricane Andrew, Tropical Storm Ernesto, and Hurricane Luis, which occurred, respectively, during the 1992, 1994, and 1995 hurricane seasons over the eastern Atlantic, have been investigated by utilizing the European Centre for Medium-Range Weather Forecasts (ECMWF) gridded data analyses. These cases were selected to illustrate the contrasting influences of the Saharan air layer (SAL) on tropical cyclogenesis.

Analyses results show that Tropical Storm Ernesto (1994) and Hurricane Luis (1995) formed from the merger of the low-level (925 hPa) and midlevel (700 hPa) vortices over the eastern Atlantic within the monsoon trough enhanced by surges in the trades. Midlevel vortices associated with each case appear to evolve from African wave troughs enhanced by cyclonic shear vorticity of the midtropospheric jet, which existed to the south of an SAL anticyclonic eddy as an elongated wind maximum. Vorticity budget calculations suggest that vortex stretching dominated the enhancement of low-level vortices, whereas positive vorticity advection (PVA) on the south and leading edge of the midlevel easterly jet (MLEJ) but ahead of the trough axis contributed to the enhancement of midlevel vortices for both cases. Persistent upper-level divergence associated with an anticyclonic circulation appears to have aided in the formation of Ernesto, whereas for Luis, no such prior forcing is evident.

Hurricane Andrew (1992), on the other hand, appears to form from a deep African wave vortex. Vortex stretching contributed to the development of low-level vortices. Although cyclonic shear vorticity to the south of the MLEJ is present in association with a deeper and wider SAL devoid of its characteristic anticyclonic eddy (unlike in Ernesto and Luis), the midlevel contribution from PVA on the south side of the jet to the maintenance of the midlevel vortex is found to be insignificant in Andrew due to negligible cross- (vorticity) contour flow to the south and ahead of the wave trough. However, the pre-Andrew growth was dominated by PVA at upper levels associated with easterly wave perturbations to the south of an anticyclonic circulation center but to the north of an upper-level easterly jet.

In at least two cases (i.e., Ernesto and Luis), the SAL directly contributed to the negative PV anomalies to the north of the MLEJ, which resulted in the sign reversal of the meridional gradient of potential vorticity (between 850- and 700-hPa levels), which satisfies the Charney and Stern criterion for barotropic and baroclinic instability across the midtropospheric jet over the eastern Atlantic. The baroclinic mechanism, proposed by Karyampudi and Carlson, is found to be valid in explaining some of the wave growth processes involved in the genesis of the same two cases. Based on these results, it is concluded that SAL had a positive influence on at least two cases [both (Ernesto and Luis) occurred in normal Sahel rainfall years], in contrast to a negative influence on Andrew, which occurred in an extremely dry year.

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John Manobianco, Steven Koch, V. Mohan Karyampudi, and Andrew J. Negri

Abstract

The present study uses a regional-scale numerical model to test the impact of dynamically assimilating, satellite-derived precipitation rates on the numerical simulations of one of the deepest extratropical cyclones to develop south of 40°N in this century. This cyclone event occurred during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) intensive observing period 4 and has been selected because of the strength of the cyclone and the availability of the special ERICA data in addition to the Special Sensor Microwave/Imager (SSM/I) and Geostationary Operational Environmental Satellite (GOES) infrared (IR) satellite data.

The unique methodology developed herein to synthesize the SSM/I and GOES IR satellite data produces precipitation estimates that have realistic spatial and temporal structure. The assimilation of satellite-derived precipitation is accomplished by scaling the internally generated model profiles of total latent heating. At points where the model is not producing precipitation, the vertical distribution of total latent heating given by satellite precipitation is specified from instantaneous model-based profiles at adjacent points using a search algorithm. The technique does not assume a priori that the satellite-estimated precipitation corresponds to either convective or stratiform model precipitation, and uses heating profiles that are consistent with the model's parameterization of either type of precipitation since they are not specified from externally based parabolic or other structure functions.

Several simulations are performed with and without satellite data assimilation at varying horizontal and vertical model resolutions. The results from the 80-km 40-layer control and assimilation runs demonstrate that the assimilation of satellite precipitation 1) does not introduce noise into the simulations at any time during or after the data assimilation period, 2) forces the model to reproduce the magnitude and distribution of satellite precipitation, and 3) improves the simulated central mean sea level pressure (MSLP) minima slightly, frontal positions, and, to a greater extent, the low-level vertical-motion patterns when compared with subjective analyses and satellite imagery. The model retains the information introduced by the assimilation of satellite-derived precipitation 8.5 h after the end of the data assimilation period.

An increase in the vertical and horizontal model resolution further reduces the errors in simulating the MSLP minima but does not consistently improve the cyclone position errors in the assimilation runs. Either the exclusion of the search algorithm, the doubling of satellite precipitation, or an eastward shift of satellite precipitation by 400 km increases the MSLP and position users; therefore, the impact of assimilating satellite precipitation depends on model resolution, the use of the search algorithm, and the magnitude and position of satellite precipitation. The increase in horizontal resolution generates the largest reduction in MSLP errors, while the shifting of satellite precipitation generates the largest increase in MSLP errors. The results confirm the findings of earlier studies that the impact of assimilating satellite precipitation on the subsequent simulations is less sensitive to errors in magnitude rather than to the distribution of satellite-derived precipitation and depends on the relative accuracy with which the model simulates the cyclone in the control run. Despite the fact that this study focuses on a single case, it does demonstrate the promise of using combined infrared and microwave satellite precipitation estimates to produce sustained positive impacts in mesoscale model forecasts of midlatitude cyclogenesis over data-sparse oceanic regions.

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V. Mohan Karyampudi, George S. Lai, and John Manobianco

Abstract

Numerical simulations were performed with the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model Version 5 (MM5) to study the impact of initial conditions, satellite-derived rain assimilation, and cumulus parameterization on Hurricane Florence (1988). A few modifications were made to the J. Manobianco et al. (MKKN) rain assimilation scheme, which was developed originally for midlatitude weather systems, to successfully simulate organized tropical weather systems such as Florence. These changes consist of replacing latent heating scaling with convective rainfall in the Kuo–Anthes scheme in areas where both the model-predicted and satellite-derived rainfall coincide, and specifying a normalized parabolic heating profile in deep convective regions where there is satellite rain but no model rain. Restoration of the original Kuo–Anthes heating distribution function in lieu of the fixed heating profile specified in the MM5 model is another change implemented in the Kuo–Anthes scheme.

Results from the sensitivity simulations made with the modified rain assimilation scheme show that 1) the enhanced initial conditions with the omega dropsonde data yield a positive impact on the development of Florence for both the Betts–Miller and the modified Kuo–Anthes schemes, 2) the effect of ingesting continuous (Special Sensor Microwave/Imager and Geostationary Operational Environmental Satellite Infrared) satellite-derived rainfall rates as latent heating by the modified rain assimilation scheme is much greater with the modified Kuo–Anthes scheme than with the Betts–Miller scheme, and 3) the combined impact of enhanced initial conditions and rain assimilation yields a superior simulation of Florence, particularly with the Kuo–Anthes scheme. The weak response of the Betts–Miller scheme to rain assimilation, compared to the large impact with the Kuo–Anthes scheme, appears to be related mainly to the differences in the treatment of convective rainfall and its latent heat release in respective cumulus parameterization schemes. Since the MKKN scheme mainly invokes latent heat scaling to ingest satellite rainfall, the Kuo–Anthes scheme responds to increased latent heating from satellite rainfall rates more favorably through conditional instability of the second kind (CISK)-type feedback effects than the Betts–Miller scheme. The latter result clearly suggests that the success of the modified rain assimilation scheme on development of organized tropical systems such as Hurricane Florence depends to a large extent on the choice of cumulus parameterization scheme.

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Steven E. Koch, Jeffery T. McQueen, and V. Mohan Karyampudi

Abstract

The effects of sensible heating and momentum mixing on the low-level structure and dynamics of a two-dimensional cold front are studied with a hydrostatic primitive equation model. Effects of inhomogeneous heating arising from a contrast in low-level cloud cover across the front are emphasized. The relative importance of grid resolution and the choice of method for parameterizing planetary boundary layer (PBL) processes in the model are also examined. Frontal updraft dynamics are studied in terms of the following inquiries: (a) the relative importance of turbulent momentum transport, differential sensible heating, and the reduction in static stability in the heated region ahead of the front; (b) the nature of the interaction between the adiabatic, semigeostrophic frontal circulation and the thermally forced circulation; and (c) possible roles played by dry symmetric instability and density current dynamics. The terms in the frontogenesis and divergence budget equations are computed to determine the relative roles played by the various physical and dynamical processes in generating the frontal secondary circulation system.

A strong, narrow updraft jet forms in the presence of uniform sensible beating across the front. Although the greatest impact on frontogenesis occurs as a response to the reduction in static stability resulting from uniform sensible heating, additional forcing results from the nonlinear interaction between the adiabatic frontal circulation and the thermally forced circulation arising from a cross-front gradient in heating (due to the introduction of an overcast low cloud deck behind the front). The relative importance of inhomogeneous heating, however, increases with the grid resolution and the use of a multilevel treatment in place of bulk mixed-layer PBL models.

Numerical experiments reveal that symmetric instability does not create the updraft jet, despite the development of negative potential vorticity ahead of the surface cold front. Highly unbalanced dynamics and a density current-like “feeder flow” behind the cold front are strongly indicated in the presence of sensible heating effects. Budget analyses show that the frontogenetical effect of sensible heating is only indirectly important through its strengthening of the confluence (convergence) field. The nonlinear and unbalanced ageostrophic vorticity terms in the divergence budget equation exert the strongest controls on the development of the updraft jet when sensible heating is nonuniform.

These results suggest that differential cloud cover across cold fronts may promote the development of frontal squall lines. Nonhydrostatic models that include explicit prognostic equations for microphysics and use improved parameterization of boundary layer fluxes in the presence of clouds are needed to more fully address this possibility.

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G. David Alexander, James A. Weinman, V. Mohan Karyampudi, William S. Olson, and A. C. L. Lee

Abstract

Inadequate specification of divergence and moisture in the initial conditions of numerical models results in the well-documented “spinup” problem. Observational studies indicate that latent heat release can be a key ingredient in the intensification of extratropical cyclones. As a result, the assimilation of rain rates during the early stages of a numerical simulation results in improved forecasts of the intensity and precipitation patterns associated with extratropical cyclones. It is challenging, however, particularly over data-sparse regions, to obtain complete and reliable estimates of instantaneous rain rate. Here, a technique is described in which data from a variety of sources—passive microwave sensors, infrared sensors, and lightning flash observations—along with a classic image processing technique (digital image morphing) are combined to yield a continuous time series of rain rates, which may then be assimilated into a mesoscale model. The technique is tested on simulations of the notorious 1993 Superstorm. In this case, a fortuitous confluence of several factors—rapid cyclogenesis over an oceanic region, the occurrence of this cyclogenesis at a time inconveniently placed in between Special Sensor Microwave/Imager overpasses, intense lightning during this time, and a poor forecast in the control simulation—leads to a dramatic improvement in forecasts of precipitation patterns, sea level pressure fields, and geopotential height fields when information from all of the sources is combined to determine the rain rates. Lightning data, in particular, has a greater positive impact on the forecasts than the other data sources.

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V. Mohan Karyampudi, Steven E. Koch, Chaing Chen, James W. Rottman, and Michael L. Kaplan

Abstract

In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection.

Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced over the Rockies. The bore and the cold front then propagated eastward for several hours and interacted with a Ice cyclone, a dryline, and a warm front, initiating severe weather over Nebraska and Kansas. Wave-ducting analysis showed that favorable wave-trapping mechanisms such as a capping inversion above a neutral layer and wind curvature from a low-level jet, which appeared to he the most dominant ducting mechanism, existed across eastern Colorado and western Nebraska to maintain the bore strength. Numerical simulations of continuously stratified shear flow specified from upstream and downstream soundings suggested that the creation of a density current along the Ice slopes, a downstream inversion height lower than the upstream inversion height, and a strong curvature in the wind profile of the low-level jet are all needed to initiate and sustain the integrity of the propagating bore.

Based on the synthesis of observational analyses and 2D nonhydrostatic model simulations, a schematic illustration of the time evolution of the bore ahead of the Pacific cold front, the hydraulic jump associated with a mountain wave, and the arctic air intrusion from the north to the Ice of the Rockies are presented in the context of severe weather occurrence over western Nebraska and Kansas.

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V. Mohan Karyampudi, Michael L. Kaplan, Steven E. Koch, and Robert J. Zamora

Abstract

In this first of a two paper series, a sequence of dynamical processes involving the evolution of a mesoscale Ice cyclone and its subsequent interaction with a mesoscale tropopause fold downstream of the Rocky Mountains is investigated. These scale-interactive phenomena, which resulted from the jet streak interaction with the topography, were examined in detail using the observational data obtained from the Program for Regional Observing and Forecasting Services' mesonetwork and wind profilers, as well as conventional surface and rawin-sonde data and Total Ozone Mapping Spectrometer satellite data over the Colorado region for the severe weather event that occurred during 13–14 April 1986.

Large-scale analysis indicated that as a baroclinic low pressure system approached the Rockies with its attendant upper-level jet streak, a typical prestorm environment over western Kansas formed in the early morning hours of 13 April. Hourly mesonet data analysis revealed the formation and eastward progression of a mesoscale Ice cyclone with a trailing wind-shift line identified as an internal bore initiated by a cold front (i.e., a prefrontal bore) in Part II. Analysis of winds and divergence including diagnostically derived temperature and height fields from Colorado wind profilers indicated that as the jet streak momentum propagated into a Acre stable region in the midtroposphere created by low-level adiabatic warming and midlevel cooling on the leeside of the Rockies, unbalanced flow conditions resulted at scales less than the Rossby radius of deformation. AS a consequence of geostrophic adjustment processes, mesoscale tropopause folding and upper-level frontogenesis occurred over the profiler network. Unbalanced upper-level frontogenesis resulted from the tilting of the isentropes by along-stream ageostrophic indirect circulations comprised of horizontal vertical velocity gradients across the tropopause fold. As the mesoscale tropopause fold extruded downwards to midlevels in association with the descending secondary upper-level jet streak forced by the geostrophic adjustment process, Ice cyclogenesis occurred due to the phasing of the upper-level front with the low-level Ice cyclone.

Synthesis of the mesonetwork and profiler observations suggest that high momentum in the midtroposphere associated with the descending branch of the jet stream just ahead of the prefrontal bore but behind the dryline. This surge of southwesterly momentum at the surface, largely responsible for blowing dust, was mostly ageostrophic and contributed to an increase in surface vorticity and moisture convergence as well as frontogenesis around the lee cyclone. A mesoscale conceptual model is proposed in order to explain the dynamical sequence of events involving lee cyclogenesis, dust stroms, and a tropopause fold that led to the severe weather environment over the Great Plains. In the companion paper (Part II), observational evidence of an internal bore occurring ahead of a cold front and comparisons with simple numerical model results are presented in order to understand the initiation and propagation of the prefrontal bore and its influence in triggering a squall line father downstream.

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Edward B. Rodgers, William S. Olson, V. Mohan Karyampudi, and Harold F. Pierce

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The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane’s rapid intensification and decay stages (3–5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal’s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal’s middle- and upper-tropospheric region during these convective burst episodes allowed Opal’s eyewall to become more buoyant, enhanced the generation of kinetic energy and, thereby, rapidly intensified the system. The observations also suggest that Opal’s intensity became more responsive to the convective burst episodes (i.e., shorter time lag between LHR and intensity and greater maximum wind increase) as Opal became more intense.

Analyses of SSM/I-retrieved parameters, sea surface temperature observations, and the European Centre for Medium-Range Weather Forecasts (ECMWF) data reveal that the convective rainband (CRB) cycles and sea surface and tropopause temperatures, in addition to large-scale environmental forcing, had a profound influence on Opal’s episodes of convective burst and its subsequent intensity. High sea surface (29.7°C) and low tropopause (192 K) temperatures apparently created a greater potential for Opal’s maximum intensity. Strong horizontal moisture flux convergence within Opal’s outer-core regions (i.e., outside 333-km radius from the center) appeared to help initiate and maintain Opal’s CRBs. These CRBs, in turn, propagated inward to help generate and dissipate the eyewall convective bursts. The first CRB that propagated into Opal’s eyewall region appeared to initiate the first eyewall convective burst. The second CRB propagated to within 111 km of Opal’s center and appeared to dissipate the first CRB, subjecting it to subsidence and the loss of water vapor flux. The ECMWF upper-tropospheric height and wind analyses suggest that Opal interacted with a diffluent trough that initated an outflow channel, and generated high values of upper-tropospheric eddy relative angular momentum flux convergence. The gradient wind adjustment processes associated with Opal’s outflow channel, in turn, may have helped to initiate and maintain the eyewall convective bursts. The ECMWF analyses also suggest that a dry air intrusion within the southwestern quadrant of Opal’s outer-core region, together with strong vertical wind shear, subsequently terminated Opal’s CRB cycle and caused Opal to weaken prior to landfall.

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V. Mohan Karyampudi, Stephen P. Palm, John A. Reagen, Hui Fang, William B. Grant, Raymond M. Hoff, Cyril Moulin, Harold F. Pierce, Omar Torres, Edward V. Browell, and S. Harvey Melfi

Lidar observations collected during the Lidar In-space Technology Experiment experiment in conjunction with the Meteosat and European Centre for Medium-Range Weather Forecasts data have been used not only to validate the Saharan dust plume conceptual model constructed from the GARP (Global Atmospheric Research Programme) Atlantic Tropical Experiment data, but also to examine the vicissitudes of the Saharan aerosol including their optical depths across the west Africa and east Atlantic regions. Optical depths were evaluated from both the Meteosat and lidar data. Back trajectory calculations were also made along selected lidar orbits to verify the characteristic anticyclonic rotation of the dust plume over the eastern Atlantic as well as to trace the origin of a dust outbreak over West Africa.

A detailed synoptic analysis including the satellite-derived optical depths, vertical lidar backscattering cross section profiles, and back trajectories of the 16–19 September 1994 Saharan dust outbreak over the eastern Atlantic and its origin over West Africa during the 12–15 September period have been presented. In addition, lidar-derived backscattering profiles and optical depths were objectively analyzed to investigate the general features of the dust plume and its geographical variations in optical thickness. These analyses validated many of the familiar characteristic features of the Saharan dust plume conceptual model such as (i) the lifting of the aerosol over central Sahara and its subsequent transport to the top of the Saharan air layer (SAL), (ii) the westward rise of the dust layer above the gradually deepening marine mixed layer and the sinking of the dust-layer top, (iii) the anticyclonic gyration of the dust pulse between two consecutive trough axes, (iv) the dome-shaped structure of the dust-layer top and bottom, (v) occurrence of a middle-level jet near the southern boundary of the SAL, (vi) transverse–vertical circulations across the SAL front including their possible role in the initiation of a squall line to the southside of the jet that ultimately developed into a tropical storm, and (vii) existence of satellite-based high optical depths to the north of the middle-level jet in the ridge region of the wave.

Furthermore, the combined analyses reveal a complex structure of the dust plume including its origin over North Africa and its subsequent westward migration over the Atlantic Ocean. The dust plume over the west African coastline appears to be composed of two separate but narrow plumes originating over the central Sahara and Lake Chad regions, in contrast to one single large plume shown in the conceptual model. Lidar observations clearly show that the Saharan aerosol over North Africa not only consist of background dust (Harmattan haze) but also wind-blown aerosol from fresh dust outbreaks. They further exhibit maximum dust concentration near the middle-level jet axis with downward extension of heavy dust into the marine boundary layer including a clean dust-free trade wind inversion to the north of the dust layer over the eastern Atlantic region. The lidar-derived optical depths show a rapid decrease of optical depths away from land with maximum optical depths located close to the midlevel jet, in contrast to north of the jet shown by satellite estimates and the conceptual model. To reduce the uncertainties in estimating extinction-to-backscattering ratio for optical depth calculations from lidar data, direct aircraft measurements of optical and physical properties of the Saharan dust layer are needed.

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