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Teddy R. Holt and Sethu Raman

Abstract

The interaction of oceanic fronts in the vicinity of the Gulf Stream with an atmospheric coastal front during the Genesis of Atlantic Lows Experiment (GALE) is examined using aircraft, satellite, and ship data. The nearshore, midshelf, and Gulf Stream oceanic fronts are readily discernible from low-level aircraft radiometer and satellite imagery data. The three-dimensional (3D) structure of the coastal front is extensively mapped by low-level aircraft transects through the frontal boundary.

Results confirm the existence of the coastal front as a very shallow (depth less than 200 m), spatially inhomogeneous, undulating material surface. Aircraft observations from 2000 to 2200 UTC (late afternoon local time) show a surface location of the coastal front that is aligned over the Gulf Stream oceanic front under conditions of very weak (2 m s−1) onshore flow, but is observed to migrate shoreward for stronger on-shore flow.

Ahead of the front in the warm air, the marine atmospheric boundary layer is characterized as well mixed with broken cumulus and stratocumulus cloud bases observed near 500 m, and tops varying from 1300 to 1900 m. The dominant scale of turbulent eddies is observed to be on the order of the boundary-layer depth. Conditional sampling statistics point to a strong direct circulation ahead of the front dominated by intense, narrow, warm updrafts, and broader, less intense, cool downdrafts.

Behind the coastal front in the cold air, visibility is much reduced by low-level fractus and layered stratocumulus clouds. The shallow subcloud layer is observed to be generally moister and more statically stable than ahead of the front. It is also characterized by an indirect circulation with more prevalent cool updrafts and warm downdrafts, particularly for the near–cloud-base region.

However, behind the front there exists a strong thermodynamic coupling of atmosphere and ocean as evidenced by the distinctly different atmospheric regimes present over the oceanic nearshore and midshelf front regions. Over the nearshore region, the horizontal wind structure is dominated by 100-m waves imbedded in a weaker 1–2-km circulation. Warm updrafts are observed over the nearshore waters, but the smaller air-sea temperature difference effectively limits large temperature perturbations. Hence, much smaller sensible heat flux is evident over the nearshore region as compared to the oceanic midshelf region. Over the midshelf region, turbulent eddies on the scale of 1.5 times the depth of the front (120 m) are solely responsible for the larger positive beat flux. The transition zone of the coastal front aloft near 150 m is remarkably confined to just the oceanic nearshore shelf, located between the nearshore waters and the midshelf region.

The frontal surface itself is observed to play an important role in the 3D atmospheric circulation in the vicinity of the front. The front causes a decoupling of the region just above the frontal surface by inhibiting the vertical transfer of fluxes from the surface. Cospectra for regions just above the front show no contributions from smaller waves generated by near-surface processes (on the order of 100–500 m) that are evident just ahead of the front. This suggests a decoupling due to the frontal boundary. Associated with this decoupling and the subsequent stabilization of the region above the front is the occurrence of buoyancy waves. These waves of wavelength approximately 840 m are believed to be a result of penetrating thermals and/or instabilities present along the frontal surface.

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Teddy R. Holt and Simon W. Chang

Abstract

Sensitivity of coastal cyclogenesis to the effects of timing of diabatic processes is investigated using the Naval Research Laboratory mesoscale model. Numerical experiments were conducted to examine the sensitivity of the intensification and propagation of a coastal cyclone to changes in the timing of latent heat release due to cumulus convection, surface fluxes, and low-level baroclinicity.

The NMC Regional Analysis and Forecast System analysis of the GALE IOP 2 coastal cyclone was unable to resolve the initial subsynoptic-wale cyclogenesis. Hence, tracking and identification of a well-defined coastal cyclone was difficult operationally. However, the control model experiment having full physics, initialized with the NMC analyses, was able to properly simulate the development of the coastal cyclone. Results from the control experiment agree with the more accurate Fleet Numerical Oceanographic Center low-level analysis. The numerical experiments suggest the development of the surface cyclone was a result of proper superposition and interaction of the upper-level forcing and the low-level baroclinic zone.

Altering the timing of latent heat release due to cumulus convection in the control experiment indicates that for the initial 12 h of cyclogenesis, cumulus convection as determined by the modified Kuo scheme has little effect on the deepening of the surface system but strongly changes the alignment of the trough by retarding the eastward propagation. It is during the second 12 h of cyclogenesis that cumulus convection is crucial for rapid cyclogenesis. Imposing a zonal sea surface temperature, in addition to withholding cumulus heating, has the most impact once the system has reached the coast. The enhanced coastal baroclinicity due to the zonal SST distribution causes the surface cyclone to propagate closer to the coast and more slowly than the control experiment. Allowing no surface fluxes, in addition to no cumulus convection, cools and stabilizes the boundary layer and inhibits surface intensification. The strong coastal baroclinicity is weakened without surface fluxes and the cyclone remains well onshore.

An experiment to modify the phasing of the low-level baroclinic zone is conducted by imposing an additional linear increase in ground surface temperature to the typical diurnal heating cycle as well as eliminating ocean surface sensible beat flux for the initial 12 h of cyclogenesis. This results in a low-level temperature field that is out of phase with the typical diurnal surface evolution. The surface cyclone deepens much more rapidly [41 mb (24 h)−1] than the control experiment and remains more onshore with relatively little movement. In addition, potential vorticity analysis suggests that the upper levels for this experiment have much weaker protrusions of high potential vorticity into the lower troposphere compared to the control experiment.

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Simon W. Chang and Teddy R. Holt

Abstract

A series of observing system simulation experiments (OSSE) and real data assimilation experiments were conducted to assess the impact of assimilating Special Sensor Microwave/Imager (SSM/I)-estimated rainfall rates on limited-area model predictions of intense winter cyclones.

For the OSSE, the slow-moving, fronto- and cyclogenesis along the cast coast of United States during the second intensive observation period (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE) (26-28 January 1986) was selected as the test case. The perfect “observed” rainfall rates were obtained by an integration of a version of the Naval Research Laboratory (NRL) limited-area model, whereas the “forecast” was generated by a degraded version of the NRL model. A number of OSSEs were conducted in which the “observed” rainfall rates were assimilated into the “forecast” model. Rainfall rates of various data frequencies, different vertical beating profiles, various assimilation windows, and prescribed systematic errors were assimilated to test the sensitivity of the impact. It was found that assimilation of rainfall rates, in general, improves the forecast in terms of sea level pressure S1 scores when either the “observed” or model-determined vertical beating profiles were used. The improvement was insensitive to the error in rainfall magnitude estimates but was sensitive to errors in geographic locations of the precipitation. More frequent observations (additional sensors in orbits) had positive but gradually diminishing benefits.

Real SSM/I-measured rainfall rates were assimilated for the rapid-moving, intense marine cyclone of IOP 4 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) (4–5 January 1989), which started from an initial offshore disturbance with a minimum pressure of 998 mb at 0000 UTC 4 January and developed into a very intense storm of 937 mb 24 h later. The NRL model simulated a well-behaved but less intense cyclogenesis episode based on the RAFS (Regional Analysis and Forecast System) initial analysis, reaching a minimum sea level pressure of 952 mb at 24 h. The first SSM/I aboard a DMSP (Defense Meteorological Satellite Program) satellite flew over the marine cyclone at 0000, 0930, and 2200 UTC 4 January and measured rainfall rates over portions of the warm and cold fronts associated with the cyclone. The SSM/I rainfall rates at 0000 and 0930 UTC were assimilated into the model as latent heating functions in ±3-h windows with model-determined vertical profiles. Two different methods were used to define the latent heating rates for the model in the assimilation experiments: 1) the model heating rates were defined by the maximum of the model computed and the SSM/I measured, and 2) the model beating rates were replaced by the SSM/I-measured rainfall rates within the SSM/I swath. Results of the assimilation experiments indicated that the assimilation in general leads to better intensity forecasts. The best forecast with assimilation predicted a 24-h minimum surface pressure of 943 mb, cutting the forecast error of the “no sat” forecast by 50%. This most efficient assimilation was carried out with assimilations of two-time SSM/I observations using the swath method. Further analysis indicated that the assimilation also resulted in better track and structure forecasts.

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Ming Liu, Douglas L. Westphal, Teddy R. Holt, and Qin Xu

Abstract

The Lut Desert of Iran is an elongated valley oriented north-northwest to south-southeast. The valley descends southward to the Jaz Murian dry lake through a pass. The Navy’s Coupled Ocean–Atmosphere Mesoscale Prediction System is used to study a northerly low-level jet in the valley and across the dry lake. The dynamics of the jet are investigated with force balance and Froude numbers to determine the contribution of various mechanisms to the jet formation and maintenance. The jet is initiated as a channeled gap flow in the convergent topography of the Lut valley by the valley-parallel pressure gradients generated by the large-scale processes and by the presence of cold air over the valley’s sloping terrain. The pressure gradient is mainly counteracted by the frictional force. The imbalance between them controls the intensity and persistence of the jet in the valley. Farther south, the jet evolves into a downslope flow resembling a hydraulic jump on the steep slope of the dry lake. A transition of subcritical situation to supercritical faster flow is found at the mountain crest between the Lut valley and dry lake. The depth of stably stratified cold layer, the static stability of upstream inversion, and magnitude of upstream winds all determine the jet configuration over the dry lake. The lee troughing over the Gulf of Oman and the Persian Gulf, as the large-scale inland flow crosses the coastal mountains, supports this low-level jet through the increased along-jet pressure gradient. The jet is also influenced by diurnal forcing, being strong at night and weak during daytime.

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Simon W. Chang, Teddy R. Holt, and Keith D. Sashegyi

Abstract

A numerical study is conducted using the Naval Research Laboratory (NRL) limited-area model to study the evolution and structure of a rapidly intensifying marine cyclone observed during intensive observing period 4 (IOP 4; 4–5 January 1989) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) over the North Atlantic Ocean.

The single grid version of the NRL model used in the study has 16 layers in the vertical with a horizontal resolution of 1/3° longitude and 1/4° latitude. The primitive equation, hydrostatic model in sigma coordinates includes parameterized physics of cumulus convection, radiation, and the planetary boundary layer. The National Meteorological Center (NMC) Regional Analysis Forecast System (RAFS) analysis is used to provide the initial and boundary conditions.

Starting from the 0000 UTC 4 January RAFS initialization, the control model simulates the ensuing cyclogenesis, deepening the initial disturbance from 998 to 952 mb in 24 h. While the simulated cyclone is about 15 mb weaker than that observed, the simulation reproduced many of the well-documented observed features of the IOP 4 cyclone, such as the remarkable comma-shaped precipitation pattern, bent-back warm front, warm-core seclusion, and secondary cold front. Control model results show that (i) the strongest temperature and water vapor gradients are aligned with the warm front and secondary cold front, not the primary cold front, (ii) the major precipitation and strongest vertical motion are along the warm front and its bent-back extension, (iii) the cyclonic circulation is displaced well to the southwest of the triple point, and (iv) the cellular convection occurs behind the secondary cold front accompanied by extreme surface sensible and latent beat transfer with a total maximum flux exceeding 3000 W m−2 over the Gulf Stream approximately 100 km offshore of the Carolinas. A detailed analysis of model results is performed and is found to be in excellent agreement with available satellite and mesoscale observations.

Sensitivity experiments are also conducted to identify the importance of various dynamical and physical processes contributing to the rapid intensification. Results from sensitivity tests show that (i) the dynamic processes are more responsible for the rapid intensification and unique structure of the marine cyclone than the physical processes, (ii) both the sea surface heat transfer and the release of latent heat in clouds contribute positively to the cyclogenesis, (iii) physical processes combine to intensify the storm in a nonlinear fashion, and (iv) the formation of unique features associated with the IOP 4 storm such as the bent-back extension of the warm front, warm-core seclusion, and westward development of the low pressure center away from the triple point are not sensitive to physical processes.

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Teddy R. Holt, Dev Niyogi, Fei Chen, Kevin Manning, Margaret A. LeMone, and Aneela Qureshi

Abstract

Numerical simulations are conducted using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to investigate the impact of land–vegetation processes on the prediction of mesoscale convection observed on 24–25 May 2002 during the International H2O Project (IHOP_2002). The control COAMPS configuration uses the Weather Research and Forecasting (WRF) model version of the Noah land surface model (LSM) initialized using a high-resolution land surface data assimilation system (HRLDAS). Physically consistent surface fields are ensured by an 18-month spinup time for HRLDAS, and physically consistent mesoscale fields are ensured by a 2-day data assimilation spinup for COAMPS. Sensitivity simulations are performed to assess the impact of land–vegetative processes by 1) replacing the Noah LSM with a simple slab soil model (SLAB), 2) adding a photosynthesis, canopy resistance/transpiration scheme [the gas exchange/photosynthesis-based evapotranspiration model (GEM)] to the Noah LSM, and 3) replacing the HRLDAS soil moisture with the National Centers for Environmental Prediction (NCEP) 40-km Eta Data Assimilation (EDAS) operational soil fields.

CONTROL, EDAS, and GEM develop convection along the dryline and frontal boundaries 2–3 h after observed, with synoptic-scale forcing determining the location and timing. SLAB convection along the boundaries is further delayed, indicating that detailed surface parameterization is necessary for a realistic model forecast. EDAS soils are generally drier and warmer than HRLDAS, resulting in more extensive development of convection along the dryline than for CONTROL. The inclusion of photosynthesis-based evapotranspiration (GEM) improves predictive skill for both air temperature and moisture. Biases in soil moisture and temperature (as well as air temperature and moisture during the prefrontal period) are larger for EDAS than HRLDAS, indicating land–vegetative processes in EDAS are forced by anomalously warmer and drier conditions than observed. Of the four simulations, the errors in SLAB predictions of these quantities are generally the largest.

By adding a sophisticated transpiration model, the atmospheric model is able to better respond to the more detailed representation of soil moisture and temperature. The sensitivity of the synoptically forced convection to soil and vegetative processes including transpiration indicates that detailed representation of land surface processes should be included in weather forecasting models, particularly for severe storm forecasting where local-scale information is important.

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Ian A. Renfrew, G. W. K. Moore, Teddy R. Holt, Simon W. Chang, and Peter Guest

This report discusses the design and implementation of a specialized forecasting system that was set up to support the observational component of the Labrador Sea Deep Convection Experiment. This ongoing experiment is a multidisciplinary program of observations, theory, and modeling aimed at improving our knowledge of the deep convection process in the ocean, and the air–sea interaction that forces it. The observational part of the program was centered around a cruise of the R/V Knorr during winter 1997, as well as several complementary meteorological research flights. To aid the planning of ship and aircraft operations a specially tailored mesoscale model was run over the Labrador Sea, with the model output postprocessed and transferred to a remote field base. The benefits of using a warm-start analysis cycle in the model are discussed. The utility of the forecasting system is illustrated through a description of the flight planning process for several cases. The forecasts proved to be invaluable both in ship operations and in putting the aircraft in the right place at the right time. In writing this narrative the authors hope to encourage the use of similar forecasting systems in the support of future field programs, something that is becoming increasingly possible with the rise in real-time numerical weather prediction.

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Jainn J. Shi, Simon W. Chang, Teddy R. Holt, Timothy F. Hogan, and Douglas L. Westphal

Abstract

In support of the Department of Defense's Gulf War Illness study, the Naval Research Laboratory (NRL) has performed global and mesoscale meteorological reanalyses to provide a quantitative atmospheric characterization of the Persian Gulf region during the period between 15 January and 15 March 1991. This paper presents a description of the mid- to late-winter synoptic conditions, mean statistical scores, and near-surface mean conditions of the Gulf War theater drawn from the 2-month reanalysis.

The reanalysis is conducted with the U.S. Navy's operational global and mesoscale analysis and prediction systems: the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). The synoptic conditions for the 2-month period can be characterized as fairly typical for the northeast monsoon season, with only one significant precipitation event affecting the Persian Gulf region.

A comparison of error statistics to those from other mesoscale models with similar resolution covering complex terrains (though in different geographic locations) is performed. Results indicate similar if not smaller error statistics for the current study even though this 2-month reanalysis is conducted in an extremely data-sparse area, lending credence to the reanalysis dataset.

The mean near-surface conditions indicate that variability in the wind and temperature fields arises mainly because of the differential diurnal processes in the region characterized by complex surface characteristics and terrain height. The surface wind over lower elevation, interior, land regions is mostly light and variable, especially in the nocturnal surface layer. The strong signature of diurnal variation of sea–land as well as lake–land circulation is apparent, with convergence over the water during the night and divergence during the day. Likewise, the boundary layer is thus strongly modulated by the diurnal cycle near the surface. The low mean PBL height and light mean winds combine to yield very low ventilation efficiency over the Saudi and Iraqi plains.

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Ming Liu, Douglas L. Westphal, Annette L. Walker, Teddy R. Holt, Kim A. Richardson, and Steven D. Miller

Abstract

Dust storms are a significant weather phenomenon in the Iraq region in winter and spring. Real-time dust forecasting using the U.S. Navy’s Coupled Ocean–Atmospheric Mesoscale Prediction System (COAMPS) with an in-line dust aerosol model was conducted for Operation Iraqi Freedom (OIF) in March and April 2003. Daily forecasts of dust mass concentration, visibility, and optical depth were produced out to 72 h on nested grids of 9-, 27-, and 81-km resolution in two-way nest interaction. In this paper, the model is described, as are examples of its application during OIF. The model performance is evaluated using ground weather reports, visibility observations, and enhanced satellite retrievals. The comparison of the model forecasts with observations for the severe dust storms of OIF shows that COAMPS predicted the arrival and retreat of the major dust events within 2 h. In most cases, COAMPS predicted the intensity (reduction in visibility) of storms with an error of less than 1 km. The forecasts of the spatial distribution of dust fronts and dust plumes were consistent with those seen in the satellite images and the corresponding cold front observations. A statistical analysis of dust-related visibility for the OIF period reveals that COAMPS generates higher bias, rms, and relative errors at the stations having high frequencies of dust storms and near the source areas. The calculation of forecast accuracy shows that COAMPS achieved a probability of dust detection of 50%–90% and a threat score of 0.3–0.55 at the stations with frequent dust storms. Overall, the model predicted more than 85% of the observed dust and nondust weather events at the stations used in the verification for the OIF period. Comparisons of the forecast rates and statistical errors for the forecasts of different lengths (12–72 h) for both dust and dynamics fields during the strong dust storm of 26 March revealed little dependence of model accuracy on forecast length, implying that the successive COAMPS forecasts were consistent for the severest OIF dust event.

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Craig H. Bishop, Teddy R. Holt, Jason Nachamkin, Sue Chen, Justin G. McLay, James D. Doyle, and William T. Thompson

Abstract

A computationally inexpensive ensemble transform (ET) method for generating high-resolution initial perturbations for regional ensemble forecasts is introduced. The method provides initial perturbations that (i) have an initial variance consistent with the best available estimates of initial condition error variance, (ii) are dynamically conditioned by a process similar to that used in the breeding technique, (iii) add to zero at the initial time, (iv) are quasi-orthogonal and equally likely, and (v) partially respect mesoscale balance constraints by ensuring that each initial perturbation is a linear sum of forecast perturbations from the preceding forecast. The technique is tested using estimates of analysis error variance from the Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS) and the Navy’s regional Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) over a 3-week period during the summer of 2005. Lateral boundary conditions are provided by a global ET ensemble. The tests show that the ET regional ensemble has a skillful mean and a useful spread–skill relationship in mass, momentum, and precipitation variables. Diagnostics indicate that ensemble variance was close to, but probably a little less than, the forecast error variance for wind and temperature variables, while precipitation ensemble variance was significantly smaller than precipitation forecast error variance.

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