Abstract

Radiosondes from Soviet ships along with dropsondes and mean and turbulence data from the National Center for Atmospheric Research (NCAR) Electra gust probe aircraft are analyzed to infer the structure of the monsoon marine boundary layer during MONEX 79. Results of mean wind profiles indicate the existence of a jetlike structure in the upper part of the boundary layer during the more suppressed “monsoon-break” conditions. The thermal structure of the monsoon boundary layer during these break conditions is characterized by near-neutral to slightly unstable conditions. There was an approximate balance of form in the monsoon boundary layer between advective acceleration, friction and geostrophic departure. Advective acceleration was found to be a significant term, especially in the lower levels of the boundary layer. This contrasts with typical trade-wind boundary layers in which acceleration is generally negligible.

Results indicate that turbulence statistics associated with wind speed components and temperature in the monsoon boundary layer during MONEX 79 are generally large. Profiles of momentum and virtual temperature flux change sign at altitudes as low as 30 to 50% of the boundary layer height. The turbulent kinetic energy budget indicates that buoyancy is not a dominant source term above, roughly, one-third the boundary layer height. Viscous energy dissipation and turbulent transport are the important sink terms in the lowest one-half of the boundary layer.

Abstract

Marine boundary-layer structure and circulation is documented for the 24 February 1986 case of offshore redevelopment of a cyclone during the Genesis of Atlantic Lows Experiment (GALE) Intensive Observing Period (IOP) 9. Mesoscale and satellite information emphasize that the onshore cyclone is not well organized as it moves offshore to the cold shelf waters with redevelopment occurring later over the Gulf Stream region. Within hours of redevelopment, low-level aircraft data were obtained in the region.

Vertical aircraft profiles down by the National Center for Atmospheric Research (NCAR) King Air in the vicinity of redevelopment over the Gulf Stream, as well as the midshelf front region and cold shelf waters, reveal two distinct boundary layers. Over the Gulf Stream region approximately 50 km south-southwest of the redeveloping cyclone, the near-neutral marine boundary layer (−h/L = 6.6) capped by layered stratocumulus is characterized by a low cloud base (360 m), relatively thick stratocumulus cloud layer (800–1200 m) and strong subcloud-layer winds (8–9 m s^-1). Associated with the developing cyclone near the Gulf Stream is shallow cyclonic flow with convergence and subsequent acceleration of the wind near the western edge.

Closer to the coast over the cold shelf waters and the midshelf front region, the relatively cloud-free boundary layer (h/L = 44.4) is characterized by a slightly shallower, new-neutral boundary layer (h = 700 to 755 m) with very light and variable winds. Boundary layer flow is strongly divergent west of the midshelf front. Them two regions are approximately 150–200 km west of the Gulf Stream region or redevelopment.

Flux profiles agree with results from other marine boundary layers under similar cloud and stability conditions and emphasize the warming and moistening of the subcloud layer from new the western edge of the Gulf Stream eastward. Temperature and moisture turbulence structure appear less well organized. The mean momentum budget emphasizes the strong baroclinicity in the MABL and the importance of horizontal advection near the western edge of the Gulf Stream. Comparison turbulent kinetic energy (TKE) budgets over the Gulf Stream and over the midshelf front show shear production and dissipation to dominate over the Gulf Stream with strong winds. Turbulent transport over the Gulf Stream is a significant term due primarily to the flux of horizontal velocity variance, which is approximately 5 times that of the flux of vertical velocity variance. Over the midshelf front, all normalized terms in the TKE budget are less active in producing, dissipating and transferring TKE for a given heat flux as compared to the Gulf Stream region, where the effects of the developing cyclone are evident.

Abstract

High-resolution numerical simulations are conducted using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) ¹ with two different urban canopy parameterizations for a 23-day period in August 2005 for the New York City (NYC) metropolitan area. The control COAMPS simulations use the single-layer Weather Research and Forecasting (WRF) Urban Canopy Model (W-UCM) and sensitivity simulations use a multilayer urban parameterization based on Brown and Williams (BW-UCM). Both simulations use surface forcing from the WRF land surface model, Noah, and hourly sea surface temperature fields from the New York Harbor and Ocean Prediction System model hindcast. Mean statistics are computed for the 23-day period from 5 to 27 August (540-hourly observations) at five Meteorological Aviation Report stations for a nested 0.444-km horizontal resolution grid centered over the NYC metropolitan area. Both simulations show a cold mean urban canopy air temperature bias primarily due to an underestimation of nighttime temperatures. The mean bias is significantly reduced using the W-UCM (−0.10°C for W-UCM versus −0.82°C for BW-UCM) due to the development of a stronger nocturnal urban heat island (UHI; mean value of 2.2°C for the W-UCM versus 1.9°C for the BW-UCM). Results from a 24-h case study (12 August 2005) indicate that the W-UCM parameterization better maintains the UHI through increased nocturnal warming due to wall and road effects. The ground heat flux for the W-UCM is much larger during the daytime than for the BW-UCM (peak ∼300 versus 100 W m⁻²), effectively shifting the period of positive sensible flux later into the early evening. This helps to maintain the near-surface mixed layer at night in the W-UCM simulation and sustains the nocturnal UHI. In contrast, the BW-UCM simulation develops a strong nocturnal stable surface layer extending to approximately 50–75-m depth. Subsequently, the nocturnal BW-UCM wind speeds are a factor of 3–4 less than W-UCM with reduced nighttime turbulent kinetic energy (average < 0.1 m² s⁻²). For the densely urbanized area of Manhattan, BW-UCM winds show more dependence on urbanization than W-UCM. The decrease in urban wind speed is most prominent for BW-UCM both in the day- and nighttime over lower Manhattan, with the daytime decrease generally over the region of tallest building heights while the nighttime decrease is influenced by both building height as well as urban fraction. In contrast, the W-UCM winds show less horizontal variation over Manhattan, particularly during the daytime. These results stress the importance of properly characterizing the urban morphology in urban parameterizations at high resolutions to improve the model’s predictive capability.

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.

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.

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.

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.

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.

Abstract

In a strongly coupled data assimilation (DA), a cross-fluid covariance is specified that allows measurements from a coupled fluid (e.g., atmosphere) to directly impact analysis increments in a target fluid (e.g., ocean). The exhaustive solution to this coupled DA problem calls for a covariance where all available measurements can influence all grid points in all fluids. Solution of such a large algebraic problem is computationally expensive, often calls for a substantial rewrite of existing fluid-specific DA systems, and, as shown in this paper, can be avoided.

The proposed interface solver assumes that covariances between coupled measurements and target fluid are often close to null (e.g., between stratospheric observations and the deep ocean within a 6-h forecast cycle). In the interface solver, two separate DA solvers are run in parallel: one that produces an analysis solution in the atmosphere, and one in the ocean. Each system uses a coupled observation vector where in addition to resident measurements in the target fluid it also includes nonresident measurements in the coupled fluid that are likely to have significant influence on the analysis in the target fluid (interface measurements). An ensemble-based method is employed and a localization function for coupled ensembles is proposed. Using a coupled model for the Mediterranean Sea (in a twin setting), it is demonstrated that (i) the solution of the interface solver converges to the exhaustive solution and (ii) that in presence of poorly known error covariances, the interface solver can be configured to produce a more accurate solution than an exhaustive solver.

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.