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

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

Cold air advection over the Gulf Stream off the Carolinas and the Appalachian Mountains is studied using idealized two-dimensional cases for the Genesis of Atlantic Lows Experiment (GALE) lop 2 conditions. An anelastic hydrostatic mesoscale model is used. Turbulent transfer in the planetary boundary layer, diurnal heating, cloud dynamics, atmospheric longwave and shortwave radiation and subgrid cumulus parameterization are included in the model.

Model results show that the geometry of the oceanic and coastal rainbands depends on the direction of the ambient flow (onshore or offshore). For onshore flows, the rainbands remain in the vicinity of the oceanic baroclinic zone. The rainbands become, transient and migrate downwind of the Gulf Stream front for offshore flows. Depths of the marine boundary layer (MBL) and the cloud (or rain) bands depend more on the ambient flow speed than its direction. The rainbands develop primarily in response to the strong low level convergence.

As expected, southward winds are produced at the eastern side of the Appalachian Mountains for onshore conditions. A significant amount of the turning, however, results from the baroclinic zone over the ocean. Upstream influence of the mountain intensifies the updrafts'in the MBL and moves the oceanic rainbands further offshore. The effects of the atmospheric longwave and shortwave radiation, subgrid cloud heating and diurnal ground heating are of secondary importance in influencing the structure of the MBL as compared to the surface turbulent beat fluxes. Diurnal effects can change the coastal inland flow regime considerably, resulting in a local breeze and the formation of another cloud (or rain) band.

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 fourth-order Crowley-type advection scheme based on the multistep Warming-Kutler-Lomax (WKL) scheme is proposed in this study. This scheme utilizes a free parameter to minimize dispersion and dissipation and can be used to represent the advection of positive-definite scalars (such as moisture).

Linear Fourier component analyses indicate that the fourth-order Crowley-type scheme can reproduce the features of other modified Crowley-type schemes of third order, such as the scheme of Schlesinger and the quadratic upstream interpolation. Using the free parameter, the scheme may illustrate the limitation of the Crowley-type schemes for which diffusion is required for numerical stability of advective quantity. For these schemes, formulations that preserve amplitude are inevitably associated with smaller time steps. Adding the first cross-space term into these schemes could eliminate marginal instability or overshooting in linear advection.

Linear and nonlinear advection tests show that the performance of the proposed scheme is comparable to the fourth-order leapfrog scheme (which requires more computer memory) and the cubic upstream spline (which requires more computer time). This two-time-level advection scheme can thus be used in a numerical model to save computer resources.

Abstract

Numerical experiments were conducted to assess the impact of Omega dropwindsonde (ODW) data and Special Sensor Microwave/Imager (SSM/I) rain rates in the analysis and prediction of Hurricane Florence (1988). The ODW data were used to enhance the initial analysis that was based on the National Meteorological Center/Regional Analysis and Forecast System (NMC/RAFS) 2.5° analysis at 0000 UTC 9 September 1988. The SSM/I rain rates at 0000 and 1200 UTC 9 September 1988 were assimilated into the Naval Research Laboratory's limited-area model during model integration.

Results show that the numerical prediction with the ODW-enhanced initial analysis was superior to the control without ODW data. The 24-h intensity forecast error is reduced by about 75%, landfall location by about 95% (reduced from 294 to 15 km), and landfall time by about 5 h (from 9 to 4 h) when the ODW data were included. Results also reveal that the assimilation of SSM/I-retrieved rain rates reduce the critical landfall location forecast error by about 43% (from 294 to 169 km) and the landfall time forecast error by about 7 h (from 9 to 2 h) when the NMC/RAFS 2.5° initial analysis was not enhanced by the ODW data. The assimilation of SSM/I rain rates further improved the forecast error of the landfall time by 4 h (from 4 to 0 h) when the ODW data were used. This study concludes that numerical predictions of tropical cyclone can benefit from assimilations of ODW data and SSM/I-retrieved rain rates.

Abstract

Numerical simulations of tropical cyclones in an axisymmetric model with the Betts convective adjustment scheme and the 1974 Kuo cumulus parameterization are compared. It is shown that the storm with the Betts scheme has a slightly more intense mature stage than the storm with the Kuo scheme. For both schemes, the parameterized heating is dominant initially, while the grid-scale heating is dominant at the mature stage. The storms begin to intensify rapidly when the grid-scale heating extends through a deep layer. The Betts scheme is more effective at removing water vapor and delays the onset of grid-scale heating. This results in later development of the storm with the Betts scheme. The storm evolution with both the Betts and Kuo schemes is sensitive to the treatment of the evaporation of liquid water in the grid-scale condensation scheme. This suggests that a prognostic equation for liquid water should be used when simulating tropical cyclones with a model resolution fine enough for grid-scale heating to be important.

Abstract

A new convective parameterization scheme proposed by Betts is tested in a tropical cyclone model. The convective adjustment scheme adjusts the local temperature and moisture structures towards the observed quasi-equilibrium thermodynamic state and includes nonprecipitating shallow convection as well as deep convection. The numerical model used for this study is an axisymmetric, primitive equation, hydrostatic, finite difference model with 15 vertical levels and a horizontal resolution of 20 km. The spectral radiation boundary condition, which uses a different gravity wave speed for each vertical mode, is implemented in the model.

It is shown that the convective scheme is capable of simulating the developing, rapidly intensifying, and mature stages of a tropical cyclone from a weak vortex. At the mature stage, the minimum surface pressure and maximum low level tangential wind speed are around 923 mb and 58 m s⁻¹. During the early developing stage, the latent heat release is from the convective parameterization, but at the mature stage the latent heat release is mainly due to the grid-scale phase change.

For comparison, an experiment is conducted with the parameterized convection excluded, leaving only the grid-scale condensation and evaporation. The results show that the development of a tropical cyclone can be modeled with crude grid-scale condensation and evaporation processes for the 20 km horizontal resolution, similar to other studies. However, the storm with the explicit convective latent heat release is considerably less intense than that with the parameterized convective latent heat release.

Abstract

Two mesoscale circulations, the Sandhills circulation and the sea breeze, influence the initiation of deep convection over the Sandhills and the coast in the Carolinas during the summer months. The interaction of these two circulations causes additional convection in this coastal region. Accurate representation of mesoscale convection is difficult as numerical models have problems with the prediction of the timing, amount, and location of precipitation. To address this issue, the authors have incorporated modifications to the Kain–Fritsch (KF) convective parameterization scheme and evaluated these mesoscale interactions using a high-resolution numerical model. The modifications include changes to the subgrid-scale cloud formulation, the convective turnover time scale, and the formulation of the updraft entrainment rates. The use of a grid-scaling adjustment parameter modulates the impact of the KF scheme as a function of the horizontal grid spacing used in a simulation. Results indicate that the impact of this modified cumulus parameterization scheme is more effective on domains with coarser grid sizes. Other results include a decrease in surface and near-surface temperatures in areas of deep convection (due to the inclusion of the effects of subgrid-scale clouds on the radiation), improvement in the timing of convection, and an increase in the strength of deep convection.

Abstract

Extensive sensitivity experiments with an axisymmetric tropical cyclone model that includes the Bets convective parameterization scheme are carded out. The sensitivity of the model storm evolution to the convective adjustment parameters is studied. These results show that the model storm leads to earlier development as the adjustment time scale becomes small and the stability weight on the moist adiabat in the lower atmosphere is increased. The model storm evolution is very sensitive to variations in the saturation pressure departure at the lowermost model integer level and the storm at mature stage has a lower central pressure as the magnitude of the saturation pressure departure is increased. The adjustment parameters affect the grid-scale precipitation as well as the convective precipitation and the precipitation is especially sensitive to changes in the saturation pressure departure.

Sensitivity of the model to variations in the sea surface temperature, latitude, initial vortex amplitude, initial moisture distribution, and radiation is also investigated. The results of the numerical simulations are similar to previous studies. Sensitivity studies with various horizontal resolutions show that the subgrid-scale heating becomes a larger fraction of the total heating as the horizontal grid size is increased.