Browse

You are looking at 11 - 20 of 14,456 items for :

  • Journal of the Atmospheric Sciences x
  • All content x
Clear All
Hyunho Lee, Ann M. Fridlind, and Andrew S. Ackerman

Abstract

Accurate numerical modeling of clouds and precipitation is essential for weather forecasting and climate change research. While size-resolved (bin) cloud microphysics models predict particle size distributions without imposing shapes, results are subject to artificial size distribution broadening owing to numerical diffusion associated with various processes. Whereas Part I of this study addressed collision–coalescence, here we investigate numerical diffusion that occurs in solving condensation and evaporation. In a parcel model framework, all of the numerical schemes examined converge to one solution of condensation and evaporation as the mass grid is refined, and the advection-based schemes are recommended over the reassigning schemes. Including Eulerian vertical advection in a column limits the convergence to some extent, but that limitation occurs at a sufficiently fine mass grid, and the number of iterations in solving vertical advection should be minimized to reduce numerical diffusion. Insubstantial numerical diffusion in solving condensation can be amplified if collision–coalescence is also active, which in turn can be substantially diminished if turbulence effects on collision are incorporated. Large-eddy simulations of a drizzling stratocumulus field reveal that changes in moments of Doppler spectra obtained using different mass grids are consistent with those obtained from the simplified framework, and that spectral moments obtained using a mass grid designed to effectively reduce numerical diffusion are generally closer to observations. Notable differences between the simulations and observations still exist, and our results suggest a need to consider whether factors other than numerical diffusion in the fundamental process schemes employed can cause such differences.

Restricted access
I. P. Chunchuzov, S. N. Kulichkov, O. E. Popov, and V. G. Perepelkin

Abstract

Infrasound parameters (amplitudes, coherences, grazing angles, azimuths, and horizontal phase speeds) derived during the passage of warm and cold fronts through the networks of microbarometers in the cities of Dubna and Moscow are presented. The significant differences observed in the temporal variations of the parameters of infrasound from warm and cold fronts are discussed. Such differences must be taken into account when detecting infrasound precursors of atmospheric storms. A possible mechanism for the generation of infrasound by the turbulent airstream flowing around the geometric irregularities of the meteorological front is proposed. The observed effect of internal gravity waves on the parameters of infrasound and its frequency spectrum is explained.

Restricted access
Hyeyum Hailey Shin, Domingo Muñoz-Esparza, Jeremy A. Sauer, and Matthias Steiner

Abstract

This study explores the response of flow around isolated cuboid buildings to variations in the incoming turbulence arising from changes in atmospheric boundary layer (ABL) stability using a building-resolving large-eddy simulation (LES) technique with explicit representation of building effects through an immersed body force method. An extensive suite of LES for a neutral ABL with different model resolution and advection scheme configurations reveals that at least 6, 12, and 24 grid points per building side are required in order to resolve building-induced vortex shedding, mean-flow features, and turbulence statistics, respectively, with an advection scheme of a minimum of third order. Using model resolutions that meet this requirement, 21 building-resolving simulations are performed under varying atmospheric stability conditions, from weakly stable to convective ABLs, and for different building sizes (H), resulting in L ABL/H ≈ 0.1–10, where L ABL is the integral length scale of the incoming ABL turbulence. The building-induced flow features observed in the canonical neutral ABL simulation, e.g., the upstream horseshoe vortex and the downstream arch vortex, gradually weaken with increasing surface-driven convective instability due to the enhancement of background turbulent mixing. As a result, two local turbulence kinetic energy peaks on the lateral side of the building in nonconvective cases are merged into a single peak in strong convective cases. By considering the ABL turbulence scale and building size altogether, it is shown that the building impact decreases with increasing L ABL/H, as coherent turbulent structures in the ABL become more dominant over a building-induced flow response for L ABL/H > 1.

Open access
Tyler Mixa, Andreas Dörnbrack, and Markus Rapp

Abstract

Horizontally dispersing gravity waves with horizontal wavelengths of 30–40 km were observed at mesospheric altitudes over Auckland Island by the airborne advanced mesospheric temperature mapper during a Deep Propagating Gravity Wave Experiment (DEEPWAVE) research flight on 14 July 2014. A 3D nonlinear compressible model is used to determine which propagation conditions enabled gravity wave penetration into the mesosphere and how the resulting instability characteristics led to widespread momentum deposition. Results indicate that linear tunneling through the polar night jet enabled quick gravity wave propagation from the surface up to the mesopause, while subsequent instability processes reveal large rolls that formed in the negative shear above the jet maximum and led to significant momentum deposition as they descended. This study suggests that gravity wave tunneling is a viable source for this case and other deep propagation events reaching the mesosphere and lower thermosphere.

Open access
Katrina L. Hui and Simona Bordoni

Abstract

Recent studies have shown that the rapid onset of the monsoon can be interpreted as a switch in the tropical circulation, which can occur even in the absence of land–sea contrast, from a dynamical regime controlled by eddy momentum fluxes to a monsoon regime more directly controlled by energetic constraints. Here we investigate how one aspect of continental geometry, that is, the position of the equatorward coastal boundary, influences such transitions. Experiments are conducted with an aquaplanet model with a slab ocean, in which different zonally symmetric continents are prescribed in the Northern Hemisphere poleward from southern boundaries at various latitudes, with “land” having a mixed layer depth two orders of magnitude smaller than ocean. For continents extending to tropical latitudes, the simulated monsoon features a rapid migration of the convergence zone over the continent, similar to what is seen in observed monsoons. For continents with more poleward southern boundaries, the main precipitation zone remains over the ocean, moving gradually into the summer hemisphere. We show that the absence of land at tropical latitudes prevents the rapid displacement into the subtropics of the maximum in lower-level moist static energy and, with it, the establishment of an overturning circulation with a subtropical convergence zone that can transition rapidly into an angular momentum–conserving monsoon regime.

Restricted access
Annette M. Boehm and Michael M. Bell

Abstract

The newly developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation–Thermodynamic Retrieval (SAMURAI-TR) is used to estimate three-dimensional temperature and pressure perturbations in Hurricane Rita on 23 September 2005 from multi–Doppler radar data during the RAINEX field campaign. These are believed to be the first fully three-dimensional gridded thermodynamic observations from a TC. Rita was a major hurricane at this time and was affected by 13 m s−1 deep-layer vertical wind shear. Analysis of the contributions of the kinematic and retrieved thermodynamic fields to different azimuthal wavenumbers suggests the interpretation of eyewall convective forcing within a three-level framework of balanced, quasi-balanced, and unbalanced motions. The axisymmetric, wavenumber-0 structure was approximately in thermal-wind balance, resulting in a large pressure drop and temperature increase toward the center. The wavenumber-1 structure was determined by the interaction of the storm with environmental vertical wind shear resulting in a quasi balance between shear and shear-induced kinematic and thermodynamic perturbations. The observed wavenumber-1 thermodynamic asymmetries corroborate results of previous studies on the response of a vortex tilted by shear, and add new evidence that the vertical motion is nearly hydrostatic on the wavenumber-1 scale. Higher-order wavenumbers were associated with unbalanced motions and convective cells within the eyewall. The unbalanced vertical acceleration was positively correlated with buoyant forcing from thermal perturbations and negatively correlated with perturbation pressure gradients relative to the balanced vortex.

Restricted access
Chanil Park, Seok-Woo Son, and Jung-Hoon Kim

Abstract

The nature of the vertical motion responsible for the summertime heavy rainfall events (HREs) in South Korea is quantitatively examined. By compositing 318 HREs from June to September in 1979–2018, it is found that the synoptic conditions of the HREs are typically characterized by a developing surface cyclone with a southwesterly low-level jet on its southeastern flank and an upper-level trough to the west of the HREs. This baroclinic environment allows for well-organized vertical motion over South Korea at the equatorward side of the upper-level jet entrance. The relative importance of dynamic and diabatic forcings in driving the vertical motion is further quantified by solving the quasigeostrophic omega equation. It turns out that the dynamic forcing, defined as Q-vector convergence, is comparable to the diabatic forcing in the developing stage of the HREs. The diabatic forcing, however, becomes more important in the mature stage as latent heating rapidly increases. The decomposition of the Q vector into the transverse (cross-isentropic) and shearwise (along-isentropic) components reveals that the dynamic uplift is largely caused by the shearwise Q-vector convergence, which is closely related to the developing trough in the upper to middle troposphere on the west of the HREs. This result indicates that the HREs in South Korea are organized by the baroclinic trough coupled to moist processes, with a minor contribution of the thermally direct secondary circulation at the entrance region of the upper-level jet.

Restricted access
R. S. Ajayamohan, Boualem Khouider, V. Praveen, and Andrew J. Majda

Abstract

The barrier effect of the Maritime Continent (MC) in stalling or modifying the propagation characteristics of the MJO is widely accepted. The strong diurnal cycle of convection over the MC is believed to play a dominant role in this regard. This hypothesis is studied here, with the help of a coarse-resolution atmospheric general circulation model (AGCM). The dry dynamical core of the AGCM is coupled to the multicloud parameterization piggybacked with a dynamical bulk boundary layer model. A set of sensitivity experiments is carried out by systematically varying the strength of the MC diurnal flux to assess the impact of the diurnal convective variability on the MJO propagation. The effects of deterministic and stochastic diurnal forcings on MJO characteristics are compared. It is found that the precipitation and zonal wind variance, on the intraseasonal time scales, over the western Pacific region decreases with the increase in diurnal forcing, indicating the blocking of MC precipitation. An increase in precipitation variance over the MC associated with the weakening of precipitation variance over the west Pacific is evident in all experiments. The striking difference between deterministic and stochastic diurnal forcing experiments is that the strength needed for the deterministic case to achieve the same degree of blocking is almost double that of stochastic case. The stochastic diurnal flux over the MC seems to be more detrimental in blocking the MJO propagation. This hints at the notion that the models with inadequate representation of organized convection tend to suffer from the MC-barrier effect.

Open access
Tsz-Kin Lai, Eric A. Hendricks, M. K. Yau, and Konstantinos Menelaou

Abstract

Intense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (aka type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddy AAM is intrinsically nonzero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes in AAM over the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the nonzero eddy radial transport of eddy AAM is an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively.

Restricted access
Bowen Zhou, Yuhuan Li, and Shiguang Miao

Abstract

A scale-adaptive model is developed for the representation of dry convective boundary layer (CBL) turbulence in numerical models operating at O(100) m to O(1) km horizontal resolution, also known as the model gray zone of the CBL. The new model is constructed based on a planetary boundary layer (PBL) scheme and a large-eddy simulation (LES) closure that are both turbulence kinetic energy–based parameterizations. Scale adaptivity is achieved by “blending” the PBL scheme with the LES closure through an inverse averaging procedure that naturally accounts for vertical variations of the dominant turbulent length scales, hence the gray zone range. High-resolution wide-domain LES benchmark cases covering a broad range of CBL bulk stability are filtered to gray zone resolutions, and analyzed to determine the averaging coefficients. Stability dependence of the dominant length scales is revealed by the analysis and accounted for in the new model. The turbulence model is implemented into a community atmospheric model, and tested for idealized cases. Compared to two established gray zone models, the new model performs equally well under strongly convective conditions, and is more advantageous for the weakly unstable and near neutral CBL.

Restricted access