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S. F. Shih and E. Chen

Abstract

Geostationary Operational Environmental Satellite (GOES) infrared data were used to study the effect of land use on the diurnal surface temperature fluctuation. Five major land use types in southern Florida: the sandy soil agricultural area; the Everglades Agricultural Area (EAA); the conservation areas; the Natural Everglades Area (NEA); and Lake Okeechobee; were observed. The average daytime and nocturnal surface temperatures of sandy soil in agricultural areas was lower than that of organic soil in agricultural areas. The average temperature of organic soil in agricultural areas was lower than that of organic soil in conservation areas. The surface temperature in the wet marsh area was much lower than that in a large water-storage lake. A land use change in the EAA, and an increase in the water storage in Lake Okeechobee and the conservation areas could influence the microclimate.

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F. Couvreux, F. Guichard, P. H. Austin, and F. Chen

Abstract

Mesoscale water vapor heterogeneities in the boundary layer are studied within the context of the International H2O Project (IHOP_2002). A significant portion of the water vapor variability in the IHOP_2002 occurs at the mesoscale, with the spatial pattern and the magnitude of the variability changing from day to day. On 14 June 2002, an atypical mesoscale gradient is observed, which is the reverse of the climatological gradient over this area. The factors causing this water vapor variability are investigated using complementary platforms (e.g., aircraft, satellite, and in situ) and models. The impact of surface flux heterogeneities and atmospheric variability are evaluated separately using a 1D boundary layer model, which uses surface fluxes from the High-Resolution Land Data Assimilation System (HRLDAS) and early-morning atmospheric temperature and moisture profiles from a mesoscale model. This methodology, based on the use of robust modeling components, allows the authors to tackle the question of the nature of the observed mesoscale variability. The impact of horizontal advection is inferred from a careful analysis of available observations. By isolating the individual contributions to mesoscale water vapor variability, it is shown that the observed moisture variability cannot be explained by a single process, but rather involves a combination of different factors: the boundary layer height, which is strongly controlled by the surface buoyancy flux, the surface latent heat flux, the early-morning heterogeneity of the atmosphere, horizontal advection, and the radiative impact of clouds.

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Tai-Jen George Chen and Lance F. Bosart

Abstract

A composite quasi-Lagrangian kinetic energy budget is constructed from four synoptically similar cases of polar air penetration into the Caribbean from off the North American continent. Computations were carried out for both the upstream anticyclone and downstream cyclone accompanying the polar outbreak.

Use of the residual technique suggests an average upscale energy exchange of 45.0 W m−2 over the anticyclone volume with a corresponding downscale energy transfer of 59.0 W m−2 over the cyclone volume for the 24 h period centered on the time of furthest southward cold air thrust as defined by the 1000–500 mb thickness patterns. The results also indicate that the vertical flux of kinetic energy ranges from 50 to 100% of the horizontal flux of kinetic energy and is of opposite sign below 400 mb in the cyclone volume. Further-more, during incipient surface cyclogenesis the horizontal boundary flux of 17.7 m m−2 is a signification of the local kinetic energy generation of 24.5 W m−2 whereas in the following 12 h time period these numbers become 32.2 and 64.6 W m−2, respectively. The corresponding figures for the anticyclone region include a horizontal export of kinetic energy of 37.2 and 55.0 W m−2 and local kinetic energy destruction of 9.5 and 6.0 W m−2 respectively, for the same 12 h time periods.

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J-F. Miao, D. Chen, and K. Borne

Abstract

In this study, the performance of two advanced land surface models (LSMs; Noah LSM and Pleim–Xiu LSM) coupled with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), version 3.7.2, in simulating the near-surface air temperature in the greater Göteborg area in Sweden is evaluated and compared using the GÖTE2001 field campaign data. Further, the effects of different planetary boundary layer schemes [Eta and Medium-Range Forecast (MRF) PBLs] for Noah LSM and soil moisture initialization approaches for Pleim–Xiu LSM are investigated. The investigation focuses on the evaluation and comparison of diurnal cycle intensity and maximum and minimum temperatures, as well as the urban heat island during the daytime and nighttime under the clear-sky and cloudy/rainy weather conditions for different experimental schemes. The results indicate that 1) there is an evident difference between Noah LSM and Pleim–Xiu LSM in simulating the near-surface air temperature, especially in the modeled urban heat island; 2) there is no evident difference in the model performance between the Eta PBL and MRF PBL coupled with the Noah LSM; and 3) soil moisture initialization is of crucial importance for model performance in the Pleim–Xiu LSM. In addition, owing to the recent release of MM5, version 3.7.3, some experiments done with version 3.7.2 were repeated to reveal the effects of the modifications in the Noah LSM and Pleim–Xiu LSM. The modification to longwave radiation parameterizations in Noah LSM significantly improves model performance while the adjustment of emissivity, one of the vegetation properties, affects Pleim–Xiu LSM performance to a larger extent. The study suggests that improvements both in Noah LSM physics and in Pleim–Xiu LSM initialization of soil moisture and parameterization of vegetation properties are important.

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Yi-Leng Chen and Norman B-F. Hui

Abstract

A relatively dry surface front during IOP-9 of the Taiwan Area Mesoscale Experiment (1600 UTC 14 June–1700 UTC 15 June) was analyzed. This surface front possessed appreciable baroclinity over southern China due to the southeastward intrusion of the polar air. As the cold air advanced the surface front over southern China moved southeastward and crossed Taiwan. Windshift was observed approximately 12 hours prior to the arrival of the cold air.

Detailed analysis of the small-scale frontal features were made based on high resolution P-3 aircraft observations. East of Taiwan the cold air boundary was rather diffuse with weak thermodynamic contrasts due to air mass modifications as the polar air traveled from northern China to the subtropics. The leading edge of the cold air resembled a density current with the following features: a vortex circulation with rising motion along the leading edge and sinking motion behind, low-level inflow from the rear with return flow above and a wavelike pattern at the top of the cold air. However, with calm winds in the environment the well-defined updraft at the nose was absent. In addition, the slope of the cold air boundary was rather gentle.

A weak mesolow formed over the, southwestern plain of Taiwan after the passage of the windshift line. It intensified after the arrival of the cold continental air. The shallow northeasterlies were blocked by the mountains leaving the warm, moist air over the southwestern plain, The relatively low pressure in this region resulted from increasing pressure elsewhere due to the passage of the windshift line followed by the arrival of the cold continental air.

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Tai-Jen George Chen and Lance F. Bosart

Abstract

A composite cyclone-anticyclone couplet is constructed from four synoptically similar cases of polar air outbreaks into the Caribbean from off the North American continent. A quasi-Lagrangian vorticity budget is then computed from these data for two consecutive 12 h time periods.

The results show that the divergence and twisting terms in the lower troposphere, the horizontal advection term in the middle troposphere, and the horizontal, vertical and system advection in the upper troposphere are of primary importance in generating negative vorticity tendencies in the area toward (from) which the surface anticyclone (cyclone) is moving. In contrast, only the divergence term in the lower troposphere and horizontal advection term in the mid and upper troposphere are primarily responsible for the intensification and movement of the downstream cyclone.

Computations suggest an apparent anticyclonic vorticity source in the mid and upper troposphere and sink in the lower troposphere for the large-scale motions over the anticyclone region with the reverse true for the downstream cyclone region due to subgrid-scale processes.

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Yi-Leng Chen and Norman B-F. Hui

Abstract

The evolution of a relatively dry front during the early-summer rainy Season of Taiwan is analyzed. Because of the synoptic subsidence associated with a subtropical high pressure cell over the northern South China Sea, prefrontal soundings over the Taiwan area exhibited a shallow, warm, moist layer in the lowest levels, capped by an inversion with extremely dry air aloft.

Over the Taiwan area, the southwest flow ahead of the surface front was more than 10 m s−1 at the 850-mb level. It interacted with the central mountain range, resulting in the windward ridge, leeside trough. Downstream of the blocked region, strong southwesterly winds (∼1 5 m s−1) developed in the lowest levels along the northwest coast, where the flow deflected by the mountain barrier merged with the undetected southwest monsoon flow.

The hilly terrain along the southeastern China coast retarded the cold air behind the surface front. The cold air was then ducted around the southeastern China coast. At the 850-mb level, a weak short-wave trough was embedded in the prefrontal monsoon flow. It moved off the southeastern China coast before cold northewterlies arrived at the surface. It deepened in the Ice side of the highlands along the southeastern China coast, with significant low-level warming and drying.

Aircraft observations of the leading edge of the shallow front revealed that a warm, moist tongue was ahead of the wind-shift line, where the winds shifted from northwesterlies to northeasterlies. Behind the leading edge, the air had a uniform equivalent potential temperature below 700 m. The stable, cold air was found 50 km north of the leading edge, with a warm, moist tongue ahead of it. East of Taiwan, the shallow, cold air behind the front appeared to be warmer than its western counterpart, with a well-mixed layer below 650 m. Since the prefrontal soundings over the Taiwan area were dry with a level of free convection (LFC) above the 800-mb level, local lifting by a shallow front was apparently not sufficient to initiate deep convection leading to heavy precipitation.

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F. H. Busse and W. L. Chen

Abstract

It is shown in the limit of small Ekman number that the preferred mode of the symmetric instability exhibits a slight angle of inclination with the direction of the mean flow. The sign of the angle depends an the sign of P − 1, where P is the Prandtl number. It is likely that owing to this effect the range of Richardson numbers for which the instability occurs is increased significantly beyond the limits derived by Kuo (1956) and by McIntyre (1970). Numerical computations are needed to establish this property quantitatively.

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Yi-Leng Chen, Yu-Xia Zhang, and Norman B-F. Hui

Abstract

A case study of a relatively dry front during TAMEX IOP-4 is presented. At 0000 UTC 27 May, the broad cloud band extended from the China plain and southern Japan to east of 150°E, along and north of the surface front. This front possessed appreciable baroclinity.

As the midtropospheric trough moved eastward, the surface front advanced southeastward and eventually separated from the quasi-stationary cloud band. The depth of the cold air decreased as it penetrated to the subtropics. In the vicinity of Taiwan, the frontal surface was shallow with a depth ≈ 1 km. The convection occurred in two different regimes; the large-scale cloud band associated with the eastward-moving trough, and cumuli along the surface front.

As the surface front approached Taiwan, the southwest flow started to increase, and a mesolow formed over the southeastern coast due to the interaction between the southwest flow and the mountainous topography. The air moved over the mountains and descended adiabatically. During the frontal passage, the mesolow deepened as the approaching front from the north also became a flow barrier. After the frontal passage, the mesolow was filled by the advancing cold air.

As the mesolow in southeastern Taiwan filled, a new mesolow formed over the southwestern plain. The surface front and isobars were distorted during the frontal passage as the shallow cold air moved around the topography. The relatively low pressure over the southwestern plain resulted hydrostatically from the cold air behind the front not reaching the southwestern plain, while surrounding areas experienced rising pressure with the arrival of the cold air.

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Dustin F. P. Grogan, Terrence R. Nathan, and Shu-Hua Chen

Abstract

The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume.

The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.

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