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.

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.

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.

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.

Abstract

During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities.

Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection.

Second, the southward branch of the circulation, with its rising motion in the frontal region and equatorward sinking motion, is driven by frontal vertical deep convection. The return flow of this circulation at low levels can produce an LLJ through geostrophic adjustment. The intensification of the LLJ is sensitive to the presence of convection.

Third, there is a circulation that develops from low to middle levels that has a slantwise rising and sinking motion in the pre- and postfrontal regions, respectively. From an absolute momentum surface analysis, this slantwise circulation is maintained by conditionally symmetric instability located at low levels ahead of the front. The presence of both the LLJ and moisture is an essential ingredient in fostering this conditionally symmetric unstable environment.

Abstract

This study evaluates, for the first time, the impact of airborne global positioning system radio occultation (ARO) observations on a hurricane forecast. A case study was conducted of Hurricane Karl during the Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) field campaign in 2010. The assimilation of ARO data was developed for the three-dimensional variational (3DVAR) analysis system of the Weather Research and Forecasting (WRF) Model version 3.2. The impact of ARO data on Karl forecasts was evaluated through data assimilation (DA) experiments of local refractivity and nonlocal excess phase (EPH), in which the latter accounts for the integrated horizontal sampling along the signal ray path. The tangent point positions (closest point of an RO ray path to Earth’s surface) drift horizontally, and the drifting distance of ARO data is about 2 to 3 times that of spaceborne RO, which was taken into account in these simulations.

Results indicate that in the absence of other satellite observations, the assimilation of ARO EPH resulted in a larger impact on the analysis than local refractivity did. In particular, the assimilation of ARO observations at the actual tangent point locations resulted in more accurate forecasts of the rapid intensification of the storm. Among all experiments, the best forecast was obtained by assimilating ARO data with the most accurate geometric representation, that is, the use of nonlocal EPH operators with tangent point drift, which reduced the error in the storm’s predicted minimum sea level pressure (SLP) by 43% beyond that of the control experiment.

Abstract

A coupled land surface–atmospheric model that permits grid-resolved deep convection is used to examine linkages between land surface conditions, the planetary boundary layer (PBL), and precipitation during a 12-day warm-season period over the central United States. The period of study (9–21 June 2002) coincided with an extensive dry soil moisture anomaly over the western United States and adjacent high plains and wetter-than-normal soil conditions over parts of the Midwest. A range of possible atmospheric responses to soil wetness is diagnosed from a set of simulations that use land surface models (LSMs) of varying sophistication and initial land surface conditions of varying resolution and specificity to the period of study.

Results suggest that the choice of LSM [Noah or the less sophisticated simple slab soil model (SLAB)] significantly influences the diurnal cycle of near-surface potential temperature and water vapor mixing ratio. The initial soil wetness also has a major impact on these thermodynamic variables, particularly during and immediately following the most intense phase of daytime surface heating. The soil wetness influences the daytime PBL evolution through both local and upstream surface evaporation and sensible heat fluxes, and through differences in the mesoscale vertical circulation that develops in response to horizontal gradients of the latter. Resulting differences in late afternoon PBL moist static energy and stability near the PBL top are associated with differences in subsequent late afternoon and evening precipitation in locations where the initial soil wetness differs among simulations. In contrast to the initial soil wetness, soil moisture evolution has negligible effects on the mean regional-scale thermodynamic conditions and precipitation during the 12-day period.