Abstract

Six cases of prefrontal squall lines were observed over the Taiwan Strait and western Taiwan during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The mean propagation speed was 10 m s⁻¹, and the mean life span was 11.4 h for the six squall lines. All the lines occurred ahead of the Mei-Yu front and moved away from the front with time. The mean environmental conditions associated with the squall lines were analyzed by compositing the six cases. The environmental conditions observed during the TAMEX squall lines were found with characteristics between tropical and midlatitude squall lines. The steering level was near 7 km during the mature stage. A low-level jet at 3–4 km was present, with strong vertical shear in the presquall environment below 700 hPa. The squall lines were oriented 45° to the shear in the 1–3-km layer, like midlatitude cases. The CAPE, however, is similar to the tropical squall lines. The inflow ahead of the squall lines was deeper and stronger below 400 hPa, and the CAPE was higher during the mature stage as compared to the intensifying stage. The squall-line collapse is correlated with decreasing CAPE and low-level inflow ahead of the lines.

Abstract

In this paper we investigate the interrelationships between objective and subjective temperature forecasts. An information-content approach is adopted within the overall context of a general framework for forecast verification. This approach can be used to address questions such as whether the subjective forecasts contain information regarding the corresponding observed temperatures that is not included in the objective forecasts. Two methods of analysis are employed: 1) ordinary least squares regression analysis and 2) a Bayesian information-content analysis.

Maximum and minimum temperature forecasts formulated operationally for six National Weather Service offices during the period 1980–86 are analyzed. Results produced by the two methods are quite consistent and can be summarized as follows: 1) the subjective forecasts contain information not included in the objective forecasts for all cases (i.e., stratifications) considered and 2) the objective forecasts contain information not included in the subjective forecasts in a substantial majority of these cases. Generally, the incremental information content in the subjective forecasts considerably exceeds the incremental information content in the objective forecasts. The implications of these results for operational short-range temperature forecasting are briefly discussed.

Abstract

Low-level monsoonal cross-equatorial airflow is investigated using a simple Lagrangian model for the tropical atmosphere. With the use of Jacobian elliptic functions, analytical solutions for the dynamic system are obtained to an extent that enables the most interesting features of the model to be demonstrated. It is found that there exists some unpredictability with the equatorial airflow, in the sense that the trajectory of the airflow can either remain in one hemisphere or cross the equator from one hemisphere to another depending upon the value of a model parameter λ _c being negative or positive, even when the corresponding external forcings are of little difference. By means of qualitative analysis of the dynamic system, some more esoteric properties of the model solutions are investigated. It is found that the model exhibits a very interesting bifurcation, which explains the unpredictability. The bifurcation is a characteristic attributable to the nonlinearity of the Coriolis term in the Lagrangian model. Since the Coriolis term is linear in a dynamically identical Eulerian model, the characteristic would not be explicitly revealed by Eulerian modeling.

By numerical method, the model is extended and applied to the simulation of cross-equatorial airflow in the context of Australian monsoon. The exercise provides an example of how to bridge the gap between a simplified analytical model and the realities of the actual tropical meteorology.

Abstract

Optically thin cirrus play a key role in the earth’s radiation budget and global climate change. Their radiative effects depend critically on the thin cirrus optical and microphysical properties. In this paper, inhomogeneous hexagonal monocrystals (IHMs), which consist of a pure hexagon with spherical air bubble or aerosol inclusions, are applied to calculate the single-scattering properties of individual ice crystals. The multiangular polarized characteristics of optically thin cirrus for the 0.865- and 1.38-μm spectral bands are simulated on the basis of an adding–doubling radiative transfer program. The sensitivity of total and polarized reflectance at the top of the atmosphere (TOA) to different aerosol, cirrus, and surface parameters is studied. A new sensitivity index is introduced to further quantify the sensitivity study. The TOA polarized reflectance measured by the Polarization and Directionality of the Earth’s Reflectance (POLDER) instruments is compared to simulated TOA total and polarized reflectance. The test results are reasonable, although small deviations caused by the change of aerosol properties and thin cirrus optical thickness do exist. Finally, on the basis of the sensitivity study, a conceptual approach is suggested to simultaneously retrieve thin cirrus clouds’ optical thickness, ice particle shape, and the underlying aerosol optical thickness using the TOA total and polarized reflectance of the 0.865- and 1.38-μm spectral bands measured at multiple viewing angles.

The World Climate Research Programme Project for Intercomparison of Land Surface Parameterization Schemes (PILPS) is moving into its second and third phases that will exploit observational data and consider the performance of land surface schemes when coupled to their host climate models. The first stage of phase 2 will focus on an attempt to understand the large differences found during phase 1. The first site from which observations will be drawn for phase 2 intercomparisons is Cabauw, the Netherlands (51 °58′N, 4°56′E), selected specifically to try to reduce one of the causes of the divergence among the phase 1 results: the initialization of the deep soil moisture. Cabauw's deep soil is saturated throughout the year. It also offers a quality controlled set of meteorological forcing and 160 days of flux measurements. PILPS phase 2 follows the form of the phase 1 intercomparisons: simple off-line integrations and comparisons, but in phase 2 participating schemes' results will be compared against observed fluxes. Preliminary results indicate that between model variability persists (i) in better specified experiments and (ii) in comparison with data. Although median values are consistent with observations, there is a large range among models. Phase 3, in which the intercomparison of PILPS schemes as a component of global atmospheric circulation models, is being conducted jointly with the Atmospheric Model lntercomparison Project (AMIP) as diagnostic subproject number 12. Preliminary results suggest that results differ by about the same range as in the off-line experiments in phases 1 and 2. Incomplete diagnostics suggest that bucket and canopy models differ and that variability among models can be tracked to the soil moisture parameterization. This paper offers a review of the PILPS project to date and an invitation to participate in PILPS' current and future activities.

Abstract

The East Asian summer monsoon (Mei-yu) disturbance of 17–25 June 1992 was the most intense 850-hPa low center of such systems during a 7-yr period. Due to the moisture fluxes associated with the southwesterlies from the warm tropical oceans, diabatic heating has generally been considered the main energy source of these heavy-precipitation disturbances as they propagate eastward from the eastern flank of the Tibetan Plateau across southeastern China and move into the East China Sea. In this study piecewise potential vorticity inversion is used to analyze the physical mechanisms of this intense case, particularly the possible roles of midlatitude baroclinic processes in its development and evolution.

The development of the low-level vortex involved the coupling with two upper-level disturbances, one at 500 hPa that also originated from the eastern flank of the Tibetan Plateau, and another at 300 hPa. Both disturbances appeared later than and upstream of the low-level vortex. Faster eastward movements allowed them to catch up with the low-level vortex and led to a strong vertical coupling and deep tropopause folding. Initially, diabatic heating was the dominant mechanism for the low-level vortex while the tropopause process opposed it. Both mechanisms supported the 500-hPa disturbance, and tropopause folding was the dominant mechanism for the 300-hPa disturbance. As the vertical coupling developed, the tropopause process reversed its earlier role in the low-level disturbance and contributed to its development. Boundary layer and adiabatic effects also became contributive as the disturbance moved out of eastern China to the oceanic region.

The vertical coupling of the three disturbances was a major factor in the development. The timing and position of the middle-tropospheric disturbance was critical in bridging the upper- and lower-level disturbances and a deep tropopause folding. This midlatitude-originated process compounded the diabatic heating effect that was sustained by tropical moist air, leading to the strong intensification.

Abstract

A computationally inexpensive ensemble transform (ET) method for generating high-resolution initial perturbations for regional ensemble forecasts is introduced. The method provides initial perturbations that (i) have an initial variance consistent with the best available estimates of initial condition error variance, (ii) are dynamically conditioned by a process similar to that used in the breeding technique, (iii) add to zero at the initial time, (iv) are quasi-orthogonal and equally likely, and (v) partially respect mesoscale balance constraints by ensuring that each initial perturbation is a linear sum of forecast perturbations from the preceding forecast. The technique is tested using estimates of analysis error variance from the Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS) and the Navy’s regional Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) over a 3-week period during the summer of 2005. Lateral boundary conditions are provided by a global ET ensemble. The tests show that the ET regional ensemble has a skillful mean and a useful spread–skill relationship in mass, momentum, and precipitation variables. Diagnostics indicate that ensemble variance was close to, but probably a little less than, the forecast error variance for wind and temperature variables, while precipitation ensemble variance was significantly smaller than precipitation forecast error variance.

Abstract

Black carbon aerosol (BC) has a significant influence on regional climate changes because of its warming effect. Such changes will feed back to BC loadings. Here, the interactions between the BC warming effect and the East Asian monsoon (EAM) in both winter (EAWM) and summer (EASM) are investigated using a regional climate model, RegCM4, that essentially captures the EAM features and the BC variations in China. The seasonal mean BC optical depth is 0.021 over East Asia during winter, which is 10.5% higher than that during summer. Nevertheless, the BC direct radiative forcing is 32% stronger during summer (+1.85 W m⁻²). The BC direct effect would induce lower air to warm by 0.11–0.12 K, which causes a meridional circulation anomaly associated with a cyclone at 20°–30°N and southerly anomalies at 850 hPa over East Asia. Consequently, the EAM circulation is weakened during winter but enhanced during summer. Precipitation is likely increased, especially in southern China during summer (by 3.73%). Relative to BC changes that result from EAM interannual variations, BC changes from its warming effect are as important but are weaker. BC surface concentrations are decreased by 1%–3% during both winter and summer, whereas the columnar BC is increased in south China during winter. During the strongest monsoon years, the BC loadings are higher at lower latitudes than those during the weakest years, resulting in more southerly meridional circulation anomalies and BC feedbacks during both winter and summer. However, the interactions between the BC warming effect and EAWM/EASM are more intense during the weakest monsoon years.

Abstract

In the Different Models, Same Initial Conditions (DIMOSIC) project, forecasts from different global medium-range forecast models have been created based on the same initial conditions. The dataset consists of 10-day deterministic forecasts from seven models and includes 122 forecast dates spanning one calendar year. All forecasts are initialized from the same ECMWF operational analyses to minimize the differences due to initialization. The models are run at or near their respective operational resolutions to explore similarities and differences between operational global forecast models. The main aims of this study are 1) to evaluate the forecast skill and how it depends on model formulation, 2) to assess systematic differences and errors at short lead times, 3) to compare multimodel ensemble spread to model uncertainty schemes, and 4) to identify models that generate similar solutions. Our results show that all models in this study are capable of producing high-quality forecasts given a high-quality analysis. But at the same time, we find a large variety in model biases, both in terms of temperature errors and precipitation. We are able to identify models whose forecasts are more similar to each other than they are to those of other systems, due to the use of similar model physics packages. However, in terms of multimodel ensemble spread, our results also demonstrate that forecast sensitivities to different model formulations still are substantial. We therefore believe that the diversity in model design that stems from parallel development efforts at global modeling centers around the world remains valuable for future progress in the numerical weather prediction community.

Abstract

The Lagrangian Submesoscale Experiment (LASER) was designed to study surface flows during winter conditions in the northern Gulf of Mexico. More than 1000 mostly biodegradable drifters were launched. The drifters consisted of a surface floater extending 5 cm below the surface, containing the satellite tracking system, and a drogue extending 60 cm below the surface, hanging beneath the floater on a flexible tether. On some floats, the drogue separated from the floater during storms. This paper describes methods to detect drogue loss based on two properties that distinguish drogued from undrogued drifters. First, undrogued drifters often flip over, pointing their satellite antenna downward and thus intermittently reducing the frequency of GPS fixes. Second, undrogued drifters respond to wind forcing more than drogued drifters. A multistage analysis is used: first, two properties are used to create a preliminary drifter classification; then, the motion of each unclassified drifter is compared to that of its classified neighbors in an iterative process for nearly all of the drifters. The algorithm classified drifters with a known drogue status with an accuracy of virtually 100%. Drogue loss times were estimated with a precision of less than 0.5 and 3 h for 60% and 85% of the drifters, respectively. An estimated 40% of the drifters lost their drogues in the first 7 weeks, with drogue loss coinciding with storm events, particularly those with steep waves. Once the drogued and undrogued drifters are classified, they can be used to quantify the differences in material dispersion at different depths.