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Frank L. Martin and Wallace C. Palmer

Abstract

Based upon four years of complete data spanning 1957 through 1960 at Ship P, “clear-sky” multiple regression equations were obtained relating computed long-wave flux from water vapor over Ship P to shipboard measurements of the black-body flux and the square root of vapor pressure, as the two independent variables. The water-vapor radiative flux computations were made using the “total water vapor” flux table in the recent Elsaaser-Culbertson Meterological Monograph. In order to limit the noise, only those clear-sky cases synoptic with the filing time of radiosondes were selected. The multiple correlation coefficient was of the order of 0.95. Statistical tests indicate, at high confidence levels, that both independent variables gave significant contributions to the explained variance.

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C. L. KIBLER and R. H. MARTIN

Abstract

No Abstract Available.

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Martin L. M. Wong, Johnny C. L. Chan, and Wen Zhou

Abstract

The intensity change of past (1976–2005) tropical cyclones that made landfall along the south China coast (110.5°–117.5°E) is examined in this study using the best-track data from the Hong Kong Observatory. The change in the central pressure deficit (environmental pressure minus central pressure) and maximum surface wind after landfall are found to fit fairly well with an exponential decay model. Of the various potential predictors, the landfall intensity, landward speed, and excess of 850-hPa moist static energy have significant influence on the decay rates. Prediction equations for the exponential decay constants are developed based on these predictors.

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Martin L. M. Wong and Johnny C. L. Chan

Abstract

The structure and intensity changes of tropical cyclones (TCs) in environmental vertical wind shear (VWS) are investigated in this study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Triply nested domains of 36-, 12-, and 4-km resolution are used with fully explicit moisture physics in the 4-km domain. Idealized environments with easterly shears of 2, 4, 6, 8, and 10 m s−1 between 800 and 200 hPa are applied on an f plane. Under small values of VWS (2 and 4 m s−1), the TC intensities are similar to that of the control (CTRL; i.e., no VWS) after initial adjustments. The TCs under 6 and 8 m s−1 of VWS are not as intense, although they do not weaken during the simulation. On the other hand, the TC in 10 m s−1 of VWS weakened significantly.

Given the same VWS, the TC intensity is also found to be sensitive to TC size. Experiments with TCs with a smaller radius of 15 m s−1 wind reveal that while the TC in 2 m s−1 of VWS remains as intense as the CTRL, the TC in the 4 m s−1 VWS case weakened significantly to a minimal hurricane by the end of the simulation. A VWS of 6 m s−1 is strong enough to cause dissipation of the TC in 72 h. These results indicate that the size of a TC has to be taken into account in determining the intensity change of a TC in VWS.

In the 10 m s−1 VWS case, the average temperature over the lower half of the troposphere within 50 km from the TC surface center is higher than that of the CTRL throughout the simulation. Such a warming, though of a small magnitude, is also observed for a brief period in the upper half of the troposphere before the rapid weakening of the TC and is related to the asymmetry of temperature required for a tilt of the vortex axis. The evolution of the vortex tilt is found to be similar to the dry simulations in previous studies, with the midlevel center (σ = 0.525) located mainly in the southeast quadrant of the surface center. A tendency for the midlevel center to rotate about the surface center is also observed. These results support the idea that the resistance to vertical tilt by the mutual rotation between the low-level and midlevel centers is also valid in the moist simulations.

It is hypothesized that the secondary circulation and the associated diabatic heating reduce the vertical tilt and the weakening. Condensation heating offsets the anomalous cooling effect due to the anomalous rising motion ahead of the vortex tilt. For small VWS, the vertical motion asymmetry is not strong enough to destroy the complete secondary circulation and the eyewall. As a result, a large temperature asymmetry and the associated vortex tilt cannot develop. Furthermore, there is no entrainment of cool/dry air in the upper troposphere. Therefore, TCs under small shears can be as intense as the CTRL.

Large-scale asymmetries in the form of anticyclones found in previous studies are also observed. These asymmetries are apparently related to the change of shears near the TCs. While the shears at outer radii stay roughly constant with time, the shears near the TC centers can have large temporal fluctuations both in magnitude and orientation. This result suggests that the location at which the VWS is estimated in observational studies could be important in determining the relationship between VWS and TC intensity change.

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Martin L. M. Wong and Johnny C. L. Chan

Abstract

Numerical experiments are performed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to study the effects of surface-moisture flux and friction over land on the movement of tropical cyclones (TCs). On an f plane, the TCs are initially placed 150 km due east of a north–south-oriented coastline in an atmosphere at rest. It is found that a TC could drift toward land when the roughness length is 0.5 m over land, with an average drift speed of ∼1 m s−1. Friction, but not surface-moisture flux over land, is apparently essential for the movement toward land. The friction-induced asymmetry in the large-scale flow is the primary mechanism responsible for causing the TC drift. The mechanism responsible for the development of the large-scale asymmetric flow over the lower to midtroposphere (∼900–600 hPa) appears to be the creation of asymmetric vorticity by the divergence term in the vorticity equation. Horizontal advection then rotates the asymmetric vorticity to give a northeasterly flow in the TC periphery (∼500–1000 km from the TC center). The flow near the TC center has a more northerly component because of the stronger rotation by the tangential wind of the TC at inner radii. However, the TC does not move with the large-scale asymmetric flow. Potential vorticity budget calculations indicate that while the horizontal advection term is basically due to the effect of advection by the large-scale asymmetric flow, the diabatic heating and vertical advection terms have to be considered in determining the vortex landward drift, because of the strong asymmetry in vertical motion. Two mechanisms could induce the asymmetry in vertical motion and cause a deviation of the TC track from the horizontal asymmetric flow. First, the large-scale asymmetric flow in the upper troposphere differs from that in the lower troposphere, both in magnitude and direction, which results in a vertical shear that could force the asymmetry. A vertical tilt of the vortex axis is also found that is consistent with the direction of shear and also the asymmetry in rainfall and vertical motion. Second, asymmetric boundary layer convergence that results from the internal boundary layer could also force an asymmetry in vertical motion.

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Martin L. M. Wong and Johnny C. L. Chan

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate tropical cyclone (TC) wind distribution near landfall. On an f plane at 15°N, the effects of the different surface roughness between the land and sea on the wind asymmetry is examined under a strong constraint of a dry atmosphere and time-invariant axisymmetric mass fields. The winds are found to adjust toward a steady state for prelandfall (50, 100, and 150 km offshore), landfall, and postlandfall (50, 100, and 150 km inland) TC positions.

The TC core is asymmetric even when it lies completely offshore or inland. The surface (10 m) wind asymmetry at the core for pre- (post) landfall position is apparently related to the acceleration (deceleration) of the flow that has just moved over the sea (land) as a response to the sudden change of surface friction. For prelandfall TC positions, the resulted strong surface inflow to the left and front left (relative to the direction pointing from sea to land) also induces a tangential (or total) wind maxima at a smaller radius, about 90° downstream of the maximum inflow, consistent with the absolute angular momentum advection (or work done by pressure). The surface maximum wind is of similar magnitude as the gradient wind. There is also a small region of weak outflow just inside the wind maxima. For postlandfall TC positions, inflow is weakened to the right and rear right associated with the onshore flow. Both onshore and offshore flows affect the surface wind asymmetry of the core in the landfall case. Above the surface and near the top of the planetary boundary layer (PBL), the wind is also asymmetric and a strongly supergradient tangential wind is primarily maintained by vertical advection of the radial wind. Much of the steady-state vertical structure of the asymmetric wind is similar to that forced by the motion-induced frictional asymmetry, as found in previous studies.

The associated asymmetry of surface and PBL convergences has radial dependence. For example, the landfall case has stronger PBL convergence to the left for the 0–50-km core region, due to the radial inflow, but to the right for the 100–500-km outer region, due to the tangential wind convergence along the coastline.

The strong constraint is then removed by considering an experiment that includes moisture, cumulus heating, and the free adjustments of mass fields. The TC is weakening and the sea level pressure has a slightly wavenumber-1 feature with larger gradient wind to the right than to the left, consistent with the drift toward the land. The asymmetric features of the wind are found to be very similar to those in the conceptual experiments.

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C. L. KIBLER, C. M. LENNAHAN, and R. H. MARTIN

Abstract

No Abstract Available.

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Arnold L. Gordon, Martin Visbeck, and Josefino C. Comiso

Abstract

Shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the Weddell Sea. During the years 1974–1976, a 250 × 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice. This feature, the Weddell Polynya, was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer. Though smaller, less persistent polynyas associated with topographically induced upwelling at Maud Rise frequently form in the area, there has not been a reoccurrence of the Weddell Polynya since 1976. Archived observations of the surface layer salinity within the Weddell gyre suggest that the Weddell Polynya may have been induced by a prolonged period of negative Southern Annular Mode (SAM). During negative SAM the Weddell Sea experiences colder and drier atmospheric conditions, making for a saltier surface layer with reduced pycnocline stability. This condition enables Maud Rise upwelling to trigger sustained deep-reaching convection associated with the polynya. Since the late 1970s SAM has been close to neutral or in a positive state, resulting in warmer, wetter conditions over the Weddell Sea, forestalling repeat of the Weddell Polynya. A contributing factor to the Weddell Polynya initiation may have been a La Niña condition, which is associated with increased winter sea ice formation in the polynya area. If the surface layer is made sufficiently salty due to a prolonged negative SAM period, perhaps aided by La Niña, then Maud Rise upwelling meets with positive feedback, triggering convection, and a winter persistent Weddell Polynya.

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Laurel L. DeHaan, Andrew C. Martin, Rachel R. Weihs, Luca Delle Monache, and F. Martin Ralph

Abstract

Accurate forecasts of atmospheric rivers (ARs) provide advance warning of flood and landslide hazards and greatly aid effective water management. It is, therefore, critical to evaluate the skill of AR forecasts in numerical weather prediction (NWP) models. A new verification framework is proposed that leverages freely available software and metrics previously used for different applications. Specifically, AR detection and statistics are computed for the first time using the Method for Object-Based Diagnostic Evaluation (MODE). In addition, the measure of effectiveness (MoE) is introduced as a new metric for understanding AR forecast skill in terms of size and location. The MoE provides a quantitative measure of the position of an entire forecast AR relative to observation, regardless of whether the AR is making landfall. In addition, the MoE can provide qualitative information about the evolution of a forecast by lead time, with implications about the predictability of an AR. We analyze AR forecast verification and skill using 11 years of cold-season forecasts from two NWP models: one global and one regional. Four different thresholds of integrated vapor transport (IVT) are used in the verification, revealing differences in forecast skill that are based on the strength of an AR. In addition to MoE, AR forecast skill is also addressed in terms of intensity error, landfall position error, and contingency-table metrics.

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Martin C. Todd, Eric C. Barrett, Michael J. Beaumont, and Joanna L. Green

Abstract

As part of the U.S. Agency for International Development/National Oceanic and Atmospheric Administration project to develop an improved monitoring, forecasting, and simulation system for the river Nile, the Remote Sensing Unit of the University of Bristol has been investigating and developing satellite infrared techniques for small-scale estimation of rainfall over the region of the upper Nile basin. In this paper, the need for variable IR rain/no-rain temperature thresholds as a basis for reliable satellite identification of rain areas over small scales is explained, and the spatially and temporally variable nature of optimum IR rain/no-rain threshold temperatures is examined.

Meteosat IR data covering a period of 17 months have been analyzed along with daily rain gauge reports for calibration and validation. Analyses have been carried out on a monthly basis. Optimum IR rain/no rain threshold temperatures over the study area in the east Africa region are shown to have exhibited a marked seasonal trend, with an annual variation approaching 40 K. Minimum threshold temperature values were found at the onset of the summer wet season, and maximum threshold temperature values during the driest winter months. Generally, summer threshold temperatures were low, around 230 K, and winter thresholds high, in the range of 240–260 K.

During the wet season, optimum IR rain/no-rain threshold temperatures exhibited a distinct pattern of spatial variation. This was modeled as a function of pixel latitude, longitude, and surface elevation. This threshold temperature model was then used to generate threshold temperature estimates at the pixel scale from an independent Meteosat dataset for 1992. Compared with the performance of spatially uniform threshold methods, marked improvements in rain-area classification accuracy were obtained. Optimum IR rain/no-rain threshold temperature variation is therefore seen to be a result of a complex interaction of climatology, meteorology, and topography, and as such the implications of this for the design and use of regional-scale rainfall monitoring techniques are discussed.

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