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Xin Lin and Richard H. Johnson

Abstract

Rawinsonde, surface, and satellite data collected from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate the distributions of beating, moistening, precipitation, and evaporation over the western Pacific warm pool. The behavior of the atmosphere and the response of the ocean surface before, during, and after the passages of westerly wind bursts are examined. The tropospheric vertical wind shear associated with tropical low-frequency oscillations strongly modulated convective beating and moistening. Heavy precipitation usually fell 1 to 3 weeks prior to the peak westerly wind bursts. SSTs reached their maximum during the undisturbed phase of the intraseasonal oscillations (ISOs), gradually decreased as convection intensified, and reached their minimum during the periods of peak westerly winds when deep convection was generally suppressed over the intensive flux array (IFA).

Surface latent heat fluxes were positively correlated with surface wind speed and varied between 50 and 100 W m−2 during light winds to more than 200 W m−2 during strong westerly wind bursts. Surface sensible heat fluxes, however, did not follow the pattern of surface wind speed and usually peaked during organized deep convection over the IFA.

Intensive observing period (IOP)-mean evaporation and sensible heat flux over the large-scale array (LSA) were obtained by adjusting the European Centre for Medium-Range Weather Forecasts (ECMWF) fields toward buoy estimates over the IFA. These values were then used to determine IOP-mean rainfall distribution over the LSA from the moisture budget. The results generally compare well with two satellite remote-sensing estimations, SSM/I retrievals and GPI, and the ECMWF model forecast. All four methods indicate an east-west zone of minimal rainfall along the equator. Two heavy rainfall bands coinciding with the double-ITCZ structure were located north and south of the equator. The IFA was mainly located within the minimum rainfall band. Budget-diagnosed rainfall rates over the IFA agree with SSM/I retrievals, but the GPI values are excessive, particularly at times of extensive cirrus.

Comparison of the apparent heat source Q 1 profiles suggests that the long-term mean heating rates over the warm pool have large positive values at all levels of the atmosphere and peak between 400 and 450 hPa. In contrast, the apparent moisture sink Q 2 profiles show distinctly different features over the warm pool. Positive values (indicating drying) exist in the ITCZ bands north and south of the IFA. However, low-level moistening is evident in the profiles over the IFA, probably due to strong evaporation and upward transport of moisture by shallow cumuli during high winds.

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Lin Jiang and Paul H. Leblond

Abstract

Submarine landslides are a common cause of tsunamis in coastal and estuarine areas. To study this phenomenon, a numerical model is developed to simulate tsunami generation due to a viscous mudslide on a gentle uniform slope. A formulation of the dynamics of the problem is presented, where the mudslide is treated as an incompressible three-dimensional viscous flow. Seawater is treated as an inviscid fluid, and the water motion is assumed irrotational. The long-wave approximation is adopted for both water waves and the mudslide. The resulting differential equations are solved by a finite-difference method. The focus of this paper is to examine the effects of the longitudinal spreading as well as the transversal spreading of the slide upon surface wave generation, and the spreading of water waves sideways. Three-dimensional pictures are presented for successive profiles of the mud surface, the horizontal velocities of the mudslide, the evolution of the surface elevations, and the velocities of the water motion. Comparisons of the present three-dimensional calculations with previously published two-dimensional results indicate small differences for large length/width ratios for a small time after the initiation of the slide. Generally, however, the water surface profiles deviate significantly from the two-dimensional results. Adequate simulations thus require accurate representation of the aspect ratio of the sliding mass.

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Robert H. Weisberg and Lin Qiao

Abstract

Horizontal divergence and vertical velocity (w) are estimated at 0°, 140°W using an array of five subsurface moored acoustic Doppler current profilers deployed from May 1990 to June 1991 during the Tropical Instability Wave Experiment. The record-length mean flow is divergent within the near-surface region and convergent within the thermocline, with maximum convergence located at the high speed core of the Equatorial Undercurrent (EUC). This pattern of divergence results in upwelling at and above the EUC core (with maximum value of 2.3 × 10−5 m s−1 located at 60-m depth) and downwelling below the core. The relative slopes in the zonal plane between the mean velocity vectors and the isotherms suggest a net diffusive heat flux. Assuming that this occurs vertically, an entrainment velocity parameterization provides an estimate of the “diapycnal vertical velocity” profile that reverses sign at the EUC core depth. Several kinematical and dynamical consistency checks are developed on both the time-dependent and the mean motions to supplement a discussion of errors for the mean w profile. The time-dependent fluctuations in w may be an order of magnitude larger than the mean values, and on synoptic timescales w may be directed either up or down over the entire upper 250-m region sampled.

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Richard H. Johnson and Xin Lin

Abstract

The western Pacific warm pool experiences the greatest rainfall of any oceanic region on earth. While the SST is everywhere high over the warm pool, there is great spatial and temporal variability in rainfall. Sounding data from the recent TOGA COARE are used to document this variability. In particular, the vertical distributions of heating and moistening at different phases of the 30–60 day or intraseasonal oscillation are determined for different areas within the warm pool.

While heating and moistening distributions near the equator over the warm pool are often similar to those observed over the western Pacific within the convectively active ITCZ, these profiles do not prevail at all times. In particular, during westerly wind bursts and suppressed, light-wind periods, heating and moistening distributions over the COARE Intensive Flux Array frequently resemble those observed in the trade wind belts. Such profiles are characterized by relatively large negative values of apparent moisture sink Q 2 in the lowest 2–3 km, reflecting the important moistening effects of shallow, mostly nonprecipitating cumulus clouds. A maximum in moistening commonly occurs in the lower part of the cloud layer during the westerly wind bursts, indicating many “forced” cumuli that are extensions of boundary-layer turbulence. During suppressed, light-wind periods the moistening peak shifts to the upper part of the cloud layer, indicating a larger proportion of “active” trade cumulus at that time (consistent with higher SSTs and weaker vertical wind shear).

A spectral cloud diagnostic model is used to determine the properties of the shallow cloud fields. Computed profiles of mass fluxes, entrainment, detrainment, and heat and moisture balances during these periods generally resemble those for the western Atlantic trade wind regime. These findings lead to the conclusion that, in association with the intraseasonal oscillation, the western Pacific warm pool boundary layer periodically develops trade-wind-like characteristics with abundant shallow cumulus, and these episodic “tradelike” regimes are frequent enough to impact the seasonal-mean distributions of boundary layer heating and moistening.

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Xin Lin and Richard H. Johnson

Abstract

Rawinsonde and satellite infrared radiation (IR) data from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western Pacific warm pool. Infrared data for the 4-mo Intensive Observing Period (IOP) and vertical motion fields indicate that the intensity of convection, height of maximum upward motion, and SST all increased from west (140°E) to east across the COARE domain. IOP-mean IR data show a double ITCZ (Intertropical Convergence Zone) structure north and south of the Intensive Flux Array (IFA, centered at 2°S, 156°E), although marked variability in the patterns occurred on a month to month basis.

Three prominent westerly wind bursts occurred over the IFA during the 4-mo IOP in association with the intraseasonal oscillations (ISOs). Strong upward motion usually occurred 1–3 weeks prior to the peak low-level westerlies. Subsidence dominated when the westerly winds prevailed. COARE data reveal that the vertical wind shear (more than 50 m s−1 from 850 to 100 hPa) and the vertical extent of westerlies during the peak westerly wind bursts were far greater than previously recognized. The mean low-level equatorial flow over the western Pacific was westerly, interrupted occasionally by brief periods of easterly flow. The perturbation westerlies to the west of the disturbance associated with the ISO were usually stronger than the perturbation winds to the east. Maximum surface latent beat flux usually occurred during the peak westerlies, whereas the surface sensible heat flux peaked prior to the strongest westerlies.

The IOP-mean divergence profile over the IFA shows a very weak divergence near the surface and weak convergence at middle and low levels. The ITCZ-band divergence profiles show strong low-level convergence from the surface to about 700 hPa. The striking difference between the divergence profiles along the equator over the IFA and those north and south in the ITCZ bands suggests that, although the divergence and vertical motion profiles tend to look alike whenever and wherever the convection is strong, great care should be exercised in generalizing divergence and vertical motion profiles from one region to another over the western Pacific warm pool.

Correlations between cold clouds and vertical motion indicate that cold clouds are a good indicator of upper-level upward vertical motion but not low-level vertical motion. In a significant number of cases, low-level downward motion occurred under very cold cloud tops over the warm pool, indicating extensive optically thick anvil cloud and nonprecipitating high cirrus are a common occurrence over the warm pool.

The IOP-mean relative humidity profile over the IFA shows a primary peak at low levels at the top of the mixed layer and a secondary peak near 550 hPa (near the 0°C level). The secondary peak is not present in either ECMWF or NMC operational analyses, and the midtroposphere is much drier in the two model-assimilated results.

A synthesis of the kinematic and thermodynamic characteristics of the December–early January westerly wind burst as it passed the IFA is presented.

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G. T. Diro and H. Lin

Abstract

Accurate and skillful subseasonal forecasts have tremendous potential for sectors that are sensitive to hazardous weather and climate events. Analysis of prediction skill for snow water equivalent (SWE) and near-surface air temperature (T2m) is carried out for three (GEPS, GEFS, and FIM) global models from the subseasonal experiment (SubX) project for the 2000–14 period. The prediction skill of SWE is higher than the skill of T2m at week-3 and week-4 lead times in all models. The GEPS forecast tends to yield higher (lower) prediction skill of SWE (T2m) compared to the other two systems in terms of correlation skill score. The snow–temperature relationship in reanalysis is characterized by a strong negative correlation over most of the midlatitude regions and a weak positive correlation over high-latitude Arctic regions. All forecast systems reproduced well these observed features; however, the snow–temperature relationship is slightly weaker in the GEPS model. Despite the apparent lack of skill in temperature forecasts at week 4, all three models are able to predict the sign of temperature anomalies associated with extreme SWE conditions albeit with reduced intensity. The strength of the predicted temperature anomaly associated with extreme snow conditions is slightly weaker in the GEPS forecast compared to reanalysis and the other two models, despite having better skill in predicting SWE. These apparent disparities suggest that weak snow–temperature coupling strength in the model is one of the contributing factors for the lower temperature skill.

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Xin Lin and Richard H. Johnson

Abstract

Heat and moisture budgets and mesoscale circulation features for the developing, mature, and dissipating stages of an intense frontal squall line that occurred in the central United States are investigated. The slow propagating behavior of the squall line made the dataset unique since observations covered a large fraction of the squall line life cycle. Budgets have been performed at six different times at intervals of 90 minutes using 1985 OK PRE-STORM rawinsonde data.

The squall line was followed by a low-level cold front. The flow pattern normal to the squall line was generally similar to previous squall line studies except that a low-level rear inflow associated with the cold front was superimposed upon expected squall line FTR/RTF (front to rear/rear to front) flows. The midlevel RTF flow was quite weak well behind the squall line during the developing and mature stages and significantly strengthened during the dissipating stage as the stratiform region developed, suggesting that internal processes within the expanding stratiform region played an important role in RTF flow development.

A convergence band resulting from system RTF and FTR flows extended upward and rearward from low levels near the leading edge of the system. During the developing and mature stages, peak convergence was located at low levels around the leading edge. At the dissipating stage, midlevel convergence behind the convective region intensified as the stratiform region developed, while low-level convergence near the leading edge gradually weakened.

Both the apparent heat source Q 1, and apparent moisture sink Q 2 showed an increasing upshear tilt when the stratiform region developed, as did the vertical velocity field. The system-averaged heating peak Q 1 was located at middle levels between 500 and 550 hPa throughout the evolution. The moisture sink Q 2 exhibited a single drying peak, which resulted from the convective region, at low levels around 700 hPa through most of the developing and early mature stages. During the late mature and dissipating stages, a double-peak structure in Q 2 become very pronounced. The coexistence of convective and stratiform drying appears to be the causal mechanism for the double peak in Q 2 at these stages. At later stages, a single drying peak resulting from the stratiform region was present at middle levels around 475 pHa.

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Y. J. Lin, T. C. Wang, and J. H. Lin

Abstract

Some dynamic and thermodynamic properties of a convective cell within a squall line that occurred on 6 June 1979 were studied based on dual-Doppler observations. The domain under investigation had a horizontal dimension of 27 km × 27 km with 12 levels in the vertical. The grid spacing used was 1 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Fields of deviation perturbation pressure, density and virtual temperature were recovered from a three-dimensional wind field using the thermodynamic retrieval method. These retrieved fields were then subjected to internal consistency checks to determine the level of confidence.

Our findings demonstrate that thermodynamic retrieval is feasible when random errors inherent in the radial wind components are minimized by proper smoothing. Errors in the computation of vertical velocity can be substantially reduced when a variational approach is used with the anelastic continuity equation applied to the vertically integrated horizontal mass divergence as an integral constraint. Results show that the gust front (GF) is primarily responsible for vigorous convection in the storm. Distinct features of strong wind shear, pressure change and temperature contrast are evident across the GF. The derived pressure and temperature perturbations are closely related to the updraft–downdraft structure. In particular, high pressure forms on the upshear side of an updraft with low pressure on the downshear side. The orientation of maximum pressure gradient across an updraft is in the direction of the environmental shear vector. Strong perturbation temperature gradients occur in the vicinity of an updraft with warning on its upwind side and cooling on its downwind side. The appearance of a downdraft in the immediate vicinity of an updraft is of importance in affecting the magnitude and distribution of pressure and temperature perturbations within the storm.

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Lin H. Chambers, Bing Lin, and David F. Young

Abstract

New data products from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission Satellite have been examined in the context of the recently proposed adaptive tropical infrared Iris hypothesis. The CERES Single Scanner Footprint data products combine radiative fluxes with cloud properties obtained from a co-orbiting imaging instrument. This enables the use of cloud property–based definitions of the various regions in the simple Iris climate model. Regardless of definition, the radiative properties are found to be different from those assigned in the original Iris hypothesis. As a result, the strength of the feedback effect is reduced by a factor of 10 or more. Contrary to the initial Iris hypothesis, most of the definitions tested in this paper result in a small positive feedback. Thus, the existence of an effective infrared iris to counter greenhouse warming is not supported by the CERES data.

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H. Nakamura, G. Lin, and T. Yamagata

Decadal wintertime variability in the North Pacific climate system observed over the last few decades is documented. The decadal sea surface temperature (SST) variability is found to be concentrated around two major oceanic fronts. The variability around the subtropical front, accompanied by the anomalous subtropical high, exhibits strong negative simultaneous correlation with the tropical SST variability, but that around the subarctic front does not. In fact, cooling around the subarctic front in the mid-1970s cannot be attributed to the influence through the atmosphere of tropical warming that occurred about two years later. During the coolest period around the subarctic front in the mid-1980s, the enhanced surface westerlies associated with the intensified Aleutian low seemed to reinforce the underlying SST anomalies. The westerlies tended to be substantially weaker during the warmest period around 1970. These findings are suggestive of self-maintaining mechanisms inherent to the northern North Pacific climate system for the decadal variability.

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