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Fang Li and David M. Lawrence

Abstract

Fire is a global phenomenon and the primary form of terrestrial ecosystem disturbance on a global scale. It is tightly coupled with climate, ecosystems, carbon and water cycles, and human activities. Through biomass burning and fire-induced plant-tissue mortality, current and historical fires significantly affect terrestrial ecosystems, which can alter hydrology fluxes. This study provides the first quantitative assessment and understanding about the influence of fire on the global land water budget due to changing terrestrial ecosystems during the twentieth century. This is done by quantifying the difference between twentieth-century fire-on and fire-off simulations using the Community Earth System Model (CESM). Results show that fire significantly reduces the annual evapotranspiration (ET) over the global land by 0.6 × 103 km3 yr−1 and increases global total of runoff in almost the same quantity, while having almost no impact (0.0 × 103 km3 yr−1) on annual precipitation amount. Fire also weakens both the significant upward trend in total ET over global land prior to the 1950s and the downward trend from 1950 to about 1985 by approximately 35%. For the twentieth-century average, fire impact on ET and runoff is most clearly seen in the tropical savannas, African rain forests, and some boreal forests and southern Asian forests. Fire affects global ET and runoff through reducing vegetation canopy and vegetation height, which interact with fire-induced changes in biogeochemical cycle and result in drier and warmer surface air and higher wind speed. Globally speaking, reducing the vegetation canopy is the main pathway of fire’s impact on ET and runoff.

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Lawrence Coy and David C. Fritts

Abstract

The effect of a vertically propagating, internal gravity wave on the vertical flux of potential temperature (heat) is considered by averaging the local heat flux vector over a potential temperature surface. This approach gives the wave heat flux a simple physical picture which is not readily apparent from the more common Eulerian formulation. This method also allows the eddy diffusion coefficient to be a function of the phase of the wave. Such a phase dependent eddy diffusion has been previously considered from an Eulerian viewpoint as a model of a convectively unstable gravity wave. Here, the Lagrangian method confirms and corrects the Eulerian results. Earlier work is extended by modeling a constant amplitude “breaking” wave, as well as by considering eddy diffusion coefficients that are asymmetric with respect to the wave breaking region. In all cases studied, 1ocalizing the eddy diffusion to the region of wayebreaking decreases the average heat flux.

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Lawrence Cheng and David C. Rogers

Abstract

Observational evidence from an Alberta hailstorm was examined in an attempt to demonstrate the link between feeder clouds and hailfalls. Radar data, time resolved surface collections of hail, and cloud photographs from a storm were analyzed. It was found that the streak events in the surface hailfall can he linked to small-scale radar reflectivity maxima in the new growth region of the storm. The results suggest that the hail growth process began with packets of hail embryos in distinct feeder clouds, and that the separation between feeder clouds was eventually manifested as distinct hail streak events at the surface. The feeder clouds formed approximately in a line parallel to the vertical ambient wind shear near the cloud base level. The spacings between feeder clouds were almost equal and estimated to he 3 km. Theoretical predictions indicate that convective spacing in a horizontally uniform atmosphere is determined by environmental wind shear, stability, and depth of the shear layer. The results of this and other observational studies lead to the speculation that the spacing between distinct hail streak events may be controlled by the same factors in the vicinity of the new growth zone of hailstorms.

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David R. Dowling and Lawrence F. Radke

Abstract

A review of existing literature is made to determine typical values for the physical properties cirrus clouds. The properties examined (with typical values and measured ranges) are cloud-center altitude (9 km, 4 to 20 km), cloud thickness (1.5 km, 0.1 to 8 km), crystal number density (30 L−1, 10−4 to 10−4 L−1), condensed water content (0.025 g m −3, 10−4 to 1.2 g m −3), and crystal size (250 μm, 1 to 8000 μm). A typical crystal size distribution is also reported.

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David M. Lawrence and Peter J. Webster

Abstract

The summertime intraseasonal oscillation (ISO) is an important component of the south Asian monsoon. Lagged regressions of intraseasonally filtered (25–80 days) outgoing longwave radiation (OLR) reveal that centers of convection move both northward and eastward from the central equatorial Indian Ocean subsequent to the initiation of an ISO. Eastward movement of convection is also seen at Indian subcontinent latitudes (10°–20°N). Based on the regression results, the summertime ISO convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 20°N. Viewed along any meridian, convection appears to propagate northward while equatorial convection propagates to the east. To examine the robustness of the connection between eastward and northward movement, individual ISOs are categorized and composited relative to the strength of the large-scale eastward component of convection in the central equatorial Indian Ocean. It is found that the majority of ISOs that exhibit northward movement onto the Indian subcontinent (42 out of 54 ISOs, or 78%) also exhibit eastward movement into the western Pacific Ocean. It is also found that when convection in the central Indian Ocean is not followed within 10–20 days by convection in the western Pacific Ocean (12 out of 54 ISOs, or 22%), the independent northward movement of convection in the Indian Ocean region is somewhat stunted.

The link between the eastward and northward movement of convection is consistent with an interpretation of the summertime ISO in terms of propagating equatorial modes. The northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the Rossby cell that is excited by equatorial ISO convection. A similar convergence pattern is seen for the northern winter ISO, but it does not generate poleward movement due to relatively cool SSTs underlying the surface convergence.

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Stanley David Gedzelman and James R. Lawrence

Abstract

Precipitation samples were collected at stations in the Eastern United States for two extratropical cyclones during the Genesis of Atlantic Lows Experiment (GALE) of 1986 and analyzed for their δ18O values. They represent the first synoptic-scale datasets of isotopic values.

Measured isotope ratios are explained in terms of physical principles and meteorological processes. They are shown to be related to vertical profiles of ω cloud-top temperatures, evaporation beneath cloud base, isotope equilibration, and water vapor sources for the precipitation. Measured isotope ratios are then compared to values obtained from simple models of convective and stratiform precipitation.

Both storms are shown to exhibit a consistent pattern of isotope ratios, with lowest δ18O values occurring in the stratiform precipitation well within the cold air, and highest values associated with the convective precipitation of the warm sector. A pronounced-amount effect, in which δ18O values decrease as rainfall totals increase, is also identified at many stations. The isotopic datasets from these storms may prove useful in deriving physical calibrations for climatological relationships between mean annual surface temperature or precipitation amount and the δ18O value of its precipitation for both present and past climate patterns.

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David M. Lawrence and Peter J. Webster

Abstract

It is noted that the behavior of the intraseasonal oscillation (ISO) of the south Asian monsoon varies from year to year. An index representing seasonally averaged ISO activity is developed using outgoing longwave radiation data for the period 1975–97. Interannual variations in ISO activity are found to be related to year-to-year changes in the number of discrete events rather than to changes in the characteristic period.

Summertime ISO activity exhibits a reasonably strong inverse relationship with Indian monsoon strength but not with total south Asian monsoon strength primarily because of a lack of correlation between ISO activity and the Bay of Bengal component of the south Asian monsoon. Over the 22-yr period examined here, the relationship between Indian monsoon strength and ISO activity is comparable to or even stronger than the well-documented relationship with El Niño–Southern Oscillation (ENSO). However, summertime ISO activity is found to be relatively uncorrelated with ENSO except for a weakly positive correlation at the beginning of the south Asian monsoon season. Therefore, the ISO activity–Indian monsoon relationship is essentially independent of the ENSO–Indian monsoon relationship. ISO activity is uncorrelated with any other contemporaneous or leading sea surface temperature variability.

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Lawrence A. Hughes and David W. Saxton

An incipient severe polar outbreak that eventually affected practically all of the United States is considerably delayed by retrogression of a major trough in the westerlies. An investigation of this retrogression shows how long-wave principles aid in the prediction of such outbreaks.

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Lawrence Coy, David C. Fritts, and J. Weinstock

Abstract

Vertically propagating, compressible, internal gravity waves are shown to have a vertical Stokes drift which is proportional to the vertical wave energy flux. In regions of the atmosphere dominated by upward propagating waves, such as the summer mesosphere, this Stokes drift will be upward. For the Lagrangian mean parcel motion to be small, a downward mean Eulerian velocity must exist to largely oppose the upward Stokes drift. These. results may explain the downward mean Eulerian velocity observed at Poker Flat, Alaska in the summer mesosphere.

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Stanley David Gedzelman and James R. Lawrence

Abstract

The deuterium/hydrogen ratios δD of sequential samples of precipitation at Palisades, New York were measured during four separate storms during January 1978. The values ranged from a low of −177‰ to a high of −30‰. For each of the storms there was a general increase of δD values with time.

The changes of δD values are explained by means of detailed meteorological analysis assuming that the precipitation derives from a Rayleigh condensation process in which all condensation in the air column directly above the station is assumed to fall immediately to the ground. The resulting calculated values of δD usually agree to within 5‰ of the observed measured values.

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