摘要:

Six years ago, we compared the climate sensitivity of 19 atmospheric general circulation models and found a roughly threefold variation among the models; most of this variation was attributed to differences in the models' depictions of cloud feedback. In an update of this comparison, current models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the models still have physical disagreements.

ISSN:0148-0227    

DOI:10.1029/96JD00822

年代:1996

数据来源: WILEY

摘要:

A global atmospheric general circulation model (GCM) coupled to a global 1‐degree, 20‐level ocean GCM with dynamic and thermodynamic sea ice is integrated with CO2increasing at 1% per year compounded for 75 years (CO2doubles at about year 70). Flux correction is not used in the experiment. The increase of globally averaged surface air temperature at the time of CO2doubling is 3.8°C. The warm subsurface Atlantic layer at intermediate depths in the Arctic is maintained mainly by the sinking and intrusion of water from the West Spitsbergen Current in the model and the observations. With increased CO2in the model, the warmer surface waters are intruded into the upper portion of the Atlantic layer producing an anomalous warming in the model at depths between 200 and 400 m. This resembles an anomalous warm layer near those depths recently observed in the Arctic. As the climate warms and sea ice retreats, low clouds increase over the newly exposed water. Yet the consequent increase of cloud albedo over these regions is more than compensated for by the decrease of surface albedo due to the melting of sea ice. This produces a net decrease of planetary albedo in the Arctic that contributes to a strong ice‐albedo feedback and the comparatively high sensitivity of the

ISSN:0148-0227    

DOI:10.1029/96JD00505

年代:1996

数据来源: WILEY

摘要:

A regional climate model including a physically based parameterization of the subgrid effects of topography on clouds and precipitation is driven by observed meteorology on its lateral boundaries for a period of 12 months. The meteorology simulated by the model for each subgrid elevation class is distributed across a mountain watershed according to the surface elevation within the watershed. The simulated meteorology is used to drive a detailed model of hydrology‐vegetation dynamics at the topographic scale described by digital elevation data, 180 m. The watershed model, which includes a two‐layer canopy model for evapotranspiration, an energy‐balance model for snow accumulation and melt, a two‐layer rooting zone model, and a quasi‐three‐dimensional saturated subsurface flow model, is used to simulate the seasonal cycle of the accumulation and melt of snow and the accumulation and discharge of surface water within a mountain watershed in northwestern Montana. Comparisons between the simulated and the recorded snow cover and river discharge at the base of the watershed indicate comparable if not better agreement than between the recorded fields and those simulated by the watershed model driven by meteorology observed at two stations within the watershed. The agreement with the recorded discharge, precipitation, and snow water equivalent is also clearly superior to simulations driven by the regional climate model run without the subgrid parameterization but with one‐third the grid size of the simulation with the subgrid par

ISSN:0148-0227    

DOI:10.1029/96JD00441

年代:1996

数据来源: WILEY

摘要:

To test some recent theories on the nature of precipitation structure, data were collected using radar and a newly developed “sonic” gauge to investigate precipitation variability at small scales. Results show that the structure of precipitation below a few tens of meters has very different statistics than at larger scales. Interpretation of vertically pointing radar data suggests that, at small scales, a mixing‐like process occurs because of the differential fall speed of hydrometeors, which results in the destruction of small‐scale structure. It is also shown that convection in the melting layer seems to recreate some of the structure that had been lost as snow fell. Finally, over scales of meteorological interest, at least four distinct regimes in precipitation variability can be ide

ISSN:0148-0227    

DOI:10.1029/96JD00718

年代:1996

数据来源: WILEY

摘要:

A decrease in the diurnal temperature range has been identified throughout this century in many of the world's land‐based climate records. However, documentation of the seasonality, magnitude, and geography of these changes has been hampered by the lack of homogeneous maximum and minimum temperature measurements over most of the planet. In this investigation, we attempt to use satellite‐based measurements of tropospheric temperatures to assess spatial and temporal trends in the diurnal temperature range over the period 1979 to 1994. Although quite exploratory and preliminary in nature, we find that the satellite‐based estimates of the diurnal temperature range are (1) larger over land areas than ocean areas, (2) high in summer, particularly in low latitudes, and low in winter, especially in the higher latitudes, and (3) decreasing substantially in the winter season for the land areas between 30°N and 60°N. The results are potentially compromised by many factors including orbital drift problems and changes in satellites. Despite the potential pitfalls of the satellite‐based measurements in this investigation our findings support other results showing a reduction in diurnal temperature range in the midlatitudes of the northern hemisphere during the low

ISSN:0148-0227    

DOI:10.1029/96JD00446

年代:1996

数据来源: WILEY

摘要:

We present analyses of data collected at Christmas Island in the mesosphere and lower thermosphere using a meteor echo detection and collection system (MEDAC) in addition to data collected in the troposphere and lower stratosphere using a clear air stratosphere/troposphere (ST) radar. These data, which provide 5 years of semicontinuous horizontal wind velocity estimates, are analyzed to determine the temporal structure of the quasi‐two‐day wave in the middle and lower atmosphere. Our results show the presence of strong wave activity in the middle atmosphere during late January and early February as well as a secondary maxima in June/August for each of the years we have data. However, the magnitude and duration of the wave events observed in late January and early February exhibit interannual variabil

ISSN:0148-0227    

DOI:10.1029/96JD00699

年代:1996

数据来源: WILEY

摘要:

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) has provided measurements of the horizontal wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. This data set, which spans a period of more than 3 years, has facilitated an investigation of the long‐term behavior of the background circulation on a nearly global basis. At middle and high latitudes the zonal circulation is characterized by an annual oscillation. At low latitudes (±30°) the most prominent long‐term variation above the stratopause is the mesosphere semiannual oscillation (MSAO), which maximizes near the equator at an altitude of between 80 and 85 km. Further analysis of the time series reveals an additional strong variation, with an amplitude near 30 ms−1and a period of about 2 years. This feature shows the same altitude and latitude structure as the MSAO and exhibits a phase relationship with the stratospheric quasi‐biennial oscillation (QBO). Observations from the Christmas Island MF radar (2°N, 130°W) confirm the presence of this mesospheric QBO (MQBO). These observations support recent findings from a modeling study which generates an MQBO via the selective filtering of small‐scale gravity waves by the underlying winds

ISSN:0148-0227    

DOI:10.1029/96JD00575

年代:1996

数据来源: WILEY

摘要:

An analysis of the extratropical ozone sounding data shows that ozone, temperature, and wind as well as tropopause height reveal interannual variability with timescales in the range 24–30 months. This quasi‐biennial variability is noticed, as a rule, in a whole range of ozone sounding heights (0–35 km) and connected with the equatorial quasi‐biennial oscillation (QBO). The QBOs in ozone and temperature are revealed differently in the middle and lower stratosphere as well as in the troposphere. In the middle stratosphere a downward phase progression of the ozone QBO is noticed similar to that of the equatorial dynamical QBOs. The rate of the phase descent of the QBO depends on the geographical region and varies from 0.5 km/month to 1 km/month. In the northern hemisphere the rate is greatest in the polar region but lowest in the subtropics. Local maxima are noticed in the vertical structures of the ozone QBOs. In the northern midlatitudes these maxima are centered at heights of about 12, 20, and 30 km. In the lower stratosphere the QBO in ozone is in phase with that in temperature. Mean amplitudes of the temperature QBO in the troposphere are close to those in the stratosphere. The downward phase progression of the temperature QBO is noticed in the northern midlatitude stratosphere. There is an abrupt phase shift between the QBOs in ozone and temperature in the lower stratosphere and those in the middle troposphere. Over the North American stations the phase shift is that the temperature QBOs in the troposphere are in opposite phase with those in the stratosphere. In zonal and meridional winds the QBO are revealed in the northern midlatitudes. In contrast with the QBOs in ozone and temperature, the wind QBOs reveal the small rate of the phase changing with height. There are no significant phase shifts between the wind QBO in the lower stratosphere and that in the troposphere. The maximum amplitudes of the wind QBO are noticed in the vicinity of the tropopause position and at heights of 25–30 km. Amplitudes of the QBO in zonal wind are close to those in meridional wind. The QBO of tropopause height exposed in the northern hemisphere has the greatest amplitude over Japan. The tropopause height QBO is in opposite phase (in phase) with the QBOs in ozone and temperature in the lower stratosphere (troposphere). Vertical structures of the QBOs obtained for ozone, temperature, and wind in the stratosphere as well as the corresponding QBO of tropopause height support a hypothesis that the extratropical QBO is caused by quasi‐biennial forcing of the mean diabatic meridional circulation by planetary‐scale waves with the resulting modulation of verti

ISSN:0148-0227    

DOI:10.1029/95JD03434

年代:1996

数据来源: WILEY

摘要:

Observations with a 46.5‐MHz radar system at Aberystwyth, Wales (52°N, 4°W) over the period February 1990 to May 1994 have shown a pronounced seasonal variation in the height range of mesospheric echoes. These were observed over the range 64–84 km in all seasons, but those at still greater heights were confined to summer months. Coordinated radar and lidar observations at the same site during 1993 have shown that these intense echoes in summer, analogous to polar mesosphere summer echoes (PMSE), are associated with low mesopause temperatures; the weaker echoes observed in winter seem to be associated with regions of superadiabatic lapse rates; and echoes are observed both above and below the maxima in temperature inversions commonly observed below the mesopause. The types of instability implied in these associations of echo height and temperature structure are consi

ISSN:0148-0227    

DOI:10.1029/96JD00218

年代:1996

数据来源: WILEY

摘要:

The Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE) was conducted in the spring of 1993 from Moffett Field, California (NASA Ames Research Center), utilizing the NASA high‐altitude ER‐2 aircraft. These northern midlatitude aircraft flights showed laminae containing high ozone concentrations, traceable to the April 1993 polar vortex breakup and corroborated by laminae of other trace gases such as CFCs, CH4, N2O, and CO2. These laminae are clearly traceable as polar vortex breakup fragments using Rossby‐Ertel's potential vorticity and isentropic trajectory calculations. Laminae in stratospheric ozone profiles are commonly observed in the northern hemisphere from fall to spring, and are hypothesized to originate from very low frequency transverse waves, and/or via Rossby wave breaking. On the basis of these results, the ozone laminae observed during SPADE were a result of Rossby wave breaking during the breakdown of the polar vortex. In addition, it is shown that conventional once‐per‐day meteorological analyses were adequate for representing the transport of this material into the lower stratosphere midlatitudes over the course of the spring vorte

ISSN:0148-0227    

DOI:10.1029/95JD03387

年代:1996

数据来源: WILEY