Effect of latent heat release on mountain waves in a sheared mean flow

ABSTRACT. The effect of latent heat release on windward side of the mountain due to precipitation over the mountain waves has been studied assuming wind speed changing with respect to height. A single profile based on actual Peshawar data has been considered for the analysis. A thin level of heating has been chosen at medium level for the purpose of study. For non-hydrostatic case it is observed that in non-precipitation case when balanced heating/cooling takes place on the windward/leeward side or the mountain the effect of heating is negligibly small. However, for precipitation case downward displacement on the windward side, just above the level of heating. is obvious. Interference with the upstream current by the waves, produced due to elevated thermal forcing and reflected from the around surface is attributed to this phenomenon. Increase in the wave amplitude on the lee-side of the mountain as compared to non-precipitating case is also found. It is also revealed that higher the level or heating lesser the amplitude of the induced disturbance. 4.5 km agl is the level which is maximum affected by heating in general. For large and shallow mountainous terrains. hydrostatic solutions have been produced for three different levels of heating for sheared flow, Streamlines have been drawn. On comparison with no shear case, it may be inferred that shear effect is opposite to that due to thermal forcing.

Calibrating Antarctic ice sheet mass loss due to millennial-scale ocean thermal forcing

Throughout the Late Pleistocene, millennial-scale cycles in the rate of poleward heat transport resulted in repeated heating and cooling of the Southern Ocean1. Ice sheet models2 suggest that this variation in Southern Ocean temperature can force fluctuations in the mass of the Antarctic ice sheet that transiently impact sea level by up to 15 meters. However, current geologic evidence for Antarctic ice response to this ocean thermal forcing is unable to calibrate these models, leaving large uncertainty in how Antarctica contributes to sea level on millennial timescales. Here we present a >100kyr archive of East Antarctic Ice Sheet response to Late Pleistocene millennial-scale climate cycles recorded by transitions from opal to calcite in subglacial precipitates. 234U-230Th dates for two precipitates define a time series for 32 mineralogic transitions that match Antarctic climate fluctuations, with precipitation of opal during cold periods and calcite during warm periods. Geochemical evidence indicates opal precipitation via cryoconcentration of silica in subglacial water and calcite precipitation from the admixture of meltwater flushed from the ice sheet interior. These freeze-flush cycles represent changes in subglacial hydrologic-connectivity driven by ice sheet thickness response to Southern Ocean temperature oscillations around the Ross Embayment. Changes in Ross Embayment ice mass require high ocean-ice heat exchange2, and would occur only after retreat of the West Antarctic Ice Sheet3 and large portions of the East Antarctic Ice sheet margin4. These results point to high Antarctic ice sheet sensitivity to millennial-scale ocean thermal forcing throughout the Late Pleistocene, and when combined with modeling results2, predict that an Antarctic ice volume of at least 2–5 meters sea level equivalent is vulnerable to millennial-scale climate forcing.

Opposite responses of the Indian Ocean to the thermal forcing of the Tibetan Plateau before and after the onset of the South Asian monsoon

The atmospheric circulation changes dramatically over a few days before and after the onset of the South Asian monsoon in spring. It is accompanied by the annual maximum surface heating over the Tibetan Plateau. We conducted two sets of experiments with a coupled general circulation model to compare the response of atmospheric circulation and wind-driven circulation in the Indian Ocean to the thermal forcing of the Tibetan Plateau before and after the monsoon onset. The results show that the Tibetan Plateau's thermal forcing modulates the sea surface temperature (SST) of the Indian Ocean and the meridional circulation in the upper ocean with opposite effects during these two stages. The thermal forcing of the Tibetan Plateau always induces a southwesterly response over the northern Indian Ocean and weakens the northeasterly background circulation before the monsoon onset. Subsequently, wind-evaporation feedback results in a warming SST response. Meanwhile, the oceanic meridional circulation shows offshore upwellings in the north and southward transport in the upper layer crossing the equator, sinking near 15°S. After the monsoon onset, the thermal forcing of the Tibetan Plateau accelerates the background southwesterly and introduces a cooling response to the Indian Ocean SST. The response of oceanic meridional overturning circulation is limited to the north of the equator due to the location and structural evolution of the climatological local Hadley circulation. With an acceleration of the local Walker circulation, the underlying zonal currents also show corresponding changes, including a westerly drift along the equator, downwelling near Indonesia, offshore upwelling near Somalia, and a westward undercurrent.

Enhancement of ice-phase precipitation caused by thermal forcing produces ghost echoes over the Tibetan Plateau

Whether tall storms produce heavy precipitation is currently a controversial topic. Here, we used seven years of observations from the dual-frequency precipitation radar and found that there is a rare but unique vertical precipitation structure over the Tibetan Plateau. The radar echo peaks above the freezing height, which we refer to as a “ghost echo”. The existence of a ghost echo increases the echo-top height but suppresses the increase in droplet size below it, and therefore weakens the near-surface precipitation. Compared with normal echoes, ghost echoes appear more often in the afternoon. The potentially unstable environment produced by thermal forcing is the main cause of ghost echoes, rather than the dynamic factor of wind shear. The ghost echo, which is essentially a mechanism of ice-phase precipitation enhancement, represents a type of tall but weak precipitation. Its existence adds to our current perception of the nature of precipitation.