The uncertainties of residual diagnosis of atmospheric diabatic heating from ERA40 and NCEP reanalysis over Tibetan Plateau

Abstract

The Tibetan Plateau (TP), a giant elevated landscape, plays a key role in modulating the Asian summer monsoon via dynamic, thermodynamic, and mechanical lifting mechanisms. The spatial and temporal distribution of atmospheric diabatic heating (DH) is vitally important to understand the coupling of energy and moisture between the land surface and the atmosphere over TP. In this study, the residual diagnosis of DH from ERA40 and NCEP reanalysis, in combination with the precipitation and latent heating (LH) results from Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR) are investigated to reveal the uncertainties in DH over TP. Significant discrepancies of column integrated DH (CDH) between ERA40 and NCEP are found in the southern slope area of TP in summer. The CDH from ERA40 is much stronger than NCEP at high altitudes on the southern slope of TP. The area with strong ERA40 atmospheric heating extends from lower altitudes to higher altitudes exceeding 4000 m isohypse. However, in the heating diagnosis of NCEP, strong atmospheric heating is only located at altitudes lower than 4000 m isohypse. These differences are evident in DH at both the ground layer and 400–500 hPa layer, revealing a significant contribution from LH because sensible heating (SH) peaks at near-surface atmosphere and LH peaks at low to middle atmosphere. As an indicator of LH, the precipitation in southern TP estimated by ERA40 is strong (>10 mm/d) in the 25°–30°N area from April to October. The precipitation observations from TRMM PR and the diagnosis from NCEP diagnosis do not agree about this phenomenon. Instead, the precipitation detected by TRMM PR is mainly located in the area to the south of 25°N. The spatial pattern of overestimated precipitation by ERA40 mostly matches the DH overestimation. Based on the vertical cross-section of DH and the atmospheric circulation along 92.5°E, strong positive ERA40 DH (>4 K/d) over the high altitudes of southern TP (~25°–30°N) extends from surface to high troposphere (~300 hPa) vertically in summer. This fact indicates there is strong latent heating released from deep convective rains in those areas which makes a contribution to DH that is comparable with SH. The associated NCEP DH in the same area can reach only the low troposphere (~700 hPa) and the maximum heating generally is located in the near-surface atmosphere. This conclusion implies that in NCEP diagnosis, only shallow and weak rains can occur in this area and the surface SH dominates DH here. Even at the south edge of main body of TP (to north of 30°N), ERA40 DH in summer can be as high as 6–9 K/d and extend from the surface to ~350 hPa isobaric surface, while the associated NCEP DH is only less than 4 K/d. The diagnosis of vertical velocity and horizontal divergence at the 200 hPa layer from ERA40 is also stronger that from NCEP. The mean vertical profiles of ERA40 DH in summer show positive heating through the surface to 200 hPa and maximum value at 700 (500) hPa on the south (southwest) slope of TP. The shape agrees well with that of the TRMM LH profile, but the value of ERA40 DH is about 2–3 times that of TRMM LH. The NCEP DH profiles show a different shape with peaks at the near-surface layer indicating more contribution from surface SH than from LH, but its amplitude is relatively closer to TRMM LH. We hypothesize that the uncertainties of the vertical structure of precipitation and the associated LH in the southern slope area of TP are the main obstacles preventing us from understanding the real mechanisms of TP-atmosphere interaction.