Characteristics of the potential vorticity and its budget in the surface layer over the Tibetan plateau

Abstract

The variability of interior atmospheric potential vorticity (PV) is linked with PV generation at the Earth's surface. The present paper reveals the features of the surface PV and provides a stepping stone to investigate the surface PV budget. In this study, the formats of the PV and PV budget adopting a generalized vertical coordinate were theoretically examined to facilitate the calculation of the surface PV and its budget. Results show that the formats of the PV and PV budget equations are independent of the vertical coordinate. While the vertical component of the surface PV dominates over the platform of the Tibetan Plateau, the horizontal component plays an important role over the slopes of the Tibetan Plateau, especially the southern slope owing to the strong in-situ meridional gradient of the potential temperature. These results indicate that the employment of complete surface PV not only provides a finer PV structure but also more appropriately reveals its effect on atmospheric circulation. Diagnosis based on reanalysis and model output demonstrates that the surface PV budget equation is well balanced both in terms of the climate mean and synoptic process, and the surface PV budget in June has a prominent diurnal cycle. The diabatic heating with a minimum in the early morning and a maximum from evening to midnight contributes dominantly to this diurnal cycle. It is further indicated that positive PV generation due to diabatic heating is essential for the formation, development, and movement of the Tibetan Plateau vortex.

Interannual Impact of the North Atlantic Tripole SST Mode on the Surface Potential Vorticity Over the Tibetan Plateau During Boreal Summer

“…Following Sheng et al. (2021), surface PV is calculated as follows: $$$\text{PV}=g\left[\frac{\partial v}{\partial p}{\left(\frac{\partial \theta }{\partial x}\right)}_{h}-\frac{\partial u}{\partial p}{\left(\frac{\partial \theta }{\partial y}\right)}_{h}\right]-g\left[f+{\left(\frac{\partial v}{\partial x}\right)}_{h}-{\left(\frac{\partial u}{\partial y}\right)}_{h}\right]\frac{\partial \theta }{\partial p},$$$ where g is gravity and is equal to 9.8 m s −2 , p indicates pressure, $$\theta $$ indicates potential temperature, f indicates the Coriolis parameter, $$(u,v)$$ indicates horizontal wind, and h indicates that the horizontal difference is executed at the hybrid σ – p level. The surface PV is obtained from the two bottom levels at the hybrid σ – p level.…”

“…Since the TPSPV couples the dynamic and thermal information, we linearly expand the variation in TPSPV to investigate which one that dominates and reveals the relevant atmospheric features. Because the TPSPV is largely determined by its vertical component (Sheng et al., 2021), the result of the linear expansion of TPSPV is described as follows: $$$\begin{array}{c}\underset{\mathrm{A}}{\underline{\underline{{\Delta}\text{PV}}}}\approx \underset{\mathrm{B}}{{\underline{\underline{{g\overline{(f+\zeta )}{\Delta}\left(-\frac{\partial \theta }{\partial p}\right)}}}}}+\underset{\mathrm{C}}{\underline{\underline{{g{\Delta}(f+\zeta )\overline{\left(-\frac{\partial \theta }{\partial p}\right)}}}}}\end{array}$$$ in which $${\Delta}$$ indicates the time anomaly and the bar indicates the climate mean. This suggests that the variation in TPSPV is induced by both the variation in the static stability weighted by climatic absolute vorticity and the variation in absolute vorticity weighted by climatic static stability; for convenience, we denote each term in Equation in order as term A, term B and term C, respectively.…”

“…To measure the changes in the TP's forcing, Sheng et al (2022) indicated using the PV (Ertel, 1942;Rossby, 1940) framework at the TP surface could well depict both the mechanical and thermal changes in mountain forcing. Previous studies (e.g., Ma et al, 2019;Sheng et al, 2021;Yu et al, 2019;Zhang et al, 2021) revealed the significant influence of the TP surface PV (TPSPV) on the TP vortex, downstream extreme events, and East Asian general circulation and rainfall. However, how the TPSPV is controlled by other climate factors, especially the NAT mode of interest, remains unknown.…”

Potential vorticity perspective of the genesis of a Tibetan Plateau vortex in June 2016

“…The rapid increase in nighttime diabatic heating in the lower troposphere is of significant for TPV genesis near the surface since, according to Eq. ( 1), a strong increase in diabatic heating with height can lead to prominent PV generation over the TP (Sheng et al 2021), i.e., Usually, the thermally forced positive generation of PV (F > 0) over the TP at night ( 𝜕 θ∕𝜕z > 0 ) is compensated by the negative generation in the daytime ( 𝜕 θ∕𝜕z < 0 ), though not strictly symmetric, forming a typical diurnal cycle. When this diurnal cycle is interrupted with much stronger PV generation at night, the generation of a surface vortex becomes possible.…”

A dynamic and thermodynamic coupling view of the linkages between Eurasian cooling and Arctic warming

“…As addressed in Sheng et al (2021), the deduction process is as follows: the material change rate of (x, y, h, t) is θ = 𝜕 t 𝜃 + V ⋅ 𝜃 + ḣ𝜕 h 𝜃 when marking the vertical hybrid sigma-pressure coordinate of the model level as h.…”