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622 H. UEDA, M. SHINODA AND H. KAMAHORI 4.1. Seasonal evolution 4. MEAN HEAT AND MOISTURE BUDGETS To examine more quantitatively the heating and moisture processes relevant to the snow disappearance over the EEP, we examine the atmospheric heat and moisture budgets. The advective, diabatic, and adiabatic heating estimates are obtained by the thermodynamic and moisture balance equations from a 12 h sequence of upper air data: ∂T ∂t ∂q ∂t RT = −V ·∇T + ω cpP ∂q Q2 = −V ·∇q − ω − ∂p Lc ∂T − + ∂p Q1 Cp where T is temperature, q is the mixing ratio of water vapour, V is the horizontal wind, ω is the vertical p-velocity, cp is the specific heat for dry air, and Lc the latent heat of condensation. Q1 and Q2 are called the apparent heat source and moisture sink respectively, because of possible contributions resulting from unresolved eddies associated with dry thermal convection and moist convection (Yanai et al., 1973). As shown by Yanai et al. (1973), vertically integrating Equations (2) and (3) from the tropopause pressure PT to the surface pressure Ps, we obtain where 〈Q1〉 = 〈QR〉 + LcP + S (4) 〈Q2〉 = Lc(P − E) (5) 〈〉 = 1 g Ps PT () dp (6) P, S, E and QR are respectively the precipitation rate, the sensible heat flux, the evaporation rate per unit area at the surface and the radiative heating rate. We compare the horizontal advection, vertical advection, and adiabatic heating terms averaged over the key region (30–60 °E, 45–60 °N) (Figure 5(a)). We recognize that the local warming over the EEP in March and April (thin solid line) is indeed the result of horizontal (meridional plus zonal) warm advection by southwesterly winds and, particularly in mid March, the adiabatic warming by descending air contributes, in part, to the local temperature increase. Then, the horizontal warm advection tends to decrease toward the summer season. The time series of the 〈Q1〉 and 〈Q2〉 (as expressed in Equations (2) and (3)) reveal drastic seasonal changes during April–May of the snow-disappearance timing (Figure 5). Prior to this period, a large amplitude of negative 〈Q1〉 is manifested during March through to early April, in conjunction with the course of marked snowmelt (Shinoda, 2001; Shinoda et al., 2001). This fact indicates that the heat, gained through the above-mentioned horizontal warm advection and adiabatic process, is largely used for the snowmelt through the sensible heat flux included in 〈Q1〉 (see Equation (4)). Aizen (2000) also pointed out that the advective energy is the same as the heat used for snowmelt in a central area of the EEP during early April, which coincides with our results. Their estimates of the heat consumed for snowmelt have amplitudes comparable to our 〈Q1〉 values. The EEP region reveals a moisture sink (a positive 〈Q2〉) between winter and the end of April, indicating snowfall. 〈Q2〉 values abruptly turn to negative from early May, which is concurrent with the enhanced surface evaporation, as will be found in Figure 7. The moisture advection terms changed in conjunction with those of the heat budget. Namely, the large meridional transport of water vapour is recognizable in April, whereas it decreases abruptly in May. These variations are closely associated with the northward advection of warm and moist air in April and the southward intrusion of cold and dry air in May. Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003) (2) (3)
a) 80 60 40 20 0 -20 -40 -60 -80 b) 20 10 0 -10 JAN JAN EURASIAN SNOW COVER AND ATMOSPHERIC CIRCULATION 623 FEB MAR APR MAY JUN 5 10 15 20 [Wm -2 ] Q2 zonal adv meridional adv adiabatic local change 5 10 15 20 pentad number Q1 zonal adv meridional adv adiabatic local change 25 30 35 FEB MAR APR MAY JUN 25 30 35 Figure 5. Climatological (1980–90) time series of atmospheric vertically integrated (a) heat and (b) moisture budgets for the EEP (45–60 °N, 30–60 °E) in units of W m −2 . Diabatic heat source (Q1; thick line), zonal heat advection (−u∇T ; open circles), meridional heat advection (−v∇T ; filled circles), vertical advection plus adiabatic compression (ω∇T − ωRT /Cpp; dashed line) and local time change (∂T /∂t thin line) are shown. Moisture sink (Q2), zonal moisture advection (−u∇q; open circles), meridional moisture advection (−v∇q; filled circles), vertical moisture advection (ω∂q/∂p; dashed line) and local time change (∂q/∂t; thin line) are shown 4.2. Vertical cross-sections Figure 6(a) shows the mean vertical distribution of the heating rate Q1/cp in the latitudinal plane along 30–60 °E. Over the EEP region, an apparent cooling is dominant in the lower troposphere below 700 hPa, which is compensated by the advection heating (Figures, 6(b) and (c)) as well as the adiabatic heating Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003)
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