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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)