Thermodynamical interpretations of generation and decay of stable layers
over the Indochina Peninsula in the dry season*Masato I. Nodzu1, Shin-Ya Ogino1, Yoshihiro Tachibana2, 3, Manabu D. Yamanaka1, 4(1: Kobe University, 2: Tokai University, 3: FRCGC/JAMSTEC, 4: IORGC/JAMSTEC)* Graduate School of Science and Technology, Kobe University,
1-1 Rokkodai-chou, Nada, Kobe, 657 8501, Japan. e-mail: nodzu@ahs.scitec.kobe-u.ac.jp
Abstract
The thermal and moisture budget analyses were investigated on intraseasonal variations of stable layers over theIndochina Peninsula in the dry season from 1999 to 2000. The result indicates the diversity of factors of the generationand decay processes of stable layers and implies a weak tendency that a specific mechanism is important in the early
dry season.
Keyword: temperature inversion, intraseasonal oscillation, monsoon onset

1. Introduction

We have investigated seasonal variations of theatmospheric stability in the lower troposphere and theirmechanisms in order to understand the role of the strong
stable layer seen in the lower troposphere in the dryseason. We have shown that strong stable layersfrequently appear at a height of about 2 km in the early
dry season and that their mean height increases up to
about 5 km and become weak in the late dry season as aclimatological characteristic. We have also shown thatintermittent appearances of generation and decay of stablelayers with a time scale from ten to fifty days dominate ina seasonal advance of each year (Nodzu et al., 2004,
submitted to J. Climate). In this paper, we investigate thegeneration and decay mechanisms of stable layers with theintraseasonal time scale in terms of thermodynamics.

On the other hand, when it was decayed, relatively large
increase of the potential temperature was recognized below
500 hPa. It is found that the heating above 500 hPa
during the stable layer generation was caused by thehorizontal warm advection and that the heating below 500
hPa during the decay was caused by the horizontal warm
advection and the Q1 (apparent heat source) (Fig. 1b). It isinferred from a comparison of temporal and spatial
variations of Q1 and Q2 that the heating by Q1 below 500
hPa during the decay was mainly caused by the release oflatent heat. In this example, therefore, the stable layer isconsidered to have been generated by the upper horizontal
warm advection and decayed by the lower horizontal warm
advection and the latent heat release.

2. Analysis procedure

In this study, stability is defined with the verticalgradient of potential temperature. In this definition, thetemporal change of stable layer strength can be interpreted
as increase or decrease in difference between the potential
temperatures above and below the stable layer. We
consider, therefore, that heating or cooling which bring
the potential temperature difference is a cause ofgeneration and decay of a stable layer. We estimate theheating and cooling rate by the thermal and moisturebudget analysis.
NCEP/DOE AMIP-II reanalysis data (http://wesley.
wwb.noaa.gov/reanalysis2/) are utilized in the theramal
and moisture budget analysis. The budget is calculated
over the triangle region in the western inland of theIndochina Peninsula which includes six grid points ((20.0o N, 100.0o E), (17.5o N, 100.0o E), (15.0o N, 100.0oE), (17.5o N, 102.5o E), (15.0o N, 102.5o E), (15.0o N,105.0o E) ). The analysis duration is from October 1999
to May 2000.

3. Results
First, we describe an example of generation and decayof stable layers. During the period from 11 to 25 April
2000, just before the onset of rainy season, a stable layer
was generated around a height of 500 hPa and decayed(Fig. 1a). When it was generated, increase of the potential
temperature above 500 hPa dominated and the potential
temperature change below 500 hPa was not significant.

Fig. 1: An example of generation and decay eventsof stable layers over the western part of the inland
Indochina Peninsula. The horizontal axes show theperiod from 11 to 25 Apr 2000. a) Stability (shade)and changing rate of potential temperature (solidcontour). b) Same as a) but for the stability,
horizontal potential temperature advection (solidcontour) and Q1 (dotted contour).
Next we summarize the generation and decay
mechanisms for all the ten cases with a time scale fromten to fifty days. The cause of generation is lower
horizontal cold advection for six cases, upper horizontal
warm advection for two cases and upper Q1 for two cases.

The cause of decay is lower vertical warm advection for
four cases and lower Q1 for three cases including twocases in which the latent heating is considered to be themain cause of the heating. It is, therefore, said that one
mechanism did not always dominate every event. It
should be noted, however, in the early dry season stablelayers were generated frequently by the lower horizontal
cold advection and decayed frequently by the lower
vertical warm advection, on the other hand, in the late dry
season the other reasons caused the variation of stablelayers in the late dry season.

4. Summary

The thermodynamical interpretation of generation anddecay mechanisms were performed on intraseasonalvariations of stable layers over the Indochina Peninsula inthe dry season from 1999 to 2000 by the thermal andmoisture budget analysis. We did not find a common
particular mechanism explaining all the generation anddecay processes. However, we found a weak tendency thata specific mechanism becomes important in the early dry
season. These findings will be confirmed by expanding
the analysis duration into twenty-five dry seasons from1979 to 2004.