ABSTRACT - DRUM - University of Maryland

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only weak vertical motions, and this layer is dominated by radiative processes. The interface between the troposphere and the stratosphere is referred to as the tropopause. There exist several definitions of the tropical tropopause, but no consensus has been reached yet on the exact definition [Highwood and Hoskins, 1998]. Furthermore, recent research has shown that the transition from the troposphere to stratosphere in the tropics should be understood as a gradual one rather than a sudden change at a sharp material boundary [Highwood and Hoskins, 1998; Sherwood and Dessler, 2000; Thuburn and Craig, 2002]. The concept of a tropical tropopause ‘layer’ dates back to Newton and Persson [1962]. They pointed out the existence of a “secondary tropical tropopause” well below the main tropopause. Atticks and Robinson [1983] also suggested the tropical tropopause as a “transition layer” between about 140 and 60 mb instead of a discrete boundary. Sherwood and Dessler [2000, 2001] called this transition region the Tropical Tropopause Layer (TTL), which is defined as the region of the tropical atmosphere extending from the zero net radiative heating level (355K, 150 hPa, 14 km) to the highest level that convection reaches (~420 – 450 K, 70 hPa, 18 - 20 km). The TTL can be thought of as a transition layer between the troposphere and stratosphere, having characteristics of both regions. This study uses the same definition of the TTL as that used by Sherwood and Dessler [2000,2001]. Below the bottom of the TTL, the clear sky radiation shows net radiative cooling [Folkins, 2002], and this radiative cooling is balanced by the latent heating through convection. Above the top of the TTL, net radiative heating balances the upwelling 3
forces from the stratosphere by wave driven stratospheric pumping [Holton et al., 1995]. In the TTL, radiative heating balances the cooling by the mixing of overshooting cumulus clouds and the downward extension of the Brewer-Dobson circulation. However, the relative amount of their contribution is not clear. 1.2.2 Convective impact in the TTL No common agreement has yet been reached on what role convection plays in setting temperatures near the tropical tropopause, especially near the cold point. In this region, both convective and radiative effects control the temperature profile [Held, 1982]. In the classical point of view, the tropical cold point has been considered as a division between convective and radiative control of the temperature sounding. It is clear that the thermal structure is maintained by convection in the lower and middle troposphere, where the lapse rate is very close to the moist adiabat. However, in the upper troposphere, the temperature profile deviates substantially from the moist adiabat, so it is hard to verify the convective impact in this region. The question is whether convective impact can reach up to the tropical tropopause and affect properties in the region, such as temperature and humidity. Recent studies, based on a one-dimensional radiative convective equilibrium model, suggest that the cold point in the temperature profile is actually a stratospheric feature that has nothing to do with convection [Kirk-Davidoff et al., 1999; Thuburn and Craig, 2002]. Thuburn and Craig [2002] showed that a temperature minimum in radiative equilibrium can exist well 4
Page 2 and 3: the time evolution of local convect
Page 5 and 6: ACKNOWLEDGMENTS I foremost would li
Page 7 and 8: 2.3.4 Regional distribution .......
Page 9 and 10: LIST OF FIGURES 1.1 Convective impa
Page 11 and 12: 3.6 A schematic of possible errors
Page 13: This dissertation includes two stud
Page 17 and 18: Figure 1.1 describes the impact of
Page 19 and 20: Detrained air from convective cloud
Page 21 and 22: water vapor, as well as the chemica
Page 23 and 24: vapor mixing ratio with altitude in
Page 25 and 26: Instrument [H 2 O] e (ppmv) Period
Page 27 and 28: that recent lidar images of thin ci
Page 29 and 30: Troposphere Exchange Project (STEP/
Page 31 and 32: 1. Observation of tropopause coolin
Page 33 and 34: are locally associated with cold an
Page 35 and 36: channels of four weather satellites
Page 37 and 38: Figure 2.1 An example of convective
Page 39 and 40: Temperature (K) Figure 2.3 Same wit
Page 41 and 42: temperature profile. The anomalies
Page 43 and 44: Figure 2.4 Cooling rate calculation
Page 45 and 46: Figure 2.5 Sensitivity tests in (a)
Page 47 and 48: Potential temperature (K) Figure 2.
Page 49 and 50: 2.4.2 Cloud-top radiative cooling I
Page 51 and 52: the daytime and nighttime convectiv
Page 53 and 54: which implies approximately 35% of
Page 55 and 56: Chapter 3. A study of tropical deep
Page 57 and 58: not quantitatively indicate the ext
Page 59 and 60: Figure 3.1 A cartoon of overshootin
Page 61 and 62: there is, as expected, a general sy
Page 63 and 64: Figure 3.3 Geophysical distribution
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Figure 3. 5 Mean of scatter plots i
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horizontal error bars in Fig. 3.5 a
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example, if we use A = 3 km, s = 1,
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Figure 3. 7 A moist adiabatic model
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Figure 3. 8 Total heights of the ov
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Chapter 4. Summary and Conclusions
Page 77 and 78:
In order to see how fast this conve
Page 79 and 80:
is estimated from visible and near-
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-7 K/day. This cooling speed is bey
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References Abbas, M. M., H. A. Mich
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Dessler, A. E. and H. Kim, Determin
Page 87 and 88:
Hurst, D. F., G. S. Dutton, P. A. R
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Kummerow, C., W. Barnes, T. Kozu, J
Page 91 and 92:
Randel, W. J., F. Wu, and W. Rivera
Page 93 and 94:
Stenke A. and V. Grewe, Impact of d
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