Mesoscale Processes
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8 Mesoscale processes Richard H. Johnson 8.1 INTRODUCTION Within Asia and other monsoon regions of the World, most signi®cant weather events are localized or mesoscale in nature. The term mesoscale generally refers to horizontal scales between ten and several hundreds of kilometers, which lie between the scale height of the atmosphere and the Rossby radius of deformation (Ooyama, 1982). The latter scale can become quite large in equatorial regions owing to the weakness in background rotation, hence mesoscale processes in the tropics can occur over a broad range of horizontal scales. Although local weather is in¯uenced by processes ranging from the largest scales to the smallest, those on the mesoscale have the most direct impact. In the Asian monsoon region, there are a multitude of mesoscale processes that in¯uence the weather. Convection is arguably the most important, contributing through latent heat release to the energetics of the large-scale monsoon circulation. Its far-reaching eects impact short-term weather, the diurnal cycle, as well as intraseasonal, seasonal, and interannual variability of the monsoon. Convection responds to and modulates its environment through a wide range of processes that can be classi®ed as local, advective, or dynamical (Johnson and Mapes, 2001). These processes are associated with convective preconditioning and triggering, as well as the feedback of convection onto its environment and larger scales of motion. In addition to convection, a wide range of other mesoscale processes in¯uence the Asian monsoon. Notable among these are topographically forced local circula- tions (including ¯ow blocking, sea and land breezes, and mountain and valley circulations), jets, surface-atmosphere interactions, gravity currents and gravity waves, coastally trapped disturbances, and mesoscale instabilities. Most of these phenomena are signi®cantly modulated by the diurnal cycle. In this chapter we review mesoscale atmospheric processes and provide some examples that occur in B. Wang (ed.), The Asian Monsoon. # Praxis. Springer Berlin Heidelberg 2006. 332 Mesoscale processes [Ch. 8 the Asian monsoon region, many of which are relevant to other monsoon regions of the World. 8.2 CONVECTION Atmospheric convection is one of nature's most complex and multifaceted phenomena (e.g., Ludlam, 1980; Cotton and Anthes, 1991; Emanuel, 1993; and Houze, 1993). Considerable insight into tropical and monsoon convection has been gained from ®eld experiments such as the GATE (GARP Atlantic Tropical Experiment), MONEX (Monsoon Experiment), and TOGA/COARE (Tropical Ocean±Global Atmosphere/Coupled Ocean±Atmosphere Response Experiment) (Houze and Betts, 1981; Johnson and Houze, 1987; Godfrey et al., 1998); however, many aspects of convection are still not well understood. A map of the global distribution of precipitation based on data from the Tropical Rainfall Measuring Mission (TRMM) is presented in Figure 8.1 (color section). Much of the World's heaviest rainfall occurs in the regions of the Asian± Australian monsoon. The largest annual totals occur in proximity to coastlines, suggesting possible roles of sea and land breezes and topographic eects in the precipitation mechanisms. To understand this distribution or other aspects of monsoon rainfall, we need to ®rst examine the precipitation characteristics and structural properties of moist convection. 8.2.1 Distribution, organization, and structure of tropical convection There is growing evidence from ®eld experiments over the past three decades that convective systems in the various monsoon and tropical regions of the World bear a close resemblance to each other. In particular, deep convection tends to organize on the mesoscale and undergo an evolution characterized by a dominance of convective precipitation (localized heavy rainfall) early in the life cycle followed by an upscale growth and development of stratiform precipitation (lighter rainfall) on a timescale of 2±4 hours and longer (Zipser, 1977; Houze, 1977; Leary and Houze, 1979). The stratiform precipitation is partly a result of the transfer of hydrometeors from the convective region and partly a result of in situ condensation and deposition in the stratiform region. The net result is a mesoscale convective system or MCS, de®ned by Houze (1993) as a cloud system that occurs in connection with a cluster of showers and produces a contiguous precipitation area 100 km or more in horizontal scale in at least one direction. Global climatologies of MCSs in the monsoon regions were ®rst carried out using satellite studies of mesoscale convective complexes or MCCs (Maddox, 1980), the largest and longest lived of MCS populations. Laing and Fritsch (1997) present a map of MCC locations using satellite data (Figure 8.2). They found that MCCs are (1) mostly continental, (2) tend to occur in gradient zones between OLR maxima and minima (i.e., they are normally not in the most frequently raining areas), and (3) tend to occur in the lee (relative to the prevailing mid-level ¯ow) of Sec. 8.2] 8.2 Convection 333 JJA DJF OLR 225 WmÀ2 OLR 250 WmÀ2 Figure 8.2. MCC locations based on 1980s satellite data for JJA in the northern hemisphere and DJF in the southern hemisphere. Outgoing long-wave radiation (OLR) values are shaded. From Laing and Fritsch (1997) Royal Meteorological Society. elevated terrain. Figure 8.2 shows that MCCs are common not only over the region of the Asian±Australian monsoon (China, India, Bangladesh, and northern Australia) but also over South America to the lee of the Andes and West Africa. These ®ndings have recently been con®rmed and extended by TRMM microwave measurements (Nesbitt et al., 2000). The structure and dynamics of MCSs have been the subject of intensive study for the past 30 years (Houze, 1993). A recent investigation of the evolution of nearly 100 MCSs over the central USA has revealed new characteristics of such systems (Parker and Johnson, 2000). Three main patterns of MCS organization have been identi®ed (Figure 8.3). The three modes are convective lines with trailing (TS), leading (LS), and parallel (PS) stratiform precipitation. TS systems were the most common, accounting for 60% of the cases, with the LS and PS each accounting for about 20%. TS systems have received considerable attention (e.g., Houze et al., 1990), but the occurrence of LS and PS systems is not insigni®cant, and there is evidence they are important in monsoon regions. For example, MCSs in the Baiu front appear to have these precipitation structures (Ninomiya and Muarkami, 1987). Wang (2004) recently found LS organization of MCSs to be commonplace over the northern South China Sea during the 1998 South China Sea Monsoon Experiment (SCSMEX). TS systems normally propagate rapidly (10±15 m sÀ1) and, as such, produce brief, heavy rainfall but usually not ¯ash ¯oods. LS and PS systems, on the other hand, move more slowly and are often implicated in ¯ash ¯ooding as a result of slow-moving, `training', and/or back-building cells. For example, heavy rainfall over Taiwan has been attributed to back-building cells associated with the Mei-yu front (Li et al., 1997). It has long been known that the organization of tropical convection is in¯uenced predominantly by the vertical shear and convective available potential energy or CAPE (Moncrie and Green, 1972). Various observational studies in the eastern Atlantic and northern Australia have con®rmed the strong in¯uence of environ- mental winds on the structure, orientation, and propagation of convective bands 334 Mesoscale processes [Ch. 8 Initiation Development Maturity a. TS Trailing stratiform b. LS Leading stratiform c. PS Parallel stratiform 100 km Figure 8.3. Schematic re¯ectivity drawing of idealized life cycles for three linear MCS archetypes: (a) TS, (b) LS, and (c) PS. Approximate time intervals between phases: for TS 3±4 h; for LS 2±3 h; for PS 2±3 h. Levels of shading roughly correspond to, 20, 40, and 50 dBZ. From Parker and Johnson (2000). (e.g., Barnes and Sieckman, 1984; Alexander and Young, 1992; Keenan and Carbone, 1992). LeMone et al. (1998) investigated the organization of convection over the western Paci®c warm pool using aircraft data from the TOGA/COARE. In agreement with Alexander and Young (1992) they found that vertical shear in the low to mid-troposphere is a key factor in determining the orientation of convective bands, while CAPE in¯uences their depth and longevity. Their results have been recently supported by numerical simulations of convection in shear by Robe and Emanuel (2001). The results of LeMone et al. (1998) have been recently extended to the Asian summer monsoon by Johnson et al. (2005). The modes of organization of convection over the northern South China Sea during the onset of the 1998 east Asian summer monsoon have been determined using the BMRC C-POL radar located on Dongsha Island (Figure 8.4). This ®gure, adapted from LeMone et al. (1998), is a summary of the ®ndings from SCSMEX. In general, the organizational modes for SCSMEX were consistent with those determined by LeMone et al. (1998) for the western Paci®c warm pool. It is found that when the shear in the lowest 200 hPa exceeds 4 m sÀ1 and Sec. 8.2] 8.2 Convection 335 small low-level shear large low-level shear 122r (dry aloft) L L small mid-level shear small mid-level 34c M L M a L M b ML large mid-level shear large mid-level Figure 8.4. Schematic depiction adapted from LeMone et al. (1998) of four main categories of convective structures for given vertical shears in the lower troposphere (1,000±800 hPa) and at middle levels (800±400 hPa) based on COARE observations, but modi®ed to include results from SCSMEX (modes 2r and 4c added). Length of schematic convective bands is 100± 300 km; line segments in upper left frame are up to 50 km in length.