300 Part 8. Conceptual Models of Mesoscale Convective Systems (MCSs) In this chapter we focus on thunderstorm systems, which are larger in horizontal scale than the multicellular, or supercell thunderstorms we discussed in the preceding chapters. We refer to these systems as mesoscale systems because they are too small to be captured by the routine upper-air sounding network (called synoptic observations), yet they are too large to be adequately sampled by special field experiments designed to look at individual thunderstorms. Thus, observation of mesoscale convective systems requires large-scale, coordinated field. Knowledge of mesoscale convective systems is still quite limited, since we have only been able to observe them for a short period of time during special thunderstorm-scale experiments. Moreover, because they contain distinct thunderstorm elements, they are a challenge to modelers, since the models must cover a domain of several hundred kilometers yet have fine enough resolution to simulate the thunderstorm elements properly. Squall-Line Thunderstorms Ordinary Squall Lines Mesoscale convective systems organize into a variety of configurations, the easiest to recognize is the squall line. Squall-line thunderstorm systems occur throughout the tropics and mid-latitudes. An observer at the surface normally sees a sharp roll-like line of clouds followed by a sudden wind squall or gust of 12 to 25 m/s. Immediately behind the surface squall a heavy downpour starts, which may produce as much as 30 mm of rain in 30 minutes in the tropics. Often the heavy downpour is followed by several hours of steady rainfall from the stratiform anvil cloud that trails the squall front. Figure 8.1 illustrates a low-level radar depiction of an ordinary tropical squall line. Tropical squall lines are somewhat bow-shaped rather than being perfectly straight lines. Tropical301 Figure 8.1. Schematic diagram of radar reflectivity at low levels (0.5 - 1.5 km) for a tropical squall line. (Adapted from Chauzy, S., M. Chong, A. Delannoy, and S. Despiau, 1985: The June 22 tropical squall line observed during COPT 81 experiment: Electrical signature associated with dynamical structure and precipitation. J. Geophys. Res., 90, 6091-6098.) squall lines are typically embedded in easterly flow with the strongest easterly winds at lower levels. As a result, the low-level surface squall travels faster than the upper-level cloud debris spewed from the rising convective towers at the squall line. Thus the highest radar reflectivities occur along the leading edge of the line, where convective updrafts produce intense rain showers. A vertical cross section through the squall line is illustrated in Figure 8.2. This shows the strong convective updrafts along the leading edge with pronounced flow from the front of the storm at low levels toward the rear of the storm at middle and upper levels. This front-to-rear flow transports moisture, cloud droplets, ice crystals and graupel particles toward the rear of the system. The faster-falling, heavily-rimed graupel particles settle out rather quickly just behind the convective line. The slowly302 Figure 8.2. Schematic of vertical cross section of radar reflectivity along the A-A′ in Figure 8.1. Also shown are storm-relative streamlines of flow through the squall line. (Adapted from Chauzy, S., M. Chong, A. Delannoy, and S. Despiau, 1985: The June 22 tropical squall line observed during COPT 81 experiment: Electrical signature associated with dynamical structure and precipitation. J. Geophys. Res., 90, 6091-6098.) settling ice crystals and moist air lags behind the rapidly advancing squall line and forms the deep and widespread stratiform anvil cloud. Latent heat released during the vapor depositional growth of the snow crystals and freezing of supercooled raindrops warms the air in the middle and upper troposphere, causing slow, rising motion in those regions. The slow, rising motion causes adiabatic cooling, which produces more condensation, fueling further the formation of precipitation in the trailing stratiform anvil region. The vertical divergence (or change in magnitude with height) of longwave radiation at the top of the stratiform anvil cloud also contributes to the upward motion in the stratiform anvil cloud. We have seen previously that all bodies emit radiation, and the amount of energy emitted is greater, the higher the temperature of the emitting body. For the temperature of the earth's surface and the air in the troposphere, the emitted electromagnetic energy is in the infrared region. An important property of infrared radiation is that it is not absorbed appreciably in cloud-free air. As a result much of303 the infrared radiation emitted at the earth's surface passes through the atmosphere and escapes out to space (see Figure 8.3a). When clouds are Figure 8.3. Illustration of infrared radiant energy absorbed and received at different levels in the atmosphere. Left panel illustrates a cloud-free atmosphere. Right panel illustrates a stratiform-anvil cloud layer. Length of arrows is proportional to the amount of radiant energy per unit area. present the situation changes rather dramatically, as clouds are excellent absorbers of infrared radiation. Hence, upwelling radiation emitted from the earth's surface is absorbed at the base of the stratiform anvil cloud, where it is re-emitted downward towards the earth's surface and upward to levels of the cloud above where it is further absorbed and re-emitted. Radiative warming of a layer in the atmosphere is caused by a net gain in radiant energy at a given level in the atmosphere. Thus, as illustrated in Figure 8.3b, at the base of the stratiform anvil cloud, more energy is received from the radiant energy upwelling from the earth's surface than is radiated downwards from cooler cloud layers immediately above. As a result, longwave radiation causes modest warming near the base of stratiform anvil clouds. Because clouds are excellent absorbers of infrared radiation, little radiative heating occurs through much of the cloud layer. Each level in the interior of a cloud is exposed to upwelling and downward radiation from cloud levels immediately above and below, which are nearly at the same temperature as304 the given cloud level and therefore radiate about as much energy as the cloud level in question. Layers in the interior of a cloud, therefore, receive no significant net