weatherguru18 wrote:A1A wrote:Evil Jeremy wrote:Alot of energy flowing to the newly formed blob as we get ready for the D-Max. Tonight's recon should provide some answers.
What is the D-Max and what is its significance?
Ok, I'm going to try here. Somebody feel free to add to what I say...
The D-Max or Diurnial Maximum is a period just before sunrise (usually about 5 or 6 am) when thunderstorms are most prevelent. Why is this? Thunderstorms thrive off latent heat. Latent heat, the best I can describe it, is the "heat" given off when an ice cube goes from a solid to a liquid. It has to give off heat to make a phase change. This is latent heat. Latent heat is most prevelent at night because the air is cooler and more condensation can occur. The latent heat is derived from the gas (water vapor) going from a gas to a liquid. In order for this to happen, the water vapor, a gas, must give off heat to make the change. That's why we see convection or thunderstorms fire up during the overnight hours. Anybody else care to elaborate?
You are partly right. What you said about the latent heat is correct. The latent heat of fusion is the amount of energy required to melt a certain mass of ice into liquid water, or the amount released when the opposite occurs, (water freezes into ice). The latent heat of vaporization (or condensation) is the same, except for the change between liquid and vapor phases. In a thunderstorm, both latent heat of condensation, and latent heat of fusion are active, but the former is more important. Both however, contribute the updraft intensity by heating the air and making it more buoyant.
However, this has nothing to do with the diurnal max/min of convection. Latent heating is not tied to time of day, only to the phase changes of water. However, the vertical profile of temperature in the atmosphere IS largely dependent on the diurnal cycle. The steeper the lapse rate, the more buoyant thunderstorm updrafts are, because the temperature difference in the core of the storm versus outside of it will be greater. During the daytime over land is the diurnal max for land-based convection, because the sun heats up the relatively dry land quickly. For the tropical oceans, however, the sea surface temperature doesn't change much between daytime and nighttime, due to the high heat capacity of water. However, the upper-levels of the atmosphere, especially in the presence of clouds, which radiate heat effectively to space, cool off at nighttime, while the warm sea surface below (and thus the the low-levels of the atmosphere near the ocean surface) stays nearly the same temperature. The result is an increase in atmospheric instability at night over tropical oceans, enhancing convection.