Diurnal Heating

[Vinnie Frevert]

Moisturein the air comes from the oceans or large bodies of water that evaporate moisture into the air. This moisture is responsible for making clouds. Unstable air occurs when warm moist air near the ground encounters cold drier air at higher altitudes. Since warm air rises, anything that warms the air close to the ground will nudge it higher. As the warm air lifts higher and higher, it causes clouds to grow taller and tall, resulting in the towering anvil-shaped could characteristic of thunderstorms.

The heat radiated by the earth reaches its maximum by 3 to 4-hours after solar noon, which is why it feels hottest in the mid-afternoon and not at midday. This also explains why most thunderstorms occur in the afternoon when the degree of lift caused by diurnal heating reaches its peak.

If the weather forecast calls for diurnal heating with a chance of thunderstorms later in the day, it can be prudent to time your hike so that you get below treeline or undercover before mid-to-late afternoon to avoid the accompanying high winds and lightning. In the absence of a forecast, you can usually recognize that a thunderstorm is coming and take evasive action if you see the towering anvil shape forming overhead.

Once I was leading a hike to Garfield and about a quarter of a mile from the summit the sky went black in the valley. We were below tree line so we spread out on the trail. Ironically we were at a high enough elevation that we could get a radar image on our phones from Cannon cell towers. We could tell that if we sat tight, the thunderstorms in the valley would pass and we had an hour and an half to summit Garfield until the next cells moved in.

I got caught in a Derecho and only got a warning about 30 mins before it was hit by a inreach text. I did know what it was or how serious it was. Likewise, I have never heard of diurnal heating and was interested in learning more about it. Weather is no joke in the mountains and I have learned that understanding weather is and important outdoor skill.

Good explanation of Diurnal Heating, and yes 3 ingredients to Tstorms are Moisture, Instability and Lift. But while the moisture comes from the ocean, it is the amount of moisture in the specific air mass that determines likelihood of Tstorms. Also instability is not created by warm moist air rising into cold dry air but rather instability (lower pressure) of an air mass that allows the air to rise and mix. This is why when you have a large high pressure system Tstorks are unlikely. Also why do dont get Tstorms in the winter because the air is too cold to hold enough moisture.

In this paper, the authors assess the suitability of the heating fields in the latest version of the NCAR Community Climate Model (CCM2) for modeling the thermal forcing of atmospheric tides. Accordingly, diurnal variations of the surface pressure, outgoing longwave radiation, cloudiness, and precipitation are examined in the CCM2. The fields of radiative, sensible, and latent beating are similarly analyzed. These results are subjectively compared with available data.

Equatorial diurnal surface pressure tides are fairly well simulated by CCM2. The model successfully reproduces the semidiurnal surface pressure tides; however, this may result in part from reflection of wave energy at the upper boundary. The CCM2 large-scale diurnal OLR is generally consistent with observations. The moist-convective scheme in the model is able to reproduce the diurnally varying cloudiness and precipitation patterns associated with land-sea contrasts; however, the amplitudes of CCM2 diurnal continental convective cloudiness are weaker than observations. The CCM2 boundary-layer sensible heating is consistent with a very limited set of observations, and with estimates obtained from simple models of diffusive heating. Although the CCM2 tropospheric solar radiative heating is similar in magnitude to previous estimates, there are substantial differences in the vertical structures. A definitive assessment of the validity of the CCM2 diurnal cycle is precluded by the lack of detailed observations and the limitations of our CCM2 sample. Nevertheless, the authors conclude that the global-scale components of the CCM2 diurnal heating are useful proxies for the true diurnal forcing of the tides.

Temperature lag, also known as thermal inertia, is an important factor in diurnal temperature variation. Peak daily temperature generally occurs after noon, as air keeps absorbing net heat for a period of time from morning through noon and some time thereafter. Similarly, minimum daily temperature generally occurs substantially after midnight, indeed occurring during early morning in the hour around dawn, since heat is lost all night long. The analogous annual phenomenon is seasonal lag.

Diurnal temperature variations are greatest very near Earth's surface. The Tibetan and Andean Plateaus present one of the largest differences in daily temperature on the planet, as does the Western US and the western portion of southern Africa.

In the absence of such extreme air-mass changes, diurnal temperature variations typically range from 10 F (5.6 C) or smaller in humid, tropical areas, up to 40 to 50 F (22.2 to 27.8 C) in higher-elevation, arid to semi-arid areas, such as parts of the U.S. Western states' Intermountain Plateau areas, for example Elko, Nevada, Ashton, Idaho and Burns, Oregon. The higher the humidity is, the lower the diurnal temperature variation is.

In Europe, due to its more northern latitude and close proximity to large warm water bodies (such as the Mediterranean), differences in daily temperature are not as pronounced as in other continents. However, places in Southern Europe significantly far from the Mediterranean tend to have high differences in daily temperatures, some around 14 C (25 F). These include Southwestern Iberia (e.g. Alvega or Badajoz) or the high-altitude plateaus of Turkey (if considered part of Europe) (e.g. Kayseri).

Diurnal temperature variation is of particular importance in viticulture. Wine regions situated in areas of high altitude experience the most dramatic swing in temperature variation during the course of a day. In grapes, this variation has the effect of producing high acid and high sugar content as the grapes' exposure to sunlight increases the ripening qualities while the sudden drop in temperature at night preserves the balance of natural acids in the grape.[4]

Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime.

The pertinent similarity parameters are shown to be Gc, for daytime convective flow and Gd for nocturnal flow; here Gc = Hb/Hc, Gd = Hb/Hd, where Hb, is the mountain height, Hc the neutral buoyancy height of free convection. and Hd the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. The spatial and temporal structures of these motion fields are delineated, with the following being among the most important observations: (i) during the daytime, the upslope convective flow in the vicinity of the mountain tends to suppress convective turbulence over the horizontal plains; (ii) during the early evening, horizontal jets, with the principal one directed toward the mountain, develop above the mountain surface, and vortices in the vertical cross section develop both above and below the jets, following the collapse of the convective motion over the mountain; and (iii) in the evening, a downslope drainage flow is initiated following the establishment of a vertical vortex on the mountain slope and under the jet.

Quantitative experimental observations are made, which demonstrate the variation of various flow observables with the pertinent similarity parameters. These results are applied to the atmosphere following similarity relations between the physical model and the atmosphere. The predicted characteristic speeds and length scales of the daytime upslope flow and the nocturnal drainage flow for typical atmospheric parameters are in reasonable agreement with limited field observations.

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