专栏首页气象学家呕心沥血倾力巨制T-lnP图攻略——奥斯陆的气象生活

呕心沥血倾力巨制T-lnP图攻略——奥斯陆的气象生活

文章来源 | 授权转载自知乎专栏-奥斯陆的气象生活

作者 | 冰心浴血,奥斯陆大学气象研究生

Summary of T-lnP图

搞这个东西的初衷是因为我自己学的时候也被搞的很烦,而且概念乱七八糟,脑子里一团浆糊,一方面方便自己一方面也方便各位大气学子。本攻略以本题为例:

We will look at an air parcel containing 1 g of water vapor and 400 g of dry air. The temperature of the air parcel is 5 ℃ , and it is located at 900 hPa. Give a definition of, and find the following quantities

图片解答如下图:

1. Temperature

1.Potential Temperature (Θ) – 位温: Potential temperature is the temperature a parcel of air would have it were lifted (expanded) or sunk (compressed) adiabatically to 1000hpa.

Tip: The value of the potential temperature is the temperature of the dry adiabat that runs through the temperature at the pressure level of interest, at 1000hpa.

所以,如果要找到位温,只需要将原点(o)沿着干绝热线下降到1000hpa,然后看等温线此时这个点的温度是多少就是位温(Θ)了。

2.Dewpoint temperature (Td) – 露点: 是在维持空气气压不变的情况下,把空气冷却到相对于纯水面(平面)来说达到饱和时的温度。

Tip: 将原点(o)沿干绝热线上升至与湿绝热线的数值与原气块混合比相等的地方停下。然后看等温线,此点的温度即为露点。

Explain:你可以把原气块的混合比看作一个饱和湿气块的混合比,那么你需要做的就是找到这个拥有这个饱和混合比的湿气块的温度是多少,这就是你要找的原气块的露点了。

3. Wet-bulb Temperature (Tw) – 湿球温度: The minimum temperature at which a parcel of air can obtain by cooling via the process of evaporating water into it at constant pressure. It is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel.

Tip: To find the wet-bulb temperature follow the moist adiabat through the lifting condensation level and find the temperature of the intersection of this moist adiabat with the original pressure level of interest.

找到通过LCL的那条湿绝热线,然后下降到你自己原点(O)的气压线上,那个点对应的温度就是湿球温度。

Explain:在气压不变又绝热的情况下,系统内的液态水蒸发使空气降温而达到饱和时空气所具有的温度。所以我们从原点出发沿干绝热线到达LCL(即气块饱和),然后要保持等压,所以沿着湿绝热线下降到原等压面900hpa。

4. Equivalent Temperature (Te) --相当温度: The equivalent temperature is the temperature of a parcel if, via a moist adiabatic process, all moisture was condensed into the parcel. Finding the equivalent temperature is slightly more difficult.

Tip: To find Te, follow the moist adiabat that runs through the lifting condensation level at the pressure level of interest to a pressure level in which the moist adiabat and dry adiabat have similar slopes, then go down the dry adiabat at this point back down to the original pressure level of interest; this temperature is the equivalent temperature. If the dry adiabat continues beyond the boundary of the skew-t in which it cannot be determined, an alternative is to read off the temperature scale that runs diagonally in the middle of the skew-t.

原点沿着干绝热线上升到LCL然后再沿着湿绝热线上升至湿绝热线与干绝热线斜率相同时为止(此时水汽凝结完了)。然后沿着与湿绝热线斜率相同的那条干绝热线下降到原等压面900hpa,此时的温度就是你要找的相当温度。

5. Equivalent Potential Temperature (Θe)-- 相当位温: The equivalent potential temperature is similar to the equivalent temperature however after the moisture has been condensed out of the parcel, the parcel is brought down dry adiabatically to 1000hpa.

Tip: The process to find the equivalent potential temperature is the same as the regular equivalent temperature however when the parcel is brought down the dry adiabat it continues past the original pressure level and is brought down to 1000hpa. The temperature at this intersection is the equivalent potential temperature.

其实就是在相当温度的基础上继续沿着湿绝热线下降直到1000hpa,即为相当位温。

6. Saturated Equivalent Potential Temperature (Θes) -- 假相当位温: The temperature at which an unsaturated parcel would have if it were saturated. Saturation equivalent potential temperature (represented as θes) is a special case of equivalent potential temperature (θe) computed by assuming an air parcel is saturated and thus represents the theoretical maximum θe.

Tip: To find the saturated equivalent potential temperature, use a similar process used for determining the equivalent potential temperature however one must follow the moist adiabat through the environmental temperature at the necessary pressure level, unlike using the lifting condensation level for equivalent potential temperature.

气块经湿绝热过程,将所含的水汽全部凝结放出,再沿干绝热过程到达1000hPa时的温度就是假相当位温了。

7. wet-bulb pseudo temperature (Tsw) – 假湿球温度: Wet-bulb potential temperature, sometimes referred to as pseudo wet-bulb potential temperature, is the temperature that a parcel of air at any level would have if, starting at the wet-bulb temperature, it were brought at the saturated adiabatic lapse rate to the standard pressure of 1000 hpa.(所以你其实可以认为湿球温度和假湿球温度是一个东西,那么找法是一模一样的.并不是说这两个温度没有区别,而是在气象实际应用上,因为这两个差别很小,所以就没有区别。但是在理论定义和公式上还是有很大差别的!)

Tip: 气块(原点O)沿干绝热线上升到饱和后再沿湿绝热线下降到原等压面处的温度。

8. wet-bulb pseudo potential temperature (Θsw) – 假湿球位温: Wet-bulb potential temperature, sometimes referred to as pseudo wet-bulb potential temperature, is the temperature that a parcel of air at any level would have if, starting at the wet-bulb temperature, it were brought at the saturated adiabatic lapse rate to the standard pressure of 1000 hpa.(所以你其实可以认为湿球位温和假湿球位温是一个东西,那么找法是一模一样的。并不是说这两个温度没有区别,而是在气象实际应用上,因为这两个差别很小,所以就没有区别。但是在理论定义和公式上还是有很大差别的!)

Tip: 气块沿干绝热线上升到饱和后再沿 湿绝热线下降到1000hPa处的温度。

2. Levels

1. Lifting Condensation Level (LCL) – 抬升凝结高度: The level at which a parcel of air first becomes saturated when lifted dry adiabatically.

Tip: This level can be found by finding the intersection of the dry adiabat through the temperature at the pressure level of interest, and the mixing ratio through the dewpoint temperature at the pressure level of interest.

2. Convective Condensation Level (CCL) -- 对流凝结高度:The convective condensation level (CCL) represents the height (or pressure) where an air parcel becomes saturated when heated from below and lifted adiabatically due to buoyancy.

Tip: To find the convective condensation level, find the intersection of the mixing ratio through the dewpoint temperature at the pressure level of interest and the temperature sounding.

3. Level of Free Convection (LFC) – 自由对流高度:The level in which a parcel first becomes positively buoyant.

Tip: To find the level of free convection, find the lifting condensation for the level of interest, and find the intersection of the moist adiabat that goes through the LCL, and the temperature curve.

4. Equilibrium Level (EL) – 自由对流高度:The height at which a rising parcel of air is at the same temperature as its environment. The point at which a positively buoyant parcel becomes negatively buoyant, which typically will occur in the upper troposphere.

Tip: To find this level, find the level of free convection, follow the moist adiabat through this level of free convection up until it intersects the temperature sounding again. This point is the equilibrium level.

3. Vapor Pressures

1. Vapor Pressure (e)—水汽压: The amount of atmospheric pressure that is a result of the pressure from water vapor in the atmosphere.

Tip: To find the vapor pressure follow an isotherm (a line parallel to an isotherm) through the dewpoint temperature at the pressure level of interest, up to 622hpa. The value of the mixing ratio at this intersection is the vapor pressure in millibars.

根据给定的露点,在露点??等温线与 622h??等压线的交点处,读出通过该点的等饱和比 湿线的数值,即得到该状态点的实际水汽压值

2. Saturated Vapor Pressure (es) – 饱和水汽压:The amount of atmospheric pressure that is a result of the pressure of water vapor in saturated air.

Tip: This quantity can be found using similar means as the vapor pressure however one must follow a parallel isotherm through the temperature at the pressure level of interest.

根据给定的温度?,找到?等温线与622h??等压线的 交点,读出通过该点的等饱和比湿线的数值,即得 到温度?时的饱和水汽压值。

Explain:

4. Mixing Ratios

1. Mixing Ratio (r) – 混合比: The mixing ratio is the ratio of the mass of water vapor in the air over the mass of dry air.

Tip: This quantity is found by reading the mixing ratio line that goes through the dewpoint temperature at the pressure level of interest.

因实际比湿?是露点为??时的饱和比湿,故可根据给定的气压和露点值找出一点D,读出 D点的等饱和比湿线上的饱和比湿值,即为所求之实际比湿。因实际应用中比湿与混合比数值大小近似相等,可当作相等,但定义有差别!

2. Saturated Mixing Ratio (rs) – 饱和混合比:A similar mixing ratio as above, however it is the mixing ratio of a saturated parcel of air at a given temperature and pressure.

Tip: It can be found by finding the value of the mixing ratio through the temperature at a pressure level of interest. 根据给定的气压和温度,在图中找到一点A,读出通过A点的等饱和比湿线的数值,即为所求的饱和比湿值??(?)。

5. stability(大气静力稳定度判据)

薄层大气近似:

令?和Γ分别表示气块和环境气层的垂直减温率,平衡位置的大气温度是?,且假设Γ是常数0。

以下给出实际例子:

1. 绝对稳定:如果温度递减率小于饱和绝热递减率,也小于干绝热递减率,则该层是绝对稳定的(此处的减温率说的是环境减温率(即整层大气),如果大气减温率小于饱和绝热递减率(湿绝热过程)则说明气块的减温率无论如何都要比环境减温率大了,因此气块一定比环境冷,因此绝对稳定---绝对稳定大气也是冬季污染持续的成因之一)

2. 绝对不稳定:如果温度递减率大于干绝热递减率,也大于饱和绝热递减率,则该层绝对不稳定。

3.条件不稳定/稳定:如果温度递减率小于干绝热递减率,但大于饱和绝热递减率,则该层是条件不稳定的/稳定的。意思是:仅当不饱和时,该层是稳定的;如果饱和,则该层是不稳定的。

4.中性稳定:如果温度曲线平行于饱和绝热线或干绝热线,则该层与环境大气处于中性平衡态。如果平行于饱和绝热线,则饱和气块的向上移动不会受到环境的增强或抑制。同样,如果平行于干绝热线,不饱和气块的向上移动也不受环境的影响。

由于干绝热线和假绝热线同时又是等位温线和等相当位温线,所以也有以下判据:

参考

  • https://zhuanlan.zhihu.com/p/129470053

—END—

本文分享自微信公众号 - 气象学家(Meteorologist2019),作者:冰心浴血

原文出处及转载信息见文内详细说明,如有侵权,请联系 yunjia_community@tencent.com 删除。

原始发表时间:2020-04-15

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