大气科学基础：2-2-大气中的平衡运动

1、2.2 大气中的平衡运动,（1）地转风 （2）梯度风 （3）热成风,（1） 地转风,地转风， 在水平气压梯度力与水平地转偏向力达到平衡的风，无加速度、无摩擦的空气水平运动,Coriolis Force+PGF,Apply this knowledge to wind forced by horizontal pressure gradient force and balanced by the Coriolis force: Net Force = PGFH + Co,L,H,980 mb,984 mb,988 mb,Start with no velocity - what are the

2、forces on the parcel?,Coriolis Force+PGF,L,H,980 mb,984 mb,988 mb,PGF,Since there is no initial speed, the Coriolis force is zero. But There is PGF. As soon as the parcel starts to move, the Coriolis force is no longer zero. The Coriolis force is small at first, but as the speed increases, the Corio

3、lis force increases.,Coriolis Force+PGF,L,H,980 mb,984 mb,988 mb,PGF,Co,V,The PGF still exceeds the Coriolis force. The parcel continues to increase in speed, and the Coriolis force continues to increase in magnitude, further turning the parcels direction to the right (Northern Hemisphere!).,Geostro

4、phic Wind,L,H,980 mb,984 mb,988 mb,PGF,Co,V,Eventually the Coriolis and PGF balance each other. Since there is no unbalanced force, there is no acceleration. The wind continues to move at constant speed in a straight line (remember, no Friction). This is called:,A geostrophic wind flows parallel to

5、the isobars. In this model of wind flow in the Northern Hemisphere, wind begins as a flow of air perpendicular to the isobars (measured in millibars) under the primary influence of the pressure gradient force (PGF). As the movement begins, the Coriolis force (CF) begins to influence the moving air c

6、ausing it to deflect to the right of its path. This deflection continues until the pressure gradient force and Coriolis force are opposite and in balance with each other,How the wind is started Pressure gradients How the wind is turned Coriolis,Lets apply this to the winds， Net Force = PGF + G + Fr

7、+ Co Consider flow aloft， Away from the ground Friction 0 Net Force = PGF + G + Co Remember that the pressure gradient force is horizontal and vertical. We shall assume that the vertical pressure gradient force is exactly balanced by the gravity force.,地转风公式,地转风的矢量公式：,风压定律（白贝罗定律）,地转风与等压线平行，在北半球，地转风指

8、向气压梯度力的右方，在南半球，地转风指向低压梯度力的左方。即背风而立，北半球低压在左，高压在右；南半球相反，低压在右，高压在左，这就是风压定律或称白贝罗定律。,（2） 梯度风,地转风只解释了大气作等速直线运动的情况，但大气的运动通常是曲线运动。 无切向加速度，无摩擦的大气水平运动称为梯度风。,The balance of forces that create a gradient wind in the Northern Hemisphere (PGF = pressure gradient force; CF = Coriolis force; Ce = centripetal force;). In this diagram, CF = Ce + PGF for the low, and PGF = CF + Ce for the high.,梯度风方程,梯度风是水平气压梯度力、水平地转偏向力和惯性离心力三者相平衡时的风。,梯度风的方向,气旋式梯度风：围绕曲率中心作逆时针旋转； 反气旋式梯度风：围绕曲率中心作顺时针旋转,（3） 热成风,由于温度水平分布不均匀引起的地转风随高度会发生变化 热成风是上层地转风与下层地转风的矢量差 用水平位势梯度表示的地转风公式：,热成风方程,热成风的方向,在给定的两个等压面之间