Final answer:
The Coriolis force modifies the gradient force causing the wind to flow parallel to isobars above 2,000 feet, a state known as geostrophic balance.
Step-by-step explanation:
The force that modifies gradient force and causes the wind to flow parallel to the isobars above 2,000 feet is the Coriolis force. At this altitude, frictional forces are negligible, and the balance between the Coriolis force and the gradient force is known as geostrophic balance. The gradient force attempts to move air from high-pressure areas to low-pressure areas, perpendicular to the isobars. However, the Coriolis force, due to the Earth's rotation, acts at a right angle to the wind's velocity, causing the wind's path to curve. In the Northern Hemisphere, this deflection is to the right, while in the Southern Hemisphere, it is to the left. When the two forces are equal in magnitude but opposite in direction, the resultant wind flows parallel to the isobars, creating what is known as geostrophic wind.