The maxima of wind gusts which need to be considered in the structural design of aircraft to achieve certification are up- or downdrafts.
FAR part 25.341 says:
The following reference gust velocities apply:
(i) At airplane speeds between V$_B$ and V$_C$: Positive and negative gusts with reference
gust velocities of 56.0 ft/sec EAS must be considered at sea level.
The reference gust velocity may be reduced linearly from 56.0 ft/sec
EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust
velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000
feet to 26.0 ft/sec EAS at 50000 feet.
Since wind rarely flows into or out of the ground, vertical gust speeds are less of a concern during takeoff and landing. Here, lateral speeds (crosswind) and changes in horizontal speed (gradients due to microbursts) constitute the most important limits.
For crosswind limits, FAR part 25.237 states:
(a) For land planes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, demonstrated to be
safe for takeoff and landing, must be established for dry runways and
must be at least 20 knots or 0.2 V$_{SR0}$, whichever is greater,
except that it need not exceed 25 knots.
In the end, it is up to the pilot to decide whether to attempt to land in adverse conditions, to go around, or to fly to an alternate destination. This answer gives some criteria which can be used by pilots to decide if they better go around. Airports are closed rarely and only when severe weather approaches, such as thunderstorms and tornados, but then it is not due to a specific wind speed, but to the expected maximum speeds, which then will far exceed certification limits.
Vertical gusts become more stressing with speed, because at the low lift coefficient of high speed flight a vertical gust will produce a relatively higher increase in lift coefficient than at low speed. In extreme cases, the wing will stall due to the gust, which will limit the additional structural loads.
Horizontal speed gradients, which are caused by downdrafts hitting the ground and spreading out, are most harmful at low speed. First the aircraft will encounter a headwind, which slows it down, and then rapidly cross into a tailwind which will cause airspeed to drop, in extreme cases below stall speed. This can even crash a large airliner.